Fin Whale Balaenoptera physalus Historical Sightings and Strandings, Ship Strikes, Breeding Areas and Other Threats in the Mediterranean Sea: A Review (1624–2023)

Abstract

A review of the last 399 years (1624–2023) on fin whales (Balaenoptera physalus) in the Mediterranean Sea was conducted, based on an extensive compilation of records published in the scientific literature, technical reports, public databases, journals, and social media. A total of 10,716 sightings and 575 mortality events have been computed, analysed by semesters and mapped in order to compare the summer–winter seasons especially and their implications on migration–residence. Visual and acoustic detections, feedings, migrations, primary production areas (chlorophyll), threats and causes of death and their relations have been addressed, and a mini-review on heavy metals and pollutants has been carried out on fin whales in the Mediterranean Sea. Mortality events were most frequent between November and April, coinciding with the decreased sighting period. Ship strikes posed the greatest threat, peaking between May and October, when marine traffic tends to increase in the Mediterranean Sea. Two populations coexist in the Mediterranean Sea, one resident and the other migratory, the latter using the Strait of Gibraltar for its biannual movements. Two areas with a presence of calves (up to 7 m in length) between October and February were detected: one scattered in the northern Mediterranean and the Strait of Gibraltar and its surroundings. A critical zone for collisions has been established according to the results for fin whales in the Mediterranean Sea.

1. Introduction

The fin whale Balaenoptera physalus L. 1758 is the largest animal in the Mediterranean Sea and the second largest on the planet (after the congeneric species B. musculus L. 1758). It is an evolutionary jewel, protected by numerous international, national and regional administrative and legal provisions, notably the International Whaling Commission, which established a zero catch limit [1], the IUCN, where it is classified as “Endangered” [2], the Habitats Directive (92/43/EEC) under Annex IV: Animal and plant species of Community interest requiring strict protection [3] and the Convention on the Conservation of European Wildlife and Natural Habitats (Bern Convention) under Appendix II: strictly protected fauna species [4]. In addition, at the regional level, the Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS) facilitates data consolidation through projects and analyses [5]. On the other hand, management tools have been created, such as the Pelagos International Marine Sanctuary (SPAMI) [6], the Marine Protected Area (MPA): “Mediterranean Cetacean Migration Corridor“ (CMC), which covers the whole sea between the Balearic archipelago and the Spanish mainland and recently, the International Maritime Organisation (IMO), who established the north-western Mediterranean as a Particularly Sensitive Area (PSSA), in which protection measures are voluntary [7].

The lack of surveillance [8,9] and the fact that fin whales exceed the limits of the protected areas [10,11] make these conservation measures ineffective. It is only in France that vessels over 24 m are required to install whale positioning devices inside the Pelagos Sanctuary [12].

Before the 19th century, the fin whale had no commercial value given its high swimming speed, the equipment used by the whalers at the time and the fact that it produced less oil than other target species due to its relatively thin layer of blubber (unlike grey or sperm whales); furthermore, it sank once hunted [13]. It was during this century that whaling was boosted, due to advances in the development of the technologies used to capture and process whales, promoting their commercial exploitation throughout the world, including in the Mediterranean. Whales became overexploited, and consequently, hunting was regulated in the 1930s, and although it was resumed, the number of catches never reached those of the beginning of the century [14,15]. In the 1960s and 1970s, the measures adopted by the International Whaling Commission halted, to some extent, the decline of the species on a global scale. Nowadays, hunting is still allowed in some countries such as Greenland, Iceland and Antarctica [16].

Like most mysticetes, fin whales are a migratory species, remaining abundant mainly in cold, temperate offshore waters off the continental shelf [16], where they form independent breeding units [17], although they are sometimes located very close to shore. They are distributed throughout the world’s oceans, although they avoid the poles and tropical waters. In this regard, the absence of the species between approximately 20° N and 20° S was reported by [18], who found that populations in the northern and southern hemispheres do not usually mix. This suggested, together with genetic evidence, the existence of two subspecies, one from the north (B. physalus physalus) and one from the south (B. physalus quoyi) [16]. In general terms, fin whale migration is considered to follow a seasonal pattern [18], in which individuals move to feeding grounds in the summer and breeding grounds in the winter, where food is scarce [16], while there are other populations such as those in the Gulf of California, the East China Sea of Japan and the Mediterranean Sea that are apparently resident year round [19,20].

The current world population of fin whales consists of 100,000 mature individuals [21]. Therefore, an updated study of their presence and movements in the Mediterranean Sea is essential to protect this species from the threats to which it is subjected in this sea, including ship strikes, noise and chemical pollution as well as global warming [22,23,24,25,26,27].

