Strengthening Universal Jurisdiction for Maritime Piracy Trials to Enhance a Sustainable Anti-Piracy Legal System for Community Interests

: Although universal jurisdiction over piracy has long existed in customary international law and international conventions, such as the Convention on the High Seas (HSC) and the United Nations Convention on the Law of the Sea (UNCLOS), the piracy situation has been changing. The subsidence of Somali piracy provides an opportunity for rethinking how to strengthen universal jurisdiction for maritime piracy trials to enhance a sustainable anti-piracy legal system. The incidents of Somali piracy have resulted in some new developments in exercising universal jurisdiction: the separation of seizing, prosecuting, and imprisoning States; the consideration of creative piracy prosecution mechanisms; the increased focus on land-based facilitation of piracy; enhanced international cooperation; and expanded universal jurisdiction. This leads to several main challenges in existing legal frameworks, including weaknesses in UNCLOS, the disharmony among international instruments, and defects in domestic piracy legislation. In order to sustain and improve the anti-piracy legal system, universal jurisdiction over piracy should be incrementally strengthened to support the prosecution of pirates by States. To address the trends and challenges, this article explores how the legal system can be enhanced in two respects: adjusting the basic provisions of universal jurisdiction over piracy and reﬁning the relevant measures in exercising that jurisdiction to prosecute pirates.


Introduction
Piracy is the oldest international crime, and yet despite considerable global efforts, it persists and continues to evolve in various ways and contexts. The international community is equally persistent in its response, building on longstanding international legal frameworks and adopting new approaches to meet the challenge. Piracy has significant impacts on multiple aspects of sustainability.
The United Nations (UN) Sustainable Development Goal (SDG) 16 (Peace, Justice and Strong Institutions), for example, seeks to, inter alia, "significantly reduce all forms of violence and related death rates everywhere" (16.1), "significantly reduce illicit financial and arms flows, strengthen the recovery and return of stolen assets and combat all forms of organized crime" (16.4), and "strengthen relevant national institutions, including through international cooperation, for building capacity at all levels, in particular in developing countries, to prevent violence and combat terrorism and crime" (16.A) [1]. According to SDG 16, piracy, which poses a serious threat to freedom of navigation, maritime security, and human life, is the target of repression. Strengthening anti-piracy law will significantly contribute to the achievement of SDG16.
As a transnational organized crime at sea, piracy could impact other SDGs [2]. For example, maritime transport is "fundamental to sustaining economic growth and spreading prosperity throughout the world" [3]. Piracy negatively impacts on economic aspects of shipping and transport. Therefore, it directly affects economic growth in SDG 8 (Decent Work and Economic Growth) [4] and then prevents achieving the goal of poverty eradication (SDG 1: No Poverty) [4]. Since disruptions to supply chains may harm sustainable cities and communities [5], piracy also negatively impacts on SDG 11 (Sustainable Cities and Communities) [4]. Piracy threatens the lives and health of seafarers and passengers, which is related to SDG 3 (Good Health and Well-Being) [4]. When seafarers do not have a safe working environment, the goal of decent work in SDG 8 is affected. Piracy also poses a threat to food security [3]. Just as Somali pirates had an impact on the food transportation of the World Food Program [6], so anti-piracy may contribute to SDG 2 (Zero Hunger) [4]. Furthermore, piracy is potentially environmentally harmful, as vessels are attacked and lost, which leads to negative impacts on SDG 6 (Clean Water and Sanitation) and SDG 14 (Life below Water) [4]. Consequently, the anti-piracy legal system is conducive to achieving these SDGs. The enactment and implementation of anti-piracy laws and regulations needs partnerships among States and international organizations, which itself strengthens partnerships (SDG 17: Partnerships) [4,7]. Under international law, States have universal jurisdiction over piracy [8]. However, with the rapid expansion of Somali piracy, the international community faced tremendous challenges in exercising this universal jurisdiction over piracy. In many cases, approaches have involved "catch and release", and many pirates have not been prosecuted and have gone unpunished [9]. In this context, it is valuable to explore why this may be the case and how anti-piracy law can be strengthened for the future. Many scholars have researched various aspects of universal jurisdiction over piracy. They have addressed a range of issues including the history and basis of universal jurisdiction over piracy [10][11][12][13][14], definitional issues of piracy [15,16], the conflict of jurisdictions and weaknesses in domestic law [17], and the application and use of universal jurisdiction [18][19][20]. However, this scholarship frequently involves only certain aspects of the universal jurisdiction over piracy and does not analyze the anti-piracy legal system as a whole, nor does it examine the challenges brought by the current situation of piracy and the development trend of universal jurisdiction. The relationship between the anti-piracy legal system and the sustainable development goals, as well as the sustainability of the legal system itself, are not particularly concerned, either.
This article aims to enhance an anti-piracy legal system that is helpful for achieving the goals of the SDGs and that has its own sustainability issues. By investigating the legal provisions and the actual situation, this article comprehensively analyzes the problems of the lack of sustainability of the anti-piracy legal system related to universal jurisdiction and puts forward systematic solutions.
According to the definition of sustainable development in the Report "Our Common Future", which is considered to be the most accepted definition [3], sustainable development means "development that meets the needs of the present without compromising the ability of future generations to meet their own needs" [21]. The definition emphasizes the intergenerational aspects of development. It can be seen that solutions to sustainability problems need to have a long-term perspective and should be able to cope with new situations that arise, or at least should be agile enough to be adjusted in accordance with the evolving situation. The universal jurisdiction over piracy is stipulated in the Convention on the High Seas (HSC) and the United Nations Convention on the Law of the Sea (UNCLOS). Nonetheless, great changes have taken place in the situation of piracy compared with that at the time of the conclusion of the Conventions (Part Two), and new trends have emerged in the exercise of the universal jurisdiction over piracy (Part Three). In addition to the deficiencies of both international and domestic piracy law (Part Four), the existing anti-piracy legal system cannot fully and effectively respond to modern piracy and lacks sustainability to a certain extent. In this context, it is valuable to explore a long-term and comprehensive solution for the future. Therefore, various relationships must be balanced (Part Five). This article also explores approaches to enhance a sustainable, international anti-piracy legal system and how national level implementation and enforcement can be supported (Part Six).
It should be recognized that there are many factors, other than law, affecting the sustainable exercise of universal jurisdiction over piracy. States may face tremendous financial burdens and capacity issues in prosecuting and incarcerating pirates [18], and they encounter the problem that pirates may seek asylum in these States after their release [22]. Some States even lack political will to take action to counter piracy [23]. Additionally, the rise of piracy is a complicated problem, rooted in poverty and instability, which must be addressed comprehensively if long-term success is to be achieved [24]. All these issues remain important, but the scope of this article is limited to strengthening universal jurisdiction for piracy trials through legal means.
As background, the main international piracy legal framework is set out in Appendix A.

The Necessity of Strengthening Universal Jurisdiction over Piracy to Enhance a Sustainable Anti-Piracy Legal System
UNCLOS sets out the legal framework of universal jurisdiction over piracy. Its provisions on piracy originated from HSC, which has not been changed, and there is no new global convention specifically aimed at piracy (especially universal jurisdiction over piracy). Contrarily, piracy activities and contexts are always changing. Based on the yearly statistics of piracy incidents that occurred between 1984 and 2019, and statistics of piracy and armed robbery incidents in international waters since 2008 (see Table 1) (For statistical purposes, the data do not distinguish between piracy and armed robbery), the following conclusion can be drawn: Table 1. Statistics of piracy and armed robbery incidents in international waters since 2008. Out of  the Total   2008  154  306  50%  2009  250  406  62%  2010  294  489  60%  2011  330  544  61%  2012  125  341  37%  2013  66  298  22%  2014  82  291  28%  2015  36  303  12%  2016  58  221  26%  2017  52  203  26%  2018  50  223  22%  2019  34 193 18%

The Threat of Pirate Attacks Has Increased Significantly Since UNCLOS Was Formulated
The provisions on piracy in HSC and UNCLOS were once considered obsolete. As early as in the First UN Conference on the Law of the Sea, representatives of the Ukrainian Soviet Socialist Republic and Poland held that the draft provisions of the International Law Commission (ILC) on piracy were anachronistic because piracy in the strict sense was rare, and piracy had not been a real problem for a long time [37]. Czechoslovakia and Romania both believed that the draft used too many provisions to regulate the problems related to the concept of piracy in the 18th century [37]. The representative of Uruguay also said that the relevant provisions on piracy should be deleted [37]. The representative of The Union of Soviet Socialist Republics even believed that most members of the Second Committee of the First UN Conference on the Law of the Sea, which was in charge of the discussion about the general regime of high seas, supported excluding piracy-related provisions from the draft convention [37]. Likewise, in the Third UN Conference on the Law of the Sea, the representative of the Khmer Republic also pointed out that the provisions on piracy had become a dead letter due to the lack of application scenarios [38]. It can be seen from this that the relevant anti-piracy provisions were not believed to reflect the actual "dangerous forms" of maritime security threats that existed at that stage [37]. Therefore, the provisions cannot be expected to be sustainable enough to fully adapt to modern piracy (this does not mean to deny the status of the conventions as the core international instruments against piracy).
The yearly statistics of piracy incidents that occurred between 1984 and 2019 are recorded in the International Maritime Organization (IMO) Piracy Annual Report 2019 [25]. Although the accurate quantity of pirate incidents at the time of the enactment of UNCLOS in 1982 is not available here, with reference to the opinions of representatives of the above States and the situation in the years since 1984, it is judged that the piracy activities at that time were relatively infrequent. From 1984 to 1989, the average annual number of piracy incidents was only 23.8, which was in sharp contrast to the highest of 544 in 2011 [25]. Since 1995, the number has remained at a high level, hovering between 200 to 500 [25]. The number of incidents did not fall back to less than 200 until 2019 (193) [25]. Nevertheless, the number is much higher than that in 1984 (32) and the average annual number of incidents from 1984 to 1989 (23.8) [25]. Although the practical application value of the provisions on piracy in UNCLOS was not prominent at the time of its enactment, since then, piracy activities have increased significantly and are still at a high level. These provisions not only contribute to the integrity of UNCLOS as a maritime regime, but they also play the role of guiding and regulating the international law basis for anti-piracy actions, which have proven to be successful in countering Somali pirates.