1.1. Objectives

The aim of this work is to review the existing public information on fin whales in the Mediterranean in order to analyse the sightings, deaths, seasonality, feedings, calving grounds, presence near or far from the coast and their main threats, in order to contribute to the improvement of their protection and conservation in the Mediterranean basin.

1.2. Feeding

In the Mediterranean, fin whales usually feed in the water column up to 470 m [28] during the day, but at night, their feeding activity is limited to the surface. In addition, they can filter about 70 m³ of water per gullet [29]. Krill is the main food source of fin whales in the Mediterranean Sea, with two euphausiid species standing out: Nyctiphanes couchi as the common prey [30] and Meganyctiphanes norvegica as the main prey in this sea [16,31,32]. The latter shows very specific vertical movement patterns, settling between 75 m and 800 m depths during the day [33] and migrating to the surface at dusk, settling at depths of 30–50 m [19]. The crustacean M. norvegica is partially phytophagous in the spring and summer [34,35,36,37], which is of great interest, since as we review and discuss in this paper, it is related to the greater presence of the species in areas with higher levels of chlorophyll production. Therefore, the abundance and greater presence of the species appears to be highly dependent on the magnitude of local primary production in the spring.

In the Mediterranean Sea, krill availability during the summer is more confined, making it a more challenging task in the winter to find the feeding grounds. It is now known that during the spring, fin whales concentrate in the Catalan-Balearic Sea [11,38], moving eastwards during the summer towards feeding areas located in the Liguro–Corsico–Provençal basin [38], where upwellings produced by the permanent Ligurian frontal system serve high concentrations of euphausiids.

Some winter-feeding areas of the resident population have been described, such as the waters off the island of Lampedusa [30], but the feeding grounds of northeast Atlantic (NENA) fin whales in the Mediterranean are still uncertain [15]. The intense feeding activity of fin whales in the Liguro–Corsico–Provençal basin during the summer has been confirmed by numerous individual sightings [39,40]. Furthermore, as evidenced by numerous resightings in the area, fin whales have a certain “loyalty” to the area [41].

The Pelagos Sanctuary, located in the north-western Mediterranean, is considered a Special Area of Conservation under the Barcelona Convention of 2002 and was designated as the first internationally protected site for cetaceans [29,42]. The high concentration of individuals is due to high levels of primary production conflicts with the busy shipping routes, making it a very dangerous key area for fin whales [22]. The Tyrrhenian Sea may be the entrance and exit of this area for Mediterranean fin whales, as suggested by numerous sightings [19].

1.3. Breeding Areas

Although fin whale migration has been well described, there is controversy over the locations of specific breeding areas, as fin whales disperse to tropical waters during the winter, as reported Notarbartolo di Sciara et al. [19]. The same author revealed that the observation of calves in various locations in the Mediterranean may suggest that there are no specific breeding areas. On the other hand, Marini et al. [43] proposed that whales migrated to breeding grounds (Libyan and African coasts) from their feeding grounds during the winter, also reporting sightings of “females with calves” around the Sicilian island of Lampedusa [44]. Cotté et al. [45] suggested that Mediterranean fin whales remained at low/mid latitudes, although others remained more dispersed in the north-western feeding grounds during the winter months; the observation of young individuals in the Pelagos Sanctuary throughout the year by Orsi Relini [46] might suggest that fin whales also breed in this area. In contrast, Viale [47,48] suggested that whales entering the Strait of Gibraltar in the winter came from northwest Scotland, but the authors of [24] established that whales from the northeast Atlantic extend their wintering grounds south of the Mediterranean basin, and whales detected in the Strait during the winter belong to this population and are independent of the Mediterranean subpopulation.

1.4. Natural and Anthropogenic Hazards

Fin whales in the Mediterranean face both natural and anthropogenic threats. Over the past 25 years, several strains of dolphin morbillivirus (DMV) have caused epizootics in odontocetes and mysticetes [49], including fin whales (Balaenoptera physalus) [50,51,52], causing pulmonary and neurological diseases and triggering individual or mass strandings [53]. Killer whales (Orcinus orca) are predators of fin whales [16], although the natural threat (especially to calves) does not exist in the Mediterranean, as the presence of the predator In this sea is exceptional. Notarbartolo di Sciara [19] suggested that the great white shark may be a predator, but no attacks have been documented in the Mediterranean.

It is known that 220,000 vessels over 100 tonnes transit the Mediterranean Sea [54]. The total capacity of all littoral states, including other types of vessels such as gas/chemical tankers and ferries/passenger ships, is 248,304 deadweight tonnes (dwt), representing 13% of the world’s vessel capacity. Furthermore, the Mediterranean is the second most used sea area for cruises after the Caribbean (15.8% of the global cruise fleet deployment in 2017). In addition, there are more than 400,000 berths in 940 marinas [55,56] spread across France, Spain and Italy. Recreational vessels are of increasing concern in terms of their impacts on marine mammal noise (e.g., area avoidance) and subtle effects (e.g., changes in acoustic behaviours or elevated cortisol levels) [57].

Whale-watching activities are known to have adverse effects on fin whales. Whales in the Gulf of Maine reduced their dives, duration and number of blows per surface sequence in the presence of vessels [58]. In Chile, fin whales also showed evasive responses to the presence of whale-watching vessels [59]. In the Mediterranean, an increase in speed and zigzagging movements together with a decrease in blowing frequency was reported as an avoidance response to the presence of vessels [19]. In contrast, on the island of Ischia, indifference to passing vessels at 100 m was recorded [60].

The development of their migration patterns so close to the coast means that fin whales are exposed to the effects of human activities (Figure 1), including chemical and noise pollution, maritime traffic and the consequent risk of collision [61,62,63], making them the cetacean most affected by these phenomena [42].

In the Marine Mammal Assessment Report for the Marine Strategy Framework Directive for France [64], ship strikes are recognised as an anthropogenic pressure on large cetaceans and are assessed, where possible, in sub-regions. Of these, the Mediterranean sub-region shows the highest occurrence of ship strikes with large cetaceans (fin whales and sperm whales), with a particular risk in the Gulf of Lions, west of the Pelagos Sanctuary. The area is crossed by a network of busy shipping routes connecting Italian, French and Spanish ports and the mainland with several islands, and it is the area with the highest density of fin whales during the summer [65]. Maritime traffic is expected to intensify significantly globally in the future [65,66,67,68], and consequently, pressures on marine environments will have implications for the welfare and persistence of at-risk and threatened species [69,70].

Noise pollution from anthropogenic sources has increased over the last 50 years, as all marine operations generate sound: maritime traffic, seismic exploration for the oil and gas industry, sonar use for defence and commercial activities, tourism, recreational activities, fishing, offshore mineral extraction, offshore wind energy, hydropower and coastal and near-shore activities [71,72]. Noise derived from shipping traffic, mainly from propeller cavitation as the ship moves, producing a broadband noise spectrum ranging from a few Hz to more than 100 kHz [73], has become the major source of anthropogenic noise pollution in the oceans and is responsible for a steady increase of 3 dB per decade in low-frequency (10–100 Hz) ambient noise in many ocean regions [74,75,76,77,78]. Such noise is directly masked by the frequency band of mysticetes’ acoustic communication signals [79,80,81,82], because they use low frequencies and long wavelengths in their acoustic repertoire, allowing them to communicate over long distances [82,83]. This environmental noise disturbance from shipping traffic is much more severe for right whales than for humpback and fin whales [57]. Whales in studies of ship noise exposure decreased both the duration and bandwidth of their calls [23]. In addition, under high noise conditions, altering the frequency of the call could make it more energy consuming and less efficient, which could significantly affect the reproductive rate of this species [23].

Another major treat to fin whales is climate change, which may affect their prey in the Mediterranean [84,85,86]. As mentioned above, their main prey is the northern krill Meganycthiphanes norvegica, a dominant species in the Ligurian Sea [42] due to its extreme level of thermal adaptability, as it is at the southern limit of its geographical distribution (northern hemisphere), with a northern geographical limit (“unable to ‘escape’ further north within this sea”). Consequently, rising temperatures could significantly reduce the availability of this prey, and thus, the prevalence of cetaceans in Mediterranean waters. Studies have mathematically linked the presence of fin whales to the seawater temperature, chlorophyll levels, nutrient inputs from watersheds and bathymetry, all of which are considered causal factors for the presence of krill [87].