The Number of Piracy Incidents Fluctuates in General, Often with Dramatic Changes
The number of piracy incidents has frequently expanded after a period of relatively low number of incidents. The number in 1987 (29) increased by 61% compared with 1986 (18); the number in 1990 (50) increased by 257% compared with 1989 (14); and number in 1991 (79) increased by 58% compared with 1990 (50) [25]. The growth rate was obviously high, but due to the small base at that time, the total number of incidents was relatively small. Since 1994, with the increase of the base and the surprising growth rate, the number of piracy incidents rapidly reached a new height. From 48 in 1994 to 132 in 1995 and 227 in 1996, the latter two years increased by 175% and 72%, respectively, compared with their previous year, with an overall growth rate of 373% [25]. The period from 2006 to 2011 is also a period of rapid growth, with an increase of 126.67% in 2011 (544) compared with 2006 (240) [25]. The largest increase in number occurred in 2000, with 162 more incidents than 1999 [25].
Therefore, the international community should always be vigilant against piracy and be prepared to deal with the explosive growth of piracy at any time, rather than slacken efforts with the decrease of the number of piracy incidents. Otherwise, as occurred with the outbreak of Somali piracy, the shortcomings and unsustainability of the legal system will be exposed. The temporary cessation of piracy provides an opportunity for seeking sustainable, long-term solutions to the piracy problem [39]. It is still necessary to build a sustainable anti-piracy legal system, especially in the areas that need long-term and large investment, such as legislation, law enforcement, prosecutions, and trials relating to universal jurisdiction over piracy.

The Number of Piracy Incidents in Seas Beyond Territorial Waters Is Considerable
As shown in Table 1, before 2012, when Somali piracy was rampant, the number of incidents in international waters and the total number of piracy incidents around the world were both at a high level, and the proportion of the former out of the latter was extremely considerable, reaching more than 50% from 2008 to 2011. Since 2012, with the gradual reduction of Somali piracy, the number declined accordingly. However, the number of incidents in international waters and its proportion out of the total number of incidents are still far from negligible. The average annual proportion from 2013 to 2019 is 22%. According to the definition of piracy, attacks in international waters may constitute piracy, thus universal jurisdiction will apply. Additionally, universal jurisdiction can also be applied to armed robbery in special circumstances. The UNSC resolutions have authorized navies of all States to enter Somali territorial waters to counter both piracy and armed robbery [6]. Therefore, although maritime attacks mainly occur within the scope of national jurisdiction, universal jurisdiction is still an important means to combat piracy. This implies that the universal jurisdiction over piracy is still an essential part of a sustainable anti-piracy legal system.

New Trends in the Use of Universal Jurisdiction over Piracy
Before the fight against Somali pirates, the application of universal jurisdiction was very rare. Many States had not prosecuted pirates for hundreds of years. For example, US v. Ali Mohamed Ali in 2012 was considered as the first US piracy prosecution invoking universal jurisdiction in approximately 200 years [40]. In 2012, Germany had the first piracy trial, in which 10 Somali pirates were convicted, in more than 400 years [41]. The expansion of incidents of Somali piracy and the fight against it have resulted in several new developments, each of which is examined below.

Separation of Seizing, Prosecuting, and Imprisoning States
Under Article 105 of UNCLOS, "the courts of the State which carried out the seizure may decide upon the penalties to be imposed" [42]. In this case, the seizing State is the same as the prosecuting State and, generally speaking, criminals serve their sentences in the prosecuting State. In combating Somali piracy, a new trend arose, especially in the regional piracy prosecution model, by which multilateral naval forces played the main role in seizing pirates, and other States within the region (regional States) accepted and prosecuted pirates [43]. After conviction, some pirates remained in the prosecuting States to serve sentences, and some were repatriated to Somalia [43]. Three different functions (arrest, prosecution, and imprisonment), originally performed by a same State, are now implemented by three different States. This form of separation has been positively affirmed in UNSC resolutions, facilitating prosecutions and strengthening international efforts by encouraging regional States to accept suspected Somali pirates for trials and to "incarcerate pirates in a third State after prosecution elsewhere" [44]. The Djibouti Code of Conduct concerning the Repression of Piracy and Armed Robbery against Ships in the Western Indian Ocean and the Gulf of Aden (DCoC) clearly provided that the courts of the seizing State may determine the penalties for piracy and the action relating to the ship or property and allowed that State to abandon its primary right in exercising jurisdiction and to authorize another State to "enforce its laws against the ship and/or persons on board". [24] The separation also led to a cooperative mode of allowing the prosecuting States' officers to board the ship of the seizing State for investigation [45].

Consideration of Creative Piracy Prosecution Mechanisms
To fight against impunity for pirates, the international community has called for creative piracy prosecution mechanisms. The UN Secretary-General proposed seven options for piracy prosecutions with varying degrees of international participation (The seven options are: (1) enhancing UN assistance to build capacity of regional States to prosecute and imprison pirates; and establishing (2) a Somali court sitting in another regional State, either with or without UN participation; (3) a special chamber within the national jurisdiction of a State or States in the region, without UN participation; (4) that special chamber with UN participation; (5) a regional tribunal on the basis of a multilateral agreement among regional States, with UN participation; (6) an international tribunal on the basis of an agreement between a State in the region and the UN; (7) an international tribunal by SC resolution under Chapter VII of the UN Charter) [46]. Apart from the first one, which was to strengthen the existing domestic trial mechanism, the other six options required new courts, chambers, or tribunals [46]. In addition to those options, others suggested the expansion of the jurisdiction of existing international courts, such as the International Criminal Court and International Tribunal for the Law of the Sea, to include piracy prosecutions [46][47][48]. The courts in some new piracy trial mechanisms, such as international and regional tribunals, are not purely domestic courts, and their jurisdiction will extend beyond universal jurisdiction exercised by a single State.

Increased Focus on Land-Based Facilitation of Piracy
Article 101 (c) of UNCLOS, and land-based piracy, have gained greater attention. Whereas Article 101 (a) and (b) refer to piracy occurring at sea, subparagraph (c) refers to "any act of inciting or of intentionally facilitating an act described in subparagraph (a) or (b)", literally, without the geographical restriction. This is also reflected in judicial practice. In US v. Ali, the court noted that the UNCLOS Article 101(c) does not refer to a geographical limitation [49]. Universal jurisdiction over piracy originally aimed to prevent pirates on the high seas from escaping punishment because their activities occurred beyond any given State's jurisdiction [50]. However, for Somali piracy, its character as organized crime has become increasingly obvious. In addition to the "foot soldiers" (usually the persons in subparagraph (a) and (b)), the piracy network includes, at least, organizers, planners, leaders, financiers, facilitators, and ransom negotiators [51]. Some local Islamist militia and government officers also supported Somali piracy and participated in ransom allocation [52]. Those persons almost always acted on land. Thus, UNSC resolutions repeatedly reiterated the necessity to exercise jurisdiction over, and to prosecute, not only suspects caught at sea but also the above-mentioned persons [53], as well as the necessity to prevent the illicit financing and laundering for piracy [44].

Enhanced International Cooperation
International cooperation in anti-piracy law enforcement and judicial processes has been strengthened. Piracy at sea itself has a wide range of international characteristics, as the people, property, and ships involved in an attack may come from different States or may relate to interests of individuals or entities in different States. In order to reduce difficulties in collecting evidence, the UNSC urged States to allow their citizens and vessels to undergo forensic investigations at the first port of call immediately following an act or attempted act or release from captivity [45]. It also called upon all States to cooperate in the determination of jurisdiction and in the investigation and prosecution stages [54]. Simultaneously, the onshore part of piracy has gradually developed, and transnational criminal networks have formed. For example, part of the ransom money was laundered through the khat trade in Kenya [55]. It is necessary for Somalia and regional States to cooperate with other States, the UN Office on Drugs and Crime (UNODC), and the International Criminal Police Organization (INTERPOL) in strengthening their onshore anti-piracy law enforcement capabilities, such as implementing anti-money laundering laws and establishing Financial Investigation Units [56]. Moreover, due to the inadequate capacity of Somalia and regional States, external support is needed. IMO, INTERPOL, the UN Office of Legal Affairs, the UN Development Programme, UNODC, other States, and regional organizations have assisted Somalia and regional States in developing their capacity for arresting, detaining, prosecuting and incarcerating pirates, as well as for sharing information [57]. These developments both signal the need for enhanced cooperation and demonstrate how greater collaboration can be achieved.

Expanded Universal Jurisdiction
The scope of universal jurisdiction may be expanded under certain conditions. Under UNCLOS, the scope of anti-piracy actions by States is limited to the high seas and in a place outside the jurisdiction of any State. Since UNSC resolution 1816 (2008) came into effect, States have been authorized to enter Somali territorial waters to fight piracy [6]. Resolution 1851 (2008) further authorized States to enter the land of Somalia [45]. Although the UNSC resolutions repeatedly stressed that it was temporary, occurring only at the request of the TFG, and did not constitute customary international law [6,45], the practice established by the resolutions provides a model that might be followed again under certain conditions. In such cases, actions in Somali territorial waters should be consistent with such actions permitted by relevant international law to combat piracy on the high seas [6]. That means it is necessary to consider the compatibility of existing legal provisions with that situation.