The effects of pollution are also evident in fin whales. The high toxicity, persistence and bioaccumulation of contaminants (e.g., POPs such as organochlorines and polybrominated diphenyl ethers) combined with the cetacean´s biomagnification capacity due to the amount of blubber, their high metabolic rate and poor ability to excrete these compounds [88] leads to high levels of accumulation in marine predators [89], resulting in the most significant toxic effects [90]. Fin whales are excellent indicators for contaminant assessment, because they migrate long distances, consume substantial amounts of water and are relatively abundant in the ocean [16]. Although they have lower contaminant concentrations than odontocetes, since fin whales feed on planktonic crustaceans with lower contaminant levels [91], fin whales in the Ligurian Sea are more exposed to organochlorine compounds than those in the Atlantic [92]. These pollutants act as hormonal [93,94,95] and endocrine disruptors [96,97], are neurotoxic [94,98,99,100,101,102], immunotoxic [103,104], reproduction toxic [105,106,107] and carcinogenic [108,109,110] disruptors. In addition, epigenetic changes have recently been described that may be associated with adverse health effects of POPs and other classes of contaminants in fin whales [111]. On the other hand, heavy metals of natural [112] or anthropogenic [113,114,115] origin are highly bioaccumulative and toxic [116]. These include mercury and lead [117,118] as well as cadmium, zinc and copper because of their toxicological effects and their relative abundance of metal inputs to the marine environment [119]. High concentrations of copper can enhance cell membrane Na⁺/K⁺-ATPase and limit potassium regulation and osmotic balance; Zn protects against ultraviolet light, which destroys DNA. Mercury affects the endocrine and nervous systems, causing dysfunctions in osmoregulation, prey location, orientation and interspecific communication, while Pb causes behavioural abnormalities that impair survival, metabolism and learning [120,121,122,123,124,125,126,127]. The accumulation of heavy metals in mysticete prey is low compared to that of odontocetes [128]. In addition, an estimated 100,000 objects per km float in the Mediterranean², and given the constant flow of water from the Atlantic, these floating pollutants tend to remain in the Mediterranean basin [29]. The filter-feeding activity of fin whales makes them susceptible to ingesting microscopic plastic particles each day [129], indirectly through contaminated prey [130,131] or directly [132,133]. Recent studies have correlated the feeding grounds used by fin whales with areas of high microplastic concentrations, showing a clear relationship [29]. One such area is the Pelagos Sanctuary [134].

In addition, plastics can release harmful particles such as polyethylene, polypropylene and phthalates [134], which is a serious threat given the lifespan of fin whales, and also act as vectors for the dispersal of invasive species [135], which could introduce disease-causing pathogens, posing another major threat, especially to newborns and calves with weak immune systems [134].

2. Materials and Methods

A literature review was carried out for the period 1624–2023 using scientific search engines (Web of Science, Scopus, Research Gate, Academia and Google Scholar), cetacean-related databases such as MEDACES (Mediterranean Database of Cetacean Strandings), OBIS-SEAMAP, the Biodiversity Data Bank of the Valencian Community and REDIAM (Environmental Information Network of Andalusia), as well as news, social networks and the rich bibliography on the subject, which was consulted previously in numerous sources. Information on B. physalus was collected on sightings and mortalities (location, number, origin, length at stranding, decomposition code and cause of death) in different sectors of the Mediterranean. In order to achieve the highest possible scientific rigour and to facilitate the traceability of the search and the location of sources for future authors, anonymous sources were first removed, as well as the sources with a high lack of information (Figure 2).

Histograms of sightings, strandings, collisions and other selected mortality descriptors were plotted, classified as “May–October” and “November–April” semesters to detect differences between the periods. Only the minimum number of individuals for which sightings were certain was taken into account. A distinction was made between information from strictly scientific sources (following a pure scientific methodology and species identification was accurate) and information from other sources that did not fit into this category. In the histogram of sightings, the descriptor “Sightings of cetaceans” was introduced, providing data from cetacean sighting platforms. The category “Other sources” included citations outside the context of scientific publications, such as from databases, reports, social media, news, volunteers and fishermen’s records. In addition, journalistic and social media citations accompanied by reference images or videos were considered in order to identify the species. Scientific sources that did not provide enough information as required for the strictly scientific category were included in this group. If an article reported “three groups sighted” without specifying the number of individuals, 3 specimens were counted. When, for example, the presence of “more than three individuals” was mentioned, 4 individuals were counted (the minimum number of animals observed with certainty). Many papers provided information on sightings in the Mediterranean Sea but lacked sufficient information to include them in the count, e.g., [136], or mixed sightings in different sectors of the Mediterranean, e.g., [137]. Similarly, other papers contained valuable information on B. physalus sightings but were outside the study area of interest, e.g., [138]. Information that did not provide specific coordinates was approximated according to the description of the location provided by the authors. If the data lacked location information, they were disregarded. Data that could correspond to other data already recorded, for example, those included in the REDIAM database, which included data on deaths and sightings, were substituted and excluded. As for deaths, data that coincided with others already identified or that belonged to areas outside the study area were discarded.