Newly Involved Private Sector
The legitimacy of privately contracted armed security personnel (PCASP) on board ships has been recognized at the international level. The UN Security Council resolution commends the measures taken by flag States to allow vessels flying their flags transiting the High-Risk Areas to deploy PCASP [53]. IMO's attitude is relatively conservative. While formulating relevant regulations for PCASP, IMO has emphasized that this does not constitute an endorsement or institutionalization of their application [58]. As for its legitimacy at the national level, IMO and its member States agree that the decision to allow the deployment of PCASP on board ships should be decided by the flag State [41]. The activities of PCASP should be managed by States in accordance with international law [53]. The task of PCASP is defined as applying necessary and reasonable force to protect the ship and persons on board from illegal attacks [59]. For this reason, the way and degree of its use of force are strictly limited. They are required to take all reasonable measures to avoid the use of force, and PCASP should only use weapons in self-defense or protection of others [59]. The force should be used in a manner that is in accord with applicable law [60]. The use of force should not exceed the extent strictly necessary and reasonable in such circumstances, and attention should be paid to minimizing damage and injury, respecting and protecting human life [59]. There is also a viewpoint that due to the limited number of guards with limited weapons, the function of PCASP can only be self-defense [61]. In conjunction with Article 107 of UNCLOS on "ships and aircraft that have the right to arrest due to piracy", although PCASP is considered to be a sustainable way to combat piracy [61], they are not authorized to exercise enforcement jurisdiction. However, in theory, improper behavior of PCASP may still occur. Once the actions go beyond the necessary and reasonable use of weapons, the nature of the behavior and the applicable jurisdiction should be identified. As Article 94 of UNCLOS requires States to exercise effective jurisdiction and control over administrative, technical, and social matters over ships flying their flags [42], the interim guide related to PCASP issued by IMO holds that the incidents occurring on ships served by PCASP are subject to flag State jurisdiction and applicable domestic law (including criminal law) of the flag State [59]. However, if the behavior of PCASP is beyond reasonable self-defense and meets the definition of piracy itself, the flag State jurisdiction conflicts with the universal jurisdiction over piracy, which has not been resolved in the existing anti-piracy legal system.

Main Challenges in Exercising Universal Jurisdiction over Piracy
Based on the evolution of international law before, and developments after, the spike in Somali piracy, many current challenges can be identified.

Weaknesses in UNCLOS
The definition of piracy in UNCLOS (Piracy is defined in UNCLOS Art. 101 as follows:" (a) any illegal acts of violence or detention, or any act of depredation, committed for private ends by the crew or the passengers of a private ship or a private aircraft, and directed: (i) on the high seas, against another ship or aircraft, or against persons or property on board such ship or aircraft; (ii) against a ship, aircraft, persons or property in a place outside the jurisdiction of any State; (b) any act of voluntary participation in the operation of a ship or of an aircraft with knowledge of facts making it a pirate ship or aircraft; (c) any act of inciting or of intentionally facilitating an act described in subparagraph (a) or (b)" [42]), especially the elements of private ends, two ships requirement, and geographical restrictions, has been widely criticized for its limitations [62][63][64]. These limitations have been dealt with by other scholars but are explored again here in light of the developments highlighted in the previous section. Some provisions of UNCLOS relating to piracy are ambiguous. Article 101(a) mentioned "any illegal acts of violence or detention", without specifying the meaning of the word "illegal". What kind of law does it refer to, international law or domestic law? If it refers to international law, what are the sources of it? If it refers to domestic law, which State's domestic law is it? There is no clear answer. There is a view that the word "illegal" here means the absence of either "grounds negating criminal liability despite the use of violence" (e.g., self-defense) or "situations in which private detention may be lawful" (e.g., citizen arrest) [65]. This position lists two situations that should be understood as "legal", but it does not answer the above questions, such as the reasons for exemption from criminal responsibility and the legal basis for allowing private detention. The draft guidelines for national legislation on maritime criminal acts, proposed by the Comité Maritime International (CMI), has already considered that two categories of acts that may contain violence but with justifications should not be held as maritime criminal acts or piracy. Justifiable violence proposed by CMI includes: (1) "reasonable acts to rescue a person or to recover stolen property or regain lawful control of a ship or maritime structure"; and (2) "reasonable or proportionate acts to protect a person, ship or maritime structure, or related property, against a maritime criminal act or act of piracy". [66] Nevertheless, these provisions have not been formally endorsed. With the increasing use of privately contracted armed security personnel on board ships to counter piracy, both situations may be more likely to occur as the defensive capabilities of the ship increase. Due to the lack of specific laws on which self-defense is based, the legal meaning is still vague. It is difficult to determine the legitimacy and moderation of self-defense and how to deal with the seized pirates and their property. When a merchant ship seizes pirates as self-defense or for another "legal" reason, does it have the right to hand them over to a warship or the authority of any State, and does the State have the obligation to accept the seized pirates? There is no relevant regulation.
Another ambiguity is whether "private ends" means "private/political" or "private/public" [65]. Recalling the process of codification of piracy-related provisions by ILC, the United States suggested that "acts of piracy are committed in pursuit of private, as contrasted with public ends" [67]. Gerald Fitzmaurice pointed out that submarines without government authorization were considered to be carrying out private ends [68]. It implied that piracy was essentially a crime committed by private individuals, rather than performing government or authorized duties. Zourek also mentioned that if the condition of "private ends" remained, then if the perpetrator could prove that his act was committed under the instigation or instruction of public authority, the act would be considered lawful [68]. Therefore, according to these views, acts for private ends should refer to acts opposite to those of government. Several cases involving environmentalists demonstrate a similar understanding of "private ends". In Castle John v. NV Mabeco (1986), a Greenpeace ship attacked other ships, claiming that the latter caused pollution [69]. The court ruled that Greenpeace's act constituted piracy because it was for private ends and "purely in support of a personal point of view concerning a particular problem" [69]. Even if the purpose of the private subject is related to a political reason, it can be classified as private ends [69]. In Institute of Cetacean Research v. Sea Shepherd Conservation Soc., licensed whalers sued environmentalists who interfered with whaling activities. The Ninth Circuit ruled that such environmental activities constituted piracy, since private ends "include those pursued on personal, moral or philosophical grounds" [70]. However, the "private/political" divide has many supporters [8,71]. It is generally agreed that UNCLOS excludes politically motivated terrorist acts from the piracy [72][73][74][75][76]. Somali pirates alleged that they attacked to defend against illegal fishing and dumping of toxic wastes in Somali waters [56], the aim of which was to protect their own rights [77], or was simply to help the poor [78]. Somali armed groups engaged in piracy once considered themselves as coastguards [77], and some Somali pirates were welcomed as national heroes [78]. If the above-mentioned purpose for attack claimed by Somali pirates was true (according to United Nations, the reason for Somali piracy is only a cover-up [53]), and the understanding of "private/political" is accepted, their acts do not constitute piracy. A similar scenario may emerge in the growing piracy tide in the Gulf of Guinea. Some commentators are concerned that maritime attacks in the Gulf of Guinea may be committed for political ends by using piracy as a weapon to influence political developments in specific States in the region [79].
A further issue is relating to the application of universal jurisdiction over piracy in the exclusive economic zone (EEZ). UNCLOS has almost absorbed the provisions on piracy of HSC. At the time of the formulation of HSC, the ocean beyond internal waters was generally divided into territorial seas and high seas, and there was no concept of exclusive economic zones. Therefore, the geographical limitation of piracy could form a logical self-consistency then. There was no intermediate zone between the high seas and the jurisdictions of States. The EEZ system was newly established in UNCLOS, but the provisions on piracy were not adjusted correspondingly. Many scholars believe the geographic scope of Article 101(a) should be read to include the EEZ of any State in line with Article 58 (2) [17,63], and many UN agencies hold the same view [46,80]. However, while acknowledging universal jurisdiction over piracy in EEZs, there are still problems to be solved. First, States may still not be obligated under Article 100 to cooperate in the repression of piracy there [17]. Second, in the process of exercising universal jurisdiction, the actions of warships may cause damage to the sovereign rights and jurisdiction of coastal States under special circumstances. For example, (1) the rescue or expulsion by warships of other States may affect the exploration, development, conservation, and management activities of coastal States; (2) the navigation or exchange of fire of warships of other States may cause pollution of the marine environment or may cause damage to artificial islands, facilities and structures, marine scientific research facilities and equipment, etc. or; (3) the actions of warships of other States may be affected by the exploration, development, conservation, management, and activities of coastal States exercising their exclusive jurisdiction. Third, the coastal State's understanding of the peaceful purpose of the EEZ may affect the enforcement of universal jurisdiction over pirates in this zone. According to UNCLOS, all States enjoy freedoms of navigation in EEZs, and these zones should only be used for peaceful purposes [42]. In other words, non-peaceful navigation should be prohibited. However, there is no unified understanding of peaceful purposes, such as whether it means banning all categories of military activities [81]. Although countering maritime piracy should be through law enforcement rather than military activities, the nature of the act is still easy to be confused because the main resource used to counter piracy is warships [82]. Some States prohibit foreign warships from engaging in military activities in their EEZs [81]. If a State considers that countering piracy has the attribute of military activities, it may be difficult for foreign warships to implement universal jurisdiction over piracy in that State.
UNCLOS does not explicitly criminalize attempts to commit piracy. Some scholars believe that in this case, corresponding acts do not apply to universal jurisdiction [16,76], and only the actual pirate attack can be prosecuted under UNCLOS [83]. The UN Division of Ocean Affairs and Law of the Sea (UN-DOALOS) also stated that attempts to commit piracy do not meet the definition of piracy in UNCLOS [84]. However, the opposite view holds that piracy in international law should include attempts to commit piracy. The UNSC called for all States to emphasize the importance of criminalizing attempts to commit acts of piracy [56]. In US v. Hasan, the court ruled that unsuccessful attempts to commit piracy acts formed piracy jure gentium [85]. Italy, the Netherlands, Germany, and Japan have also regarded attempted attacks as breaches of the law in many cases [86]. In addition, the demarcation between "preparatory" and further action is a difficult point in defining an "attempt to commit piracy" (See section I (I) of English Criminal Attempt Act 1981: "if, with intent to commit an offence to which this section applies, a person does an act which is more than merely preparatory to the commission of the offence, he is guilty of attempting to commit the offence"). It is easy to understand that if the pirates engage the target ship, their acts can be regarded as attempting to commit piracy, and thus may be punished. In US v. Hasan, the defendants mistook the navy frigate Nicholas for a merchant ship and opened fire [85]. In the cases of Topaz and Nave Atropos in Seychelles, pirate ships launched attacks on other ships, but failed [87,88]. All of those defendants were found guilty. However, the ambiguity lies in whether pursuing a ship for the purpose of carrying out a pirate attack, but not yet in contact with the chased ship, constitutes an attempt to commit piracy. At this time, the pirate ship has shown an obvious intention to attack and has begun to take action against specific targets, but it has not met the standard of "engaging criminal targets". It is suggested in Oppenheim's International Law that actually completed violence is not needed and that chasing a ship for the purpose of attacking can constitute piracy [89]. The judgement of the case of Draco in Seychelles demonstrated that a conviction can be successfully achieved for chasing a ship as an attempt to commit piracy [90]. Conversely, some scholars argue that an attempt to attack should begin with some kind of violence [16]. Another scholar holds that if offenders do not use violence but only chase ships and fail to board them, they do not commit piracy under UNCLOS Article 101(a) and are treated as pirates only when it can be proven that they know it is a pirate ship and participate in its operation voluntarily, as defined in Article 101(b) [91]. Cruising at sea with weapons in order to carry out pirate attacks could also be understood as an attempt to commit piracy. Since no specific target has been found and no specific attack has been carried out, this stage is closer to preparation to commit a crime. According to Oppenheim's International Law, it is not clear whether armed cruising with the aim of committing piracy constitutes piracy [89]. There is also the opposite view that cruising at sea only for the purpose of piracy is not piracy in State practice [16].
There are different understandings on whether the threat of violence, if not actually committed, constitutes piracy. Oppenheim's International Law holds that piracy is carried out by unauthorized acts of violence, whether such violence is the direct use of force or threat [89]. That is, the threat of violence can constitute piracy. However, in the literal sense, the threat of violence itself is not within the stipulated circumstances of piracy. If the threat of violence is accompanied by detention or depredation, this will undoubtedly constitute piracy because of detention or depredation. If the purpose of the threat of violence is to force the State, organization, individual, or other subject to carry out or not to carry out a certain act, and it is not accompanied by detention or plunder, it does not fall under the definition of piracy. Perhaps because of the lessons of UNCLOS, in order to eliminate such ambiguity, when the IMO formulated the definition of armed robbery, it added "or threat thereof" after "any illegal act of violence or detention or any act of depredation", which clearly includes the threat act [92].
There are also different opinions on whether there are territorial restrictions "on the high seas" or "beyond the jurisdiction of any State" on the act of Article 101 (b) and (c) of UNCLOS. There is a view that there are no such restrictions in the acts of subparagraph (b) and (c) [93]. Unlike Article 101(a) that clearly defines the region, subparagraph (b) and (c) themselves do not stipulate a specific geographical scope. Therefore, it should be understood that the provisions are not intended to impose geographical requirements [94]. Notwithstanding this, according to Harvard's 1932 Draft Convention (Art. 3: Piracy "is any of the following acts, committed in a place not within the territorial jurisdiction of any State: . . . any act of instigation or intentional facilitation (of piracy) . . . ."), to which the ILC mainly referred when drafting the piracy-related provisions of the HSC, all persons, including the perpetrators and facilitators of the attack, must appear on the high seas [95], which seems to reveal the original legislative intent of the provisions. US v. Ali fully reflects the collision of the two views. The defendant, Ali Mohamed Ali, did not participate in the hijacking at sea. Instead, he boarded the ship two days after it docked in Somalia and acted as an interpreter and a negotiator for remuneration. The District Court ruled that it did not have jurisdiction on the ground that his acts did not take place on the high seas [49], while the Court of Appeals held that UNCLOS does not require that subparagraph (c) have a geographical limit, and that incitement and facilitation do not need to take place on the high seas [49]. In US v. Shibin, like Ali, Mohammad Saaili Shibin only played the role of negotiator on Somali land and in territorial waters. He argued that according to UNCLOS, facilitation should take place on the high seas, and the court lacks personal jurisdiction and subject-matter jurisdiction. However, both the District Court and the Court of Appeals found that aiding and abetting ("functionally equivalent" to "facilitating") piracy can occur anywhere, even in the territory of another State, and that any State has jurisdiction under international law [96].
Further problems arise concerning the facilitation of piracy. Some acts of intentional facilitation usually need to be committed in the specific context of piracy, such as providing financial support to pirates, assisting pirates with taking care of hostages, and translating and negotiating for pirates. The other parts of intentional facilitation themselves are illegal and can constitute independent crimes. For example, providing illicit weapons for pirates involves illegal transportation and arms trafficking, legalizing piracy income involves money laundering, selling or handling depredated property for pirates involves selling stolen goods, and providing an umbrella of protection for pirates by administrative personnel involves crimes of corruption and dereliction of duty. For the latter, if a perpetrator is prosecuted for an independent crime, rather than piracy, it causes controversy as to whether universal jurisdiction can be applied.
UNCLOS does not oblige States to exercise universal jurisdiction [97]. Article 100 only obliges States to cooperate in the suppression of piracy. According to Articles 105 and 110, a State "may" rather than "shall" exercise the right to board, seize, arrest, enforce the law, and impose penalties on pirates. Moreover, UNCLOS does not mention domestic legislation on piracy at all, and this has resulted in a fragmented national legal landscape (discussed in detail below). However, in 2019, Annex C of the ILC Annual Report noted that it could be assessed whether States were obliged, or could only choose, to establish such jurisdiction [98], which means that it is necessary to reconsider whether States must or are simply permitted to adopt national law exercising universal jurisdiction over piracy.
Turning to the adjudicative jurisdiction, according to Article 105, the right to prosecute pirates only applies to the courts of the seizing State. As a result, although a large number of Somali pirates, captured by foreign escort navies, were sent to regional States for trial, some scholars believe this was contrary to UNCLOS [47,99].