Where there was evidence of different data from two different sources in the same locality, these were taken into account, such as the data from [139], which coincided with some of those published by [15]. The indications in their methodologies and the non-coincidence of the total number in each case suggested that they were independent data. On the other hand, the distinction between information from strictly scientific sources and that from other sources introduced bias, especially in the case of the histogram of sightings, since by being divided by areas, some of them (for reasons such as greater popularity or relevance, among others) tended to be more studied and published in the scientific literature than others. This is the case of the Ligurian Sea, part of the Pelagos Sanctuary, where most of the reported sightings come from strictly scientific sources.

Venn diagrams were developed to visualise and analyse the relationships between different datasets, representing all possible combinations and the 885 elements contained in each of them [140] using the generator available in [141]. These diagrams allow for the visual identification of common (overlapping circles) and unique (independent areas of the circles) descriptor spaces. For this purpose, the data inherent to all relevant fields or descriptors in the diagram literature were used, i.e., if they lacked information on their seasonality, location sector or, in the case of the mortality diagram, on the cause of death, they were excluded. Thus, the fractions of the circles in the graph that fell outside the intersections between them would be representative of the periods, locations or types of events excluded from those used in the diagram. That is, the fraction of the data outside the intersection with the area corresponding to the May–October period would report data relating to the November–April period, and thus, respectively, to the locations and types of mortality events.

The Q-GIS tool was used for mapping. Map overlays of the locations of sightings or mortalities (1624–2023) were also produced using the vessel density map for 2019 in order to visualise the most critical areas for conservation. The vessel density layers represent the total hours spent by vessels in a 1 km × 1 km resolution grid cell during 2019. These layers were consulted on the 2019 EMODnet human activities portal, created by the European Maritime Safety Agency [142]. The types of vessels included were passenger vessels, cargo, tanker, fishing, recreational, etc. The density of sightings/mortalities was provided with transparent symbology to visualise overlaps with route densities.

Moreover, to clarify the information on the areas and periods of presence of fin whale calves in the Mediterranean, the total lengths of stranded animals were reviewed. The mean length of fin whale calves in the Mediterranean is 5.2 m, as published in [19], being smaller than those in the North Pacific, which have a mean length of 6.4 m [143]. Although the length class for calves has been stipulated to be between 4 and 8 m [92], all records of stranded fin whales below 7 m were selected, as fin whales are characterised by rapid growth in the early part of their life [144] and classified by months. In addition, their state of decomposition was also recorded and classified into 5 classes, live, fresh, slightly decomposed, decomposed and mummified [145] in order to detect breeding seasons and plot the locations where live, fresh or slightly decomposed neonates and calves were stranded.

3. Results

3.1. Fin Whales in the Mediterranean Sea

Fin whales are the only regularly observed mysticete [42] in the Mediterranean, but their distribution is not homogeneous. The abundance of this cetacean is notably higher in the west than in the east, with the coast of France being the most important area, together with Corsica and Sardinia. On the other hand, on the coasts of Greece, Turkey, Israel and Egypt, it is practically absent [19], but in the Strait of Gibraltar, its eventual presence is also notable, as fin whales cross from the Atlantic to the Mediterranean between November and April and leave for the Atlantic between May and October [15]. Generally, they do not form stable groups, which is common in whales, although numerous specimens often aggregate in feeding areas. In the Bay of Biscay (Iberian Peninsula), an Atlantic area close to the Strait of Gibraltar, an abundance of 977 specimens has recently been estimated during the summer [146], a period of the year in which Walker [147] established the greatest presence of the species in August. In contrast, the authors of [10] estimated an abundance of 250 specimens during the winter.

In the Mediterranean, there is evidence of two populations of fin whales: a small resident population and a NENA population (referred to above) that migrates to the Mediterranean basin on a seasonal basis [15,17,24,30,148,149]. It has been suggested that some individuals of the NENA population may even migrate biannually, entering and leaving the Mediterranean at least twice a year [15]. On the other hand, individuals from the resident Mediterranean population also make seasonal movements, independently of those made by individuals from the NENA population [15]. These individuals remain year round in the Mediterranean and feed in both the summer and winter; they usually aggregate in the summer in feeding areas and then disperse throughout the rest of the Mediterranean basin [148]. This has been demonstrated through a study of their acoustic signals, as these differ between individuals from both subpopulations [24], determining that fin whales from the Atlantic, during the winter in the Mediterranean, were concentrated in the Alboran Sea and in the area of the Strait of Gibraltar, while the resident Mediterranean population was not present in these areas during this time of the year [148,150]. The areas occupied by both populations when they are present in the Mediterranean can be seen in Figure 3. Some data seem to indicate that there is an overlap of both populations from the Balearic Sea to the Ligurian Sea [148].