Disharmony among International Instruments
Although both UNCLOS and the Convention for the Suppression of Unlawful Acts against the Safety of Maritime Navigation (SUA) are vital legal bases for anti-piracy, their relationship is controversial. Mixed views include: (1) Crimes defined in SUA (SUA offences) "are complementary to UNCLOS provisions on piracy [78]"; (2) SUA enlarges the scope of piracy [100]; and (3) The original piracy crime has been replaced with "illegal acts endangering the safety of navigation" in SUA [101].
Relatively speaking, SUA has a broader scope of offences. As for the geographical element, unlike UNCLOS, SUA offences apply if the ships are crossing jurisdictional boundaries, including waters between States [102]. This indicates that it is applicable within the territorial seas. SUA does not stipulate the two ships requirement and private ends. Instead, it regards attempting to commit an offence as an offence [102]. Additionally, unlike UNCLOS, which does not oblige States to adopt domestic anti-piracy laws and exercise universal jurisdiction over piracy, SUA not only requires each State Party to punish the SUA offences but also establishes more detailed operational measures for this purpose, including the circumstances in which a State should and may establish its jurisdiction, as well as the obligation of "extradition or prosecution" [102]. Due to the fact that SUA more clearly stipulates the obligation of States to establish and exercise jurisdiction, it makes up for the deficiency of UNCLOS in this aspect to a certain extent. Therefore, although SUA is not specifically aimed at piracy, and the word "piracy" does not even appear in it, the UNSC resolutions on Somali piracy often require States to establish jurisdiction based on SUA [9].
However, UNCLOS and SUA actually belong to different systems. The original aim of SUA was combatting "the world-wide escalation of terrorism in all its forms" [102]. Seen from the types of SUA offences, these concern the safety of maritime navigation, which is not entirely consistent with piracy. Even if these offences (especially those stipulated in Article 3.1 (a) and (b) of SUA) overlap with piracy under UNCLOS in certain situations, it seems that the interrelation between them has not been considered in drafting SUA [78]. Article 3.1 (a) and (b) of SUA only contain the seizure or control over ships and violent activities possibly endangering the safe navigation of ships. When violent acts against persons or property are not aimed at seizing or controlling ships and are not at a level that endangers the safety of navigation, they may conform with the piracy definition under UNCLOS, but they do not constitute SUA offences.
The nature of jurisdiction determined under UNCLOS is different from that under SUA. The latter does not involve universal jurisdiction over piracy but quasi-universal jurisdiction over SUA offences (The "extradition or prosecution" established in SUA can be regarded as a quasi-universal jurisdiction. According to Anthony Aust, the "universal jurisdiction" that only applies to parties to those treaties is known as quasi-universal jurisdiction) [103]. For example, if an attack takes place on the high seas when the victim ship is scheduled to navigate into a State's territorial sea, this attack constitutes piracy under UNCLOS and the offence under SUA simultaneously. If the attack is seen as piracy and subject to jurisdiction under UNCLOS (that is, universal jurisdiction), any State has the right to seize and prosecute the pirates in its courts. If the attack is seen as a SUA offence and subject to jurisdiction on that basis, SUA does not provide that the State has the right to seize offenders on the high seas because there is no universal jurisdiction that authorizes the State to enforce the law on the high seas. Although . Furthermore, according to UNCLOS, the seizing State has the right to prosecute pirates without reporting to any other State, while according to SUA, any State Party should immediately notify the States that have established compulsory jurisdiction and any other interested States (if advisable) after taking the offender or the alleged offender, who is present in its territory, into custody [102].
Except UNCLOS and SUA, other relevant existing international legal frameworks are regional instruments, such as the Regional Cooperation Agreement on Combating Piracy and Armed Robbery against Ships in Asia (ReCAAP), DCoC, the Code of Conduct concerning the Repression of Piracy, Armed Robbery against Ships, and Illicit Maritime Activity in West and central Africa (Yaoundé Code of Conduct), and the Memorandum of Understanding on the Establishment of a Sub-regional Integrated Coast Guard Network In West and Central Africa (MOWCA MOU). Since most of their parties come from corresponding regions, their content mainly concerns the rights and obligations of respective States' parties. As of 23 February 2019, only 6 of the 20 ReCAAP contracting parties were not regional States [108]. In DCoC, Yaoundé Code of Conduct, and MOWCA MOU, there are few or no States' parties from outside the region [24,109,110]. These instruments facilitate cooperation between the State parties in each region, but cooperation with nonregional States that bring pirates back for trial, which is typically in the form of exercising universal jurisdiction, is not fully stipulated. Although the two codes of conduct request that the participants cooperate with interested States and other stakeholders to facilitate rescue, interdiction, investigation, and prosecution [109,111], there are no further specific provisions.
The two codes of conduct are comparatively consistent and have drawn lessons from the fight against Somali piracy, so that they are more advanced than ReCAAP in the breadth and details of cooperative measures. For example, in order to clarify the anti-piracy measures at the national level, the two codes of conduct elaborate which kind of national maritime security policies, domestic legislation, practices and procedures, national maritime security committees, or other systems and security plans a State should develop and implement [109,111]. They also build on the "embarked officer" mechanism entitling a State, with the authorization by another State, to nominate law enforcement or other authorized officials to embark on the latter's patrol ships or aircraft [24,109]. In contrast, ReCAAP mainly focuses on the establishment of an information-sharing center and the sharing of information, transmission, and acceptance of cooperation requests through that center, rather than the specific design of cooperation measures in other areas. It has provisions regarding extradition, mutual legal assistance, capacity building, cooperative arrangements, and protection measures for ship [112]. However, these provisions are relatively simple.