Studies on the 15 N and 13 C isotope levels in the baleens of fin whales have also contributed to the differentiation of the two subpopulations [150], in addition to results on the levels of organochlorines in their blubber layer, mainly PCBs and DDT [92]. In Atlantic individuals, the levels of the aforementioned isotopes are higher than in Mediterranean individuals [150], while PCBs and DDT are more notable in Mediterranean individuals, as the sea is almost enclosed [92,150]. On the other hand, it was observed that in the baleens of individuals belonging to the Mediterranean NENA subpopulation, the outermost layer of the baleen acquired isotopic values typical of the Mediterranean krill M. norvegica [150].

Differences in the mitochondrial DNA have been found between the two populations, as well as discrete gene flows in relation to nuclear DNA [17,42]. Although some authors consider that they may reach the Atlantic waters of southern Portugal and Morocco [42], the distribution limits of the resident population in the Mediterranean are unclear [16]. However, it seems that the results of genetic studies describe a situation that is only possible if gene flow takes place through males, suggesting that male Atlantic fin whales may reach beyond the Alboran Sea and mate with females from the resident population [42].

The data analysed for the resident population indicate that they aggregate in the Ligurian Sea during the summer and disperse during the rest of the year. In fact, strandings off the North African coast suggest that fin whales are more concentrated in the south during the winter [148]. Furthermore, the waters off the island of Lampedusa have been shown to be a winter-feeding ground [30]. Data from Panigada et al. [39] support this observation. However, some individuals remain all year round in the Ligurian Sea and others move to the Balearic Islands at the end of the summer. This makes it impossible to establish a specific migratory pattern for this population. The intense feeding activity in the Liguro–Corsico–Provençal basin during the summer is confirmed by the high number of sightings and the presence of droppings. It is also known that fin whales have a certain “loyalty” to this area, as indicated by the numerous resightings of individuals in the area. The point of entry and exit from this area for Mediterranean resident fin whales may be the Tyrrhenian Sea, as suggested by numerous sightings [19]. The specific breeding areas are unknown, so it has been assumed that because the Mediterranean is such an enclosed sea, individuals can still communicate with each other despite being dispersed, allowing mating to occur in any area of the Mediterranean basin [148].

3.2. Chronology and Analysis of Sightings (1904–2022)

A detailed review of the total recorded sightings in the Mediterranean during the period 1904–2022 is illustrated in Figure 4a by means of a histogram constructed from information obtained directly or indirectly from the following strictly scientific sources: [11,15,19,28,29,30,31,39,44,45,48,139,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207]. The category named as “Whale watching” includes information from [208], which includes data from the ecotourism company Cetáceos y Navegación S. L. [15], whose data from 1999–2001 correspond to the type of methodology shown in [209,210,211] and [212]. The rest of the sources consulted (“Other sources” category in the histogram) were quantified between 1 and 3 according to their degree of professionalism, with 3 being the maximum value. Value 1 included the following: [213,214,215,216] and citations from [217,218,219,220,221,222,223,224,225,226]. Value 2 included the following: [5,30,202,227,228,229,230,231], datasets [232,233,234,235] and OBIS-SEAMAP datasets (basic data) [236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255]. Value 3 included the following: [256,257].

The histogram (Figure 4a) confirms the distribution trend commonly considered for B. physalus: a higher presence in the western half of the Mediterranean (SG, AS, WB, LS, TS, SS and SM sectors) with respect to the eastern half (IS, SO, ADS, AGS, LBS and LVS). The Ligurian Sea sector stands out for the highest number of sightings (more than 4941) compared to the sector with the second highest number of sightings (Western Basin). The comparison between these two sectors, especially considering the sizes of both (see Figure 4a), highlights the importance of the Ligurian Sea for this species.

The Venn diagram (Figure 4b) of the sightings provides complementary information for both six-month periods. It shows more clearly that of the sightings in the Ligurian Sea, the vast majority took place in May–October. Thus, the importance of this area during this six-month period was also reinforced by the fact that most sightings in the whole Mediterranean during this period were recorded in the Ligurian Sea. This is consistent with the traditional consideration of this sea as an area where fin whales gather to feed during the summer months.

The detailed analysis of two six-month periods of the year (May–October and November–April) has allowed us to delimit the information, revealing differences, especially between the summer and winter, which deserve to be discussed. Figure 5a shows that during the May–October period, the percentage of sightings in the Ligurian Sea is much higher than that corresponding to the November–April period or to the total chart.