Defects of Domestic Piracy Legislation
Somali piracy has revealed the shortcomings of domestic piracy legislation in many States [113]. Naval forces were often forced to release suspects without any sanctions, no matter whether there was sufficient evidence to prosecute them or not [9]. That has weakened the international efforts against Somali pirates [53]. Failure to criminalize piracy and to establish universal jurisdiction in domestic law directly results in the lack of a basis for combating piracy at the national level. Even in States that have criminalized piracy and established universal jurisdiction over piracy in their domestic law, there are still various deficiencies in the legislation.
First, the definition of piracy is not uniform among States' domestic legislation and between domestic law and international law. Scholars have analyzed the definition of piracy in 19 States and regions and found that only 21.2% held two ships requirements; 31.6% required "private ends"; and 26.3% required that piracy occurs outside the scope of national jurisdictions [114]. This inconsistency will lead to operational difficulties in applying universal jurisdiction. Even if an act constitutes "piracy" under domestic law, it does not necessarily mean that universal jurisdiction applies, and there is a need to distinguish whether the so-called "piracy" conforms to the UNCLOS definition. This also means that, if the State signs a new international instrument on piracy with the requirement to implement the instrument in its domestic law, it has to distinguish the applicable part from the existing domestic "piracy" definition first and then amend the domestic law relevant to this part to comply with the instrument. That will increase the difficulty of implementation and can easily confuse.
Second, the sentences imposed on pirates vary greatly between States. The universal jurisdiction prosecutions over piracy are public goods that serve the whole international community [115]. Consequently, it is necessary to analyze the sentences for piracy in different States comparatively. Sentences for similar crimes should not differ largely, otherwise they can lead to unfairness [116]. However, as of 2010, the longest maximum sentence was a life sentence (e.g., in the US, UAE, and Kenya), and the maximum sentence in Seychelles was 30 years, while the ones in Europe were significantly shorter (e.g., 15 years in Germany; 12 or 15 in Holland; 14 or 20 in Italy) [116]. In practice, the sentences for similar piracy offences range from a life sentence to 4.5 or 5 years [116].
Third, the domestic legal basis for universal jurisdiction over piracy is not well established. Many States have not fully adopted domestic legislations and a jurisdictional framework based on the concept of universal jurisdiction stipulated in UNCLOS [117]. Some States tend to exercise universal jurisdiction only when certain links exist [61]. In this respect, it seems that other traditional jurisdiction principles are enough to meet the needs of States to combat piracy, if they are not interested in punishing piracy that has no nexus with them. According to Jack Lang, in light of general international law, the State of nationality of the pirates, the State of nationality of the victims, and the flag State of any ship concerned can claim jurisdiction over the suspected pirates [23]. However, as ships on the high seas are generally considered to be under the exclusive jurisdiction of the flag State, with the universal jurisdiction over piracy and a few other circumstances as exceptions [103], whether there is still the space for other traditional jurisdiction is confusing. Moreover, by using a flag of convenience, the flag State may not be the State of the company that genuinely owns or operates the ship. The legal link between the latter State and the piracy incident may be weak. Therefore, some States prosecute pirates having nexus with them on the ground of universal jurisdiction. For example, Japan's first piracy trial, which involved an attack on a Bahamian ship operated by a Japanese company, applied universal jurisdiction [118]. A similar but confusing situation existed in the Republic of Korea v. Araye, the first trial of Somali pirates in Korea related to the piracy attack on a Maltese ship operated by a Korean company [119]. The jurisdiction ground of the case is controversial. The court ruled that, according to the domestic criminal procedure law, it has territorial jurisdiction over the case due to the current location of the defendant [19]. One scholar believes that Korea does not recognize universal jurisdiction, and only when the suspected pirates are Korean nationals or the piracy takes place on a Korean ship can they be prosecuted for piracy in its domestic courts [120]. However, other scholars regard the trial as a typical case in which Korea exercised universal jurisdiction over piracy [19]. This issue is important because if there is a positive conflict of jurisdiction, the jurisdiction principles the State applies may affect the priority of different jurisdictions because universal jurisdiction is often considered to be subsidiary [121]. Moreover, if the seizing State does not recognize the jurisdiction ground proposed by other States, it will not cooperate [122]. For example, if the seizing State believes that only flag State jurisdiction and universal jurisdiction are applicable to piracy, and does not recognize other traditional jurisdiction claims, it will not transfer pirates. However, in the current situation of the overall negative conflict of jurisdiction over piracy, regardless of the jurisdiction ground applied, it usually will not incur protests from other States. In special circumstances, it may restrain interested States from asserting jurisdiction over a whole case. In the Republic of Korea v. Araye, because the criminal law of Korea does not clearly provide for universal jurisdiction, there was a debate about whether the criminal acts against foreign crew can be applied under Korean law [123]. Those acts against foreign crew were not ultimately prosecuted [119]. From a global perspective, universal jurisdiction is a justice-based measure [124] and an important means to protect global public goods [121]. Although it is not a legal obligation to establish and exercise universal jurisdiction over piracy, as Cedric Ryngaert claims, there are many jurisdiction principles so that at least one can be applied in any given situation [121]. Positively asserting jurisdiction can be regarded as a responsibility of States in some circumstances [122]. Without a clear stipulation of universal jurisdiction over piracy, negative conflicts of jurisdiction will undoubtedly occur, which is not conducive to the solution of global problems and the realization of SDG 16 (Peace, Justice and Strong Institutions). Fourth, domestic legislation lacks procedural provisions on extraterritorial law enforcement. For instance, in the Republic of Korea v. Araye, the pirates claimed that their transfer to Korea lacked a proper procedural basis [19]. Generally speaking, the essential procedural elements, such as time of detention, the treatment and the rights of pirates in the transfer, the procedures for boarding, and visiting and evidence collection processes, have rarely been included in the domestic law. Failure to deal with those issues may lead to violations of human rights. In the case of Hassan and Others, France was charged with illegality due to the extent of time the alleged pirates were kept in detention [125]. The French courts tried to explain this with "wholly exceptional circumstances", but the European Court of Human Rights decided that there had been a violation of the right to liberty and security in that case [125].
Fifth, domestic legislation may be unable to meet new trends in exercising universal jurisdiction. The separation of seizing, prosecuting, and imprisoning stages, the proposal of new piracy trial options, and enhanced international cooperation all mean that the exercise of universal jurisdiction is no longer limited to the conduct of a single State. Therefore, cooperation with other States and other international entities (including international organizations and international or regional tribunals) is hindered by differing domestic legislative regimes. Harmonizing the law between States will assist with maritime law enforcement cooperation, the transfer and reception of pirates, the identification of evidence collected by other States, and the application of law before and after the transfer of pirates, which are rarely reflected in existing domestic legislation. Efforts to harmonize laws between jurisdictions can have the further benefit of enhancing global best practice.

Balanced Relationships to Be Sought in Strengthening Universal Jurisdiction over Piracy to Enhance a Sustainable Anti-Piracy Legal System
In order to sustain a strong anti-piracy legal system, incremental enhancements are needed to support universal jurisdiction over piracy, including the achievement of a better balance in the three key areas explored below.