During the November–April period (Figure 5b), the relevance of the Ligurian Sea for the species in question is reduced, and straits, such as those of Gibraltar and Sicily, become more important. This, together with the fact that sightings seem to be more distributed among the different sectors of the western half, is consistent with the idea that Mediterranean resident fin whales tend to disperse during the winter months (still more frequent in the western half than in the eastern half). Among the few known winter-feeding areas, the island of Lampedusa stands out, which may explain the relevance of the Strait of Sicily during this period. On the other hand, the importance of the Strait of Gibraltar (evident in all the graphs, although especially in November–April) is obvious, as it is the obligatory passage through which the Atlantic migratory population enters and leaves the Mediterranean.

A density map was made including the revised sightings from all years and overlapped with the 2019 ship density map (all types). Three important areas of fin whale presence can be observed, which are also critical due to the high density of maritime traffic (see Figure 6).

3.3. Sightings “Near–Far” from the Shore in the Mediterranean Sea

Fin whales are mainly abundant in cold, temperate waters off the continental shelf, i.e., in deep waters far from the coast [10,16,146,258], so the probability of sightings should be higher offshore than near the coast. However, many scientific sources did not specify any exact location or whether the sightings were near or far from the coast [160,168,174,176,178,183,188,192,205,206]. Several scientific papers provided approximate information on the location of the sightings, and others used geographical features. Those that used geographical features were classified as near-shore sightings, while those that did not specify the position or reference at sea were categorised as distant.

Most sightings have been recorded “offshore” [15,28,29,31,39,48,153,159,160,161,162,163,164,165,166,167,169,171,172,173,177,179,184,185,187,189,191,196,197,198,200,201,204,205,206,207]. Comparatively, sightings in the Mediterranean “near the coast” are testimonials [11,30,44,151,152,154,155,156,157,158,181,182,186,190,194,199,200,202]. In any case, the authors consulted in this review do not explain what they mean by “offshore or near” the coast.

3.4. Why Atlantic–Mediterranean Migration?

The migration of this species is not entirely clear, as there are individuals that do not develop seasonal migration patterns, remaining at high latitudes during the colder months or at low latitudes during the warmer months [18]. Gauffier et al. [15] indicated that the migratory population enters the Mediterranean between November and April and returns to the Atlantic between May and October, often accompanied by their young, suggesting that they use the Mediterranean as a breeding area during the winter (and as a feeding area, as prey availability is lower in the summer) and the high latitudes of the Atlantic in the summer for feeding. This follows the traditional seasonal pattern of fin whales moving to feeding grounds in the summer and breeding grounds in the winter [16], but to some extent, implies that they do not feed during the winter. This pattern is not strictly followed by Atlantic fin whales migrating to the Mediterranean, as this work shows, since whales have been shown to feed during their migrations [259] that were associated with coastal fronts [260]. This was specifically observed in the Algeciras Bay with the unprecedented presence of a humpback whale (Megaptera novaeangliae) [261], exposing feeding behaviours coinciding with the detection of massive numbers of the Nordic krill M. norvegica. In this sense, a relationship has been established between sightings of fin whales in the Ligurian Sea with masses of M. norvegica, which in turn, corresponded with areas of high primary production [32,201], suggesting that in years of low primary production, their distribution would be related to short-term productive areas and thermal fronts [262,263], which would represent areas of concentration of these euphausiids [215]. This hypothesis has also been proposed in the results of [87], using neural network models on variables that define the presence of krill and coincide with the presence of fin whales [87].

By comparing the locations of the updated Mediterranean sightings with the satellite images of chlorophyll concentrations in the winter and summer (2018) produced using the Giovanni tool through the MODIS-aqua sensor at a 4 km resolution and the MODIS-Aqua MODISA L3_8d_4km_v2018 dataset [264] shown in Figure 7, we found, as a result, a high coincidence, in the Western Basin, of the presence of fin whales with the areas of highest chlorophyll concentration. This is especially relevant in the winter far from the coast (which does not occur in the summer), when fin whales are found in the Mediterranean, which seems to be explained by reasons of reproduction and food resources, as opposed to the summer season, which is characterised by a drop in primary production, clear in the aforementioned images.

Areas with higher chlorophyll-a concentrations are, in turn, hotspots of marine productivity, as they provide food for zooplankton, which form part of the diet of other links in the food chain, including fin whales [39]. Therefore, the relationship between fin whale abundance and chlorophyll levels suggests a feeding pattern, which could be one of the drivers of NENA population migration.

Recently, seasonal movements driven by feeding have been described from the Catalan–Balearic Sea, where fin whales have been identified feeding during the spring, moving eastwards towards the north-western Mediterranean Sea [38].