Balance between Benefits and Costs
The exercise of universal jurisdiction over piracy is regarded as a public good [115]. Although repressing pirates, the hostis humani generis [126], benefits all humankind, for the States that exercise universal jurisdiction over piracy, their benefits may be indirect and limited. Many States have a low passion for prosecuting pirates who do not directly harm their nationals or national interests [127] since exercising universal jurisdiction over piracy does not directly safeguard the interests of the States. Even the flag State of the attacked ship and the State of nationality of the kidnapped crew may be not active in intervening or prosecuting piracy [23,128]. In the international community, a State rarely receives financial reward for providing such public good. The State's gains in other areas are not significant either. The data show that the exercise of the legislative jurisdiction of universal jurisdiction over piracy-namely, the domestic anti-piracy legislation-does not protect the ships flying the State's flag from pirate attack [113].
The high costs of exercising universal jurisdiction is an important aspect. All the costs for piracy prosecutions, including but not limited to the costs of the trial and imprisonment, evidence and witnesses, translation and logistics, shall be borne by the State exercising universal jurisdiction, unless there are special arrangements on external assistance. The State exercising universal jurisdiction may also have to bear other risks alone, such as Kenya's intention to withdraw from the handover agreement to receive pirates in March 2010 when it claimed that it was threatened with retaliation from pirates' allies [23]. In this regard, a large number of regional States that exercise universal jurisdiction over pirates in combating Somali piracy have proposed that they need to share the burden of imprisonment with third States [46] or hope to send pirates back to Somalia to serve their sentences [128] and do not bear the costs of repatriation of suspected pirates who have not been convicted [46]. However, the State of nationality of the pirate does not always cooperate effectively on the imprisonment issue. Somaliland, a region in Somalia that was dominated by its own authorities, once withdrew from the agreement on accepting convicted pirates from Seychelles and released the pirates in Hargeysa prison without explanation [128].
The imbalance of cost-sharing is another important reason. At present, universal jurisdiction over piracy is usually understood as a kind of right, rather than an obligation. Therefore, the subject of responsibility for exercising the jurisdiction to fight against pirates is not clear. Although all States have the right to exercise universal jurisdiction over piracy, they often take a negative attitude because of the convenience of giving up their rights and their belief that they are not the most appropriate providers of the public goods. This, in turn, undermines the enthusiasm of States that have already provided public goods, thus forming a vicious circle. In 2010, Moses Wetangula, then Kenya's foreign minister, said, "We discharged our international obligations. Others shied away from doing so. And we cannot bear the burden of the international responsibility [129]." If only a few States continue to work hard without sharing the burden, problems will inevitably arise [23].
It can be predicted that it is unrealistic to expect the State to actively invest too much in exercising universal jurisdiction over piracy before the status quo of benefits and costs changes. From the perspective of benefits, the States that have relatively more benefits from universal jurisdiction over piracy are those with interests in pirate attacks. In addition to calling on all States to exercise universal jurisdiction over piracy and to prosecute and imprison pirates [56], the UNSC further points out the more appropriate States to exercise universal jurisdiction, which are particularly the flag, port, and coastal States; States of nationality of victims and perpetrators of piracy; and States with relevant jurisdiction under international law and domestic legislation [53]. However, the UNSC can only use the non-mandatory phrase "call upon", and whether to respond to the call depends on States themselves. Additionally, great powers also have a certain advantage in gaining benefits from providing public goods. Great powers are often the key factor in determining war and peace, and they also bear greater responsibility for regional and world peace and development [130], and therefore, they should make greater contributions to global governance. Although there may not be a direct interest relationship between a specific pirate attack and a great power, and there is no direct economic benefit, the exercise of universal jurisdiction over piracy helps the State to show its power and manners, so as to enhance its international image and voice. Therefore, it is necessary to encourage more interested States and great powers to provide the public goods needed in exercising universal jurisdiction over piracy.
From the perspective of costs, the absolute costs of exercising universal jurisdiction over piracy are almost fixed. It seems more appropriate to reduce the relative costs and share the costs. The same costs will put more pressure on weaker States, such as Somalia and regional States, than on more powerful States. This is not only because the same costs account for a larger proportion of the economic aggregate in weaker States but also because these States lack the infrastructure needed to exercise universal jurisdiction over piracy, such as a comprehensive legal system, qualified judicial personnel, adequate prison facilities, etc., which are needed to meet the corresponding requirements through new or large-scale improvement, with a large marginal cost. In the more powerful States, such infrastructure is usually relatively fit for purpose and need only be directed or slightly adjusted to be used for exercising universal jurisdiction over piracy; thus, the marginal cost is smaller. In terms of cost-sharing, providing financial and capacity-building support to States exercising universal jurisdiction over piracy can directly reduce and distribute the costs. During the period of countering Somali piracy, the support of the international community to Somalia and regional States, the Trust fund to Support Initiatives of States Countering Piracy off the Coast of Somalia under the auspices of the Contact Group on Piracy off the Coast of Somalia, and the IMO Djibouti Code Trust Fund initiated by Japan are all types of arrangements that objectively and effectively promoted the piracy trials in Somalia and regional States. Moreover, enabling more States to exercise universal jurisdiction over piracy can spread the burden on a global scale and avoid concentrating the costs of activities benefiting the whole international community on a few States. Jack Lang, the Special Adviser to the Secretary-General of the UN on legal issues related to piracy off the coast of Somalia, said in his report that only with the participation of all States can the commitment of each State be strengthened, and he affirmed that the Netherlands still exercises universal jurisdiction over piracy without directly involving its national interests, saying that "continuing such mobilization is essential" [23]. Although it is not realistic to require all States to exercise universal jurisdiction over piracy in the short term, it is an effective direction to take to increase the providers of such public goods as much as possible.

Balance between Right and Obligation
To a certain extent, increasing the costs of refusing to exercise universal jurisdiction over piracy will enhance the willingness of States to exercise such jurisdiction. To regard universal jurisdiction over piracy as an obligation is an approach. Different from the pure welfare rights, the necessity and urgency of exercising universal jurisdiction in antipiracy activities is obvious. The high seas have the characteristic of not belonging to the jurisdiction of a single State "but within the collective responsibility of all States"; thus, crimes on the high seas must be properly dealt with through a coordinated and comprehensive approach [131]. In other words, as far as a single State is concerned, it has no legal obligation to exercise universal jurisdiction over piracy; however, as far as the international community as a whole is concerned, the exercise of such jurisdiction is an essential measure to safeguard the common interests, which cannot be avoided blindly. While recognizing that the exercise of universal jurisdiction over piracy is an optional right, Jack Lang also pointed out in his report that although UNCLOS uses the phrase "to the fullest possible extent" to limit the obligation of cooperation in combating piracy, such flexibility should not be used as an excuse for not prosecuting [23].
However, such implied obligation attribute is not conducive to the effective implementation of universal jurisdiction over piracy. SUA has provisions on compulsory jurisdiction and facultative jurisdiction. In the case of compulsory jurisdiction, each State Party has the obligation to establish the corresponding jurisdiction. In the case of facultative jurisdiction, State Parties can decide whether to establish the jurisdiction for the corresponding offences [102], which is more similar to a right enjoyed by State Parties. According to the Legal Committee of IMO, most of the State Parties of SUA incorporate compulsory jurisdiction into their domestic legislation but lack provisions on facultative jurisdiction [117]. This comparison shows the importance of explicitly stipulating that it is an obligation to exercise jurisdiction.
Although a SUA offence does not always coincide with piracy, to some extent, the establishment of compulsory jurisdiction in domestic law still reflects the willingness of States to bear the responsibility of combating maritime crimes at sea. The increase and then reduction of Somali piracy makes the international community more aware of the importance of participating in international cooperation against piracy and undertaking corresponding international responsibilities. The signing of regional anti-piracy legal instruments, such as ReCAAP, DCoC, Yaoundé Code of Conduct, and MOWCA MOU, further reflects the initiative of the international community in combating piracy. On this basis, it is feasible to set the exercise of universal jurisdiction over piracy as an obligation under certain conditions.
In terms of the content of the obligation, there are differences in the exercise of legislative jurisdiction, law enforcement jurisdiction, and judicial jurisdiction. There is a view that the national legislation on piracy is an obvious prerequisite for the implementation of "the obligation to cooperate in the suppression of piracy" in Article 100 of UNCLOS [132]. The obligation of legislative jurisdiction is very common in international legal instruments, such as SUA, International Convention against the Taking of Hostages, and UN Convention against Transnational Organized Crime, which all require State Parties to take necessary measures to establish jurisdiction over related crimes. Therefore, if a new anti-piracy international legal instrument is concluded, there is no technical difficulty in establishing universal jurisdiction over pirates in domestic law as the obligation of State Parties.
The enforcement of universal jurisdiction over piracy has its particularity. It usually occurs on the high seas or in any other place outside the jurisdiction of any State. The vast geographical scope makes it unrealistic to obligate a State to exercise law enforcement jurisdiction over every piracy case as it exercise jurisdiction over criminal offences committed within its territory. Whether law enforcement can be actually carried out on the high seas or in any other place outside the jurisdiction of any State and its enforcement effect depends on many factors, such as the strength of a State's Navy or Coast Guard, the comprehensive strength of the State, the specific situation of the pirate attack, etc. Additionally, in the absence of a unified and authoritative global governance institution, it is obvious that there is no international treaty, customary law, and legal basis for requiring any State to undertake the obligations of "international police" on the global commons such as the high seas or any other places outside the jurisdiction of any State. Therefore, the enforcement jurisdiction should still exist in the form of rights.
The judicial jurisdiction over piracy can be compulsory in the form of "extradition or prosecution". The primary purpose is to put an end to the phenomenon of "capture release" or "only drive but not capture" and ensure that pirates are subject to judicial trials. Secondly, the "extradition or prosecution" of pirates is in line with the current State practice of transferring pirates to a third State for trial in the process of fighting against Somali pirates.

Balance between Innovation and Stability
The global governance of the ocean needs to respond to changing, new, and emerging issues through the creation of new governance rules according to new circumstances, new requirements, and new trends in ocean affairs. The development of law should be a continuous and dynamic process. The better way to protect common interests through international law is to adapt rather than to abandon the existing legal systems, and revolutionary new concepts should be constituted consistent with the recognized legal frameworks [133]. Turning to the reform of the legal system of universal jurisdiction over piracy, it is mainly reflected in the balance between the innovative anti-piracy legal measures and the existing relevant provisions of UNCLOS, as well as the balance between the exercise of universal jurisdiction and respect for national sovereignty.
Combined with the public goods attribute of universal jurisdiction over piracy, it can be anticipated that the improvement of the legal system should be cautious and gradual. To be specific, it should be noted that although the narrow definition of piracy in UNCLOS is considered by some scholars as a defect, the possibility of amending it in the near future is relatively slim. The deletion of the two ships requirement will blur the distinction between piracy and other maritime criminal offences committed within a ship under the exclusive jurisdiction of the flag State. In that case, violent crimes on board ships can be included in the scope of piracy, and only non-violent crimes remain under the exclusive jurisdiction of flag States, which is obviously contrary to the exclusive jurisdiction of flag States on the high seas with universal jurisdiction over pirates as only an exception. The deletion of private ends will have the consequence that maritime terrorism can also constitute piracy and be subject to universal jurisdiction. Although that result may be welcomed by some scholars [134,135], it would completely change the nature of the concept of piracy and may arouse much controversy. The geographic limitation is closely related to national sovereignty. It can be predicted that sovereign States would be reluctant to abolish or loosen the geographic limitation, unless they cannot address piracy by themselves [83]. The UNSC made a breakthrough in its resolution on combating Somali piracy, calling on capable States to enter the territorial waters and land of Somalia to combat piracy. However, the UNSC is conservative and is unlikely to normalize such arrangements by breaking through the existing international law system. Resolutions repeatedly emphasize the respect for national sovereignty, stating that allowing access to the territory of Somalia to combat piracy, even though with the consent of the Transitional Federal Government of Somalia, is only temporary and does not constitute customary international law [6]. This concern continued in the fight against piracy in the Gulf of Guinea. When the UN assessment mission on piracy in the Gulf of Guinea assessed the threat of piracy in the Gulf of Guinea and explored the effective response measures that UN and the international community could take, the respondents in Central Africa and West Africa had different opinions on whether to allow foreign warships to enter the Gulf of Guinea and take actions similar to those against Somali pirates [79]. As the region with the largest number of pirate attacks at present, States in the Gulf of Guinea have not fully allowed other States to enter their own territory to exercise universal jurisdiction over piracy as Somalia has. Additionally, the right of reverse hot pursuit mentioned by some scholars [100,136] is not likely to be widely implemented in a short period of time because it involves restrictions on the sovereignty of coastal States and law enforcement in other States. In addition to the above-mentioned issues on the scope of universal jurisdiction, other radical or subversive reform programs, or those requiring huge investment, are less likely to be supported. For example, the UN Secretary-General proposed seven options for a creative piracy trial mechanism, but the UNSC did not reach a consensus on which option to choose. Jack Lang believed that this meant more aggressive options should be put aside [23].

Approaches to Strengthening the Application of Universal Jurisdiction over Piracy to Enhance a Sustainable Anti-Piracy Legal System
The following measures in Sections 6.2 and 6.3 are the specific improvement objectives proposed in response to the actual needs of anti-piracy and the inadequacy of the existing legal system. They are the necessary legal technical support for international cooperation, aimed at increasing the applicability and sustainability of the anti-piracy legal system relating to universal jurisdiction. Considering the differences and diversity among States, the path in Section 6.1 will help to ensure the operability of achieving the goals of Sections 6.2 and 6.3 to the greatest extent.

Take Advantage of Both Hard Law and Soft Law
The improvement of the existing anti-piracy legal system involves the revision and new formulation of legal instruments. UNCLOS is a comprehensive treaty on maritime activities, rather than being solely aimed at piracy; reform is difficult, and it is unlikely to be supported. Other alternative approaches, such as adopting an international treaty on piracy to clarify relevant matters, is a possibility. Considering the three pairs of balancing issues mentioned above, and realizing that the reform of the anti-piracy legal system should be incremental, a soft law approach can also be contemplated.
In the existing anti-piracy legal system, there are many soft law arrangements, including but not limited to: (1) Non-legally binding intergovernmental regional cooperation arrangements, such Prosecutions submitted by UNODC, although the term "model law" is not used, aim to assist States in establishing uniform and consistent legislations on piracy [84], which is similar to the function of model law in this sense. (3) Codes of practice, guidelines, and recommendations, such as IMO's Code of Practice for the Investigation of the Crimes of Piracy and Armed Robbery against Ships, Guidelines to Assist in the Investigation of the Crimes of Piracy and Armed Robbery against Ships, and Recommendations to Governments for preventing and suppressing piracy and armed robbery against ships. On the one hand, these instruments urge governments to improve the necessary legal frameworks for universal jurisdiction over piracy; on the other hand, they also promote the unification of national legislation through detailed suggestions at the operational level.
The use of soft law is helpful in offsetting the shortcomings of the exercise of universal jurisdiction over piracy as a public good. With benefits to all States, people, and generations [137], public goods help to achieve sustainable development. However, due to the main characteristics of public goods (non-excludability and non-rivalry) [138], free-riding is easy [139], which means that individual States enjoy the benefit without contibuting to it [140]. Thus, in conjunction with the analysis of the distribution of costs in Section 5.1, public goods are often under-produced. Moreover, it is often the great powers that have the ability and willingness to provide public goods. Richer and larger States have more advantages than poorer and smaller ones in the negotiation and conclusion of agreements [122], which is not conducive to the achievement of SDG 10 (reduced inequalities) [4]. The influence of great powers, by providing the public goods of universal jurisdiction over piracy, may also cause the concern that these more powerful States police the oceans and define standard enforcement procedures. Soft law may be able to remedy these adverse effects: (1) The intervention of international organizations in soft law can improve professionalism and ensure a balance between fairness and efficiency. Many soft laws are proposed by or under the auspices of international organizations. Some international organizations have sufficient expert resources to provide professional and scientific guidance for law reform [141]. In addition, international organizations contribute to equity, fairness, and efficiency. In order to achieve sustainable development, policy formulation, decision-making, and implementation require broad participation [3]. Competing interests among a wide range of participants is inevitable. The consensual structure in international law is considered to be the main obstacle to solving the key problems of global public goods [139]. International organizations usually cover or represent the most interested stakeholders, which ensures broad participation to a certain extent. Moreover, international organizations undertake convening and coordinating functions, thus providing a forum for negotiating, reducing costs [139], and increasing the efficiency of instrument-making.
The UN and its Security Council were regarded as international organizations that guarantee the global public good related to international peace and security [142]. In terms of public goods and SDGs in the maritime domain, IMO, among others, can also be a powerful actor. IMO takes the realization of SDGs in the maritime domain as an important goal, and correspondingly created the concept of a Sustainable Transportation System, of which marine security is a vital part [143]. The IMO has contributed to the adoption of many international conventions, regulations, and policies that help in achieving the SDGs [7], which shows that it has the capability and rich experience to support the cooperation and good operation of the Maritime Transport System [3]. The IMO also emphasizes the promotion of sustainable development through full consultation and coordination. To be specific, it recognizes that one of its functions is to coordinate "maritime policies worldwide" by setting global standards for international shipping and to "ensure 'level playing fields' across the world" [3]. Therefore, IMO can be an appropriate forum for anti-piracy legislation reform and law enforcement coordination.
(2) Soft law provides flexible guidance to States to avoid the injustice caused by the imbalance of power between stronger and weaker States. Although international organizations usually cover or represent the States with the most relevant interests, the representation of diverse States is not always completely balanced due to the distribution of seats and the setting of voting mechanisms, as well as the different adoption procedures for different variations of soft law. Even so, since soft law does not have legally binding force, States will not suffer from the limitation of sovereignty or the damage of interests caused by unfair binding obligations. Under the principle of cooperation, all States have equal rights to participate, but their tasks and obligations may be differentiated according to their abilities and needs [144]. The guiding role of soft law in assisting States in forming views and shaping behavior helps to improve the practice and opinio juris of States and to urge States to comply with the soft law on their own initiative when they have the ability. This is also consistent with the important content of SDG 17: to "build upon principles and values, a shared vision, and shared goals" [145].
(3) The costs of developing soft law are lower than those of hard law, and the formation of soft law is more convenient and faster, which responds to the nature of public goods in the exercise of universal jurisdiction over piracy. Piracy is an international crime applicable to universal jurisdiction. If a hard law is established for it, its "universality" character needs to be embodied. Therefore, it is necessary to strive for global recognition by the international community and to reach a comprehensive agreement on relevant matters. Formal international lawmaking is usually costly [139], and it is obviously difficult to achieve in the short term with the current lack of a unified understanding of many problems. By comparison, making soft law is less costly [146]. It can avoid the long and arduous negotiation stage and domestic ratification process required for the conclusion of treaties, and it does not need to go through the lengthy and consistent practice process required for the formation of customary international law [147]. Furthermore, considering the exercise of the universal jurisdiction over piracy as a public good, States, as rational economic actors, are often unwilling to invest high costs to set up restrictions (e.g., hard laws related to the universal jurisdiction over piracy) themselves. Soft law can mitigate such concerns of States.
This does not mean that hard law is unnecessary. After all, the authority and binding force of hard law cannot be achieved with soft law. Some effects, such as establishing universal jurisdiction as a real legal obligation within a certain range, can only be realized through hard law. It would be best if relevant hard laws could be concluded immediately. However, as previously analyzed, in light of the practical reality, soft law can be regarded as a transitional measure before the establishment of hard law. The implementation of soft law can also provide a trial basis for the conclusion of hard law.

Adjust the Basic Provisions of Universal Jurisdiction
As mentioned above, UNCLOS is the vital foundation for universal jurisdiction over piracy, but it has its shortcomings. Therefore, it would be valuable if some basic provisions concerning universal jurisdiction could be amended or at least be further clarified.
Things to be clarified include: (1) While any State has universal jurisdiction over piracy, the seizing State has the right to transfer pirates to appropriate courts of the States or other piracy trial fora that are willing to receive them, and the place of imprisonment after the conviction may be located in a State other than the seizing State and prosecuting State. (2) The ambiguities in UNCLOS should be clarified, including but not limited to clarifying the meaning of the word "illegal" preceding "acts of violence and detention", and expounding the meaning of "private ends". (3) The attacks occurring in EEZs also can constitute piracy. Consequently, States bear the duty to cooperate in repressing piracy in EEZs, and foreign warships and law enforcement ships are enabled to enter the EEZ of another State to seize pirates. According to the provisions of Article 58 (3) and Article 56 (2) of UNCLOS, coastal States and other States are required to make joint efforts to take into account each other's rights and obligations. Thus, foreign warships and law enforcement shall regulate their own behaviors, abide by the laws and regulations formulated by coastal States in accordance with UNCLOS and other rules of international law that are not in conflict with Part V of UNCLOS, strengthen the communication with coastal States, reasonably plan navigation routes, and coordinate law enforcement action. (4) The threat of violence and an attempt to commit piracy should be regarded as piracy; to this end, the definition of "attempt to commit piracy" should also be clearly defined to articulate, for example, whether it only includes initiating violent action against the target vessel or whether it also includes chasing a vessel before using violence or even only cruising for piracy. (5) Article 101 (b) and (c) of UNCLOS do not limit the place of occurrence of acts to the high seas and a place outside the jurisdiction of any State. For this purpose, it is necessary to further clarify the extension of the "intentionally facilitating" act in subparagraph (c). For example, it should be clarified whether offences, such as arms trafficking, money laundering, sale or disposal of stolen goods, corruption, and dereliction of duty, which may constitute independent crimes, fall under universal jurisdiction because in effect they provide facilitation to pirates. (6) States have the obligation to enact domestic laws related to establishing and exercising universal jurisdiction over piracy, as well as the obligation to prosecute or extradite the seized pirates.

Refine Relevant Measures in Exercising Universal Jurisdiction over Piracy
First, harmonize the definitions of piracy in national laws as far as possible. Piracy offences established in the domestic laws of a State are the acts that the State considers should be criminalized and punished, so it is not realistic to require States to unify the definition of piracy. As a compromise, the offences that is consistent with UNCLOS and other "piracy" offences in domestic law can be stipulated in different provisions or in different subparagraphs of one provision, so as to distinguish piracy offences that attract universal jurisdiction from other "piracy" offences subject to other jurisdiction principles. However, if a State's definition of "piracy" occurring on the high seas or in a place outside the jurisdiction of other States is too broad, such as omitting the "two ships requirement" or "private ends", it may be necessary to restrict the definition.
Second, it would be beneficial to coordinate sentences imposed on pirates in domestic laws as far as possible. In determining the sentence, a State usually considers comprehensively both the nature of the crime and the gravity of its social harm. The penalty also has to be in harmony with sentences for other crimes in that jurisdiction. Therefore, it is not realistic to request the unification of the sentences imposed on pirates in different States. However, a State can evaluate its own provisions relating to sentences by an understanding of those of other States, and it can adjust its own penalties if they are not appropriate. For example, the United States prescribes a uniform sentence of life imprisonment for pirates [148], but that sentence was established in 1909, which was earlier than the signing of UNCLOS. The concept of piracy was obviously dissimilar then from the current concept of piracy, and the social background was also different from now, so there may be room for adjustment. Additionally, the States that are without domestic anti-piracy legislation and that are willing to enact it can also take into account the sentences of other States as a reference.
Third, inciting and intentionally facilitating piracy (especially by piracy sponsors and money launderers) within the territory of the State should be criminalized, and the jurisdiction over piracy sponsors and money launderers should be coordinated with other States. The destruction of land-based aspects of piracy, such as pirate finance and organizational networks, should be an essential sustainable anti-piracy approach [61]. As for land-based piracy offences within the territory of other States, a State has no power to enter the territories of the coastal States to enforce the law. At most, that State can seize and try the suspects when they enter its own territory. However, as for the land-based piracy committed within its territory, the State surely has the right to take measures against suspects. Since such land-based piracy activities usually serve the piracy activities at sea or are part of a criminal network jointly with land-based piracy activities of other States, clarification is needed in domestic legislation as to whether other States can claim jurisdiction over these land-based activities and request extradition to prosecute suspects in conjunction with other related piracy activities on the high seas or in a place outside of the jurisdiction of any States or in territories of other States.
Fourth, the jurisdictional issues should be harmonized for a variety of situations. For example, domestic anti-piracy legislation should explain under what conditions the State needs to transfer pirates to other States or other judicial mechanisms for trial, or needs to receive pirates transferred by other States for trial, and what kind of application, decision-making, and transfer procedures are required for transfer and reception.
Fifth, when seizing, prosecuting, and imprisoning pirates are not completed by the same State, the links between the relevant legal practices should be addressed. Domestic anti-piracy legislation should address problems including: (1) as a prosecuting State, how to determine the effectiveness of the evidence collected by the seizing State; (2) which State's law should be applied to determine the validity and legality of law enforcement actions taken before the seizing State transfers the pirate to the prosecuting State (such as the seize process, the time limit of detention, the treatment conditions, the protection of human rights); (3) whether the detention time in the former State needs to be deducted from the sentence determined in the latter State; (4) which State, prosecuting State or imprisoning State, has the power to decide prison management matters, such as commutation of sentence; and (5) whether the imprisoning State bears the obligation to hand over pirates to other States for trial if the pirates are found to have committed other piracy offences before imprisonment.
Sixth, the issue of mutual legal assistance among States should be considered. In addition to assistance in investigation, evidence collection, and extradition, which are usually included in judicial assistance, the content of assistance can also include urging citizens to testify, improving video and other technical means of remote testimony, and clarifying its validity. In special circumstances, a State may appropriately allow other States to investigate and obtain evidence in its territory. As mentioned, UNSC has urged States to allow their citizens and vessels to undergo forensic investigations at the first port of call immediately. If the first port of call is not the seizing State or prosecuting State, the State in which the port is located shall permit the seizing State or prosecuting State to investigate within its territory.
Seventh, the procedural law for extraterritorial law enforcement should be developed and perfected, mainly to establish the legal frameworks for boarding and inspecting ships at sea; seizing, arresting, and detaining suspects of piracy; confiscating ships, property on board, and the arms of piracy; collecting and preserving evidence, etc. This will ensure that there are reasonable procedures in domestic law, which can be followed by maritime law enforcement.
Eighth, the human rights of pirates should be guaranteed in accordance with applicable international human rights law. The issues related to human rights of pirates relate to various anti-piracy measures, such as the time of detention, guaranteeing litigation rights, the conditions of detention or imprisonment, and anti-torture, etc. Therefore, human rights should be taken into account in the authorization of relevant measures.
The above recommendations should ultimately be enacted in a State's domestic legislation to directly support anti-piracy activities. To achieve this goal, relevant antipiracy treaties that establish global obligations or soft laws on anti-piracy could be adopted at the international level to guide the development of domestic legislation.

Conclusions
The international law of piracy has a long history, and it has developed in response to challenges across hundreds of years. As a result, universal jurisdiction over piracy, which existed in customary international law, is reflected in the HSC and UNCLOS. However, it was rarely applied until the fight against Somali pirates. In combating Somali piracy, the anti-piracy legal system related to universal jurisdiction over piracy has also exposed the shortcomings in dealing with practical challenges, which means that the sustainability of the system needs to be improved. The rampancy of Somali piracy should be a trigger to rethink and develop universal jurisdiction over piracy. On the one hand, new trends in the practices of universal jurisdiction over piracy have been revealed and need to be considered in enhancing the law. On the other hand, the emphasis on universal jurisdiction over piracy has required re-examination of the relevant legal system, and it is found that there are several main challenges in exercising that jurisdiction that could usefully be focused upon. In other words, compared with the period when the HSC and UNCLOS were adopted, the current situation of piracy and the demand for combating piracy have changed. Correspondingly, the anti-piracy legal system needs to be adjusted to support sustainable marine security and promote the sustainable development of the shipping industry and the ocean. To address those trends and challenges, as well as to establish a sustainable anti-piracy legal system, it is necessary to strengthen universal jurisdiction, and this article has explored the issues and outlined how this might be achieved. Sustainability is usually associated with SDGs and the phrase "long-term" [149]. Therefore, a sustainable solution should be SDGs-oriented and durable in time or effect. In order to achieve this, a sustainable solution should be responsive to changes, comprehensive in analyzing problems, and diversified in means. It should not only be forward-looking but should also be practical and operable (e.g., both ideal goals and transitional means are needed). Meanwhile, political and economic sustainability should be taken into account [61] (e.g., considering various balance relationships is necessary). Above all, piracy is a crime subject to universal jurisdiction, which needs all States to continue to develop a "comprehensive legal regime to prosecute pirates, consistent with international law" [150]. At the same time, universal jurisdiction is a significant measure but not the whole answer to addressing piracy. As noted in the UNSC resolutions, the international community needs to adopt comprehensive measures to solve the problem of piracy and eliminate its root causes [44], which is also a significant topic related to sustainability. Data Availability Statement: The data will be made available upon request to the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.   The International Convention against the Taking of Hostages, International Convention for Safety of Life at Sea, and UN Convention Against Transnational Organized Crime are also important conventions in fighting against piracy, but they do not directly address piracy. Therefore, this article does not analyze them.