On the other hand, movements of fin whales have been related through recaptures between the waters of the Valencian Community and the Strait of Gibraltar at the beginning of the summer, establishing a correlation in the number of days between the two areas [212].

Sightings in the Nile Delta and off the coast of Israel are surprising considering how oligotrophic this area is (Figure 7). It is possible that prior to the construction of the Aswan Dam in Egypt, fin whales visited this area more frequently, and that after the construction of the dam, the flow of nutrients from the Nile River into the Mediterranean was reduced, favouring the oligotrophy that characterises the eastern Mediterranean basin [265]. However, sightings prior to the Aswan High Dam were very rare, possibly not because they did not occur, but because there has traditionally been less scientific effort made in this part of the Mediterranean [19].

3.5. Collisions, Strandings and Deaths at Sea

The likelihood of fin whale collisions with large vessels is an increasing reported threat to shipping routes in the high seas, externally to the edge of continental shelves, where the species has its preferred habitat [63,266]. To compare collision records in the Mediterranean and the extent to which the threats described in this paper merit an action plan in the Strait of Gibraltar, a review was from published records of collisions (12.52%) for individuals of this species from 1624 to 2023 and, comparatively, of strandings (74.78%), individuals found dead floating at sea (10.44%) and by-catch (2.26%).

The results of the search from the sources consulted are presented graphically in Figure 7. As strictly scientific sources, we consulted the following: [11,19,22,50,61,151,153,155,156,157,174,175,180,182,192,199,202,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,301,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,335,336,337,338,339,340,341,342,343,344,345,346,347,348] and Verona Natural History Museum in [22].

On the other hand, using the same method, the rest of the sources used in this histogram (“Other sources”) were rated between 1 and 3 according to their perceived degree of professionalism (3 being the highest value). In category 1 (several journalistic and some strictly scientific quotations that did not provide data on the season, state of decomposition and duration of most strandings) we used data from the following: [32,229,309,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,370,371,372,373,374,375,376,377,378]. In category 2, we used data from the MEDACES database (SFAX, ARION, ICBIBE, RNE-GECEM, Fundacion Aspro Natura, BIOPOLMARINE, CREMA, Fundación Museo del Mar de Ceuta y Asociación Septem Nostra- Ecologistas en Acción, CSCET, FVMUZ, CARM, EGALIBY, IMMRAC, INSTM, CREM IBIZA, Red de Rescate de Fauna Marina de la Generalitat and Zoo de Barcelona) [379], the Environmental Information Network of Andalusia [380] and UNISI- University of Siena [381]. In category 3, annual reports were included: [382], annual stranding reports from the Réseau Narional Echouages (RNE) in France [383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,405], Centro Studi Cetacei [406,407,408,409,410,411,412,413,414,415,416,417,418,419,420,421,422,423] IZS Sardegna, Museo de Tourin, Museo di Storia Naturale di Genova, Di Natale and Cagnolaro pers. comm., Universitá di Padova, Pintore, A in [331,345,424,425,426]. Strandings (where the cause of death was not publicly published or could not be determined) are the predominant mortality events for B. physalus in the Mediterranean (Figure 8a). After strandings, collisions are the second most frequent mortality descriptor.

The Venn diagram (Figure 8b) illustrates the differences found between mortality events and descriptors in the May–October and November–April semesters, with the differences between the two being clearly visible, as the areas outside the intersections with the May–October circle represent the November–April data.

Distinguishing between seasons (Figure 9), it should be noted that collisions are twice as frequent in the May–October period than in November-–April, which can be explained by a higher volume of maritime traffic, especially from fast boats and ferries. By-catch does not seem to be a serious threat to this cetacean, although it could be the origin of some strandings. These are more abundant in November–April, a period when there should be more fin whales in the Mediterranean due to the greater presence of Atlantic–Mediterranean migrants in the winter, and strandings are easy to record on the coast at any time of the year, given the size of the animals, without requiring observation campaigns or sampling efforts, nor being conditioned by the weather. On the contrary, the summary of sightings reviewed in this period (in which resident and migratory populations coincide), compared with the May–October period (see Figure 4 and Figure 5), shows the opposite situation, which is possibly explained by the lower sampling effort between November and April given the more unfavourable weather conditions during this six-month period. This may also be influenced by the fact that, as mentioned above [215], in the months between May and October, the resident population tends to cluster in feeding areas, which may facilitate a higher number of sightings. The number of collisions is almost twice as high in the May–October period as in November–April, as well as the number of mortalities at sea, which is congruent with the existence of more maritime traffic (especially fast boats) and the greater ability to detect floating carcasses at the beggining of both semesters.

Figure 10a shows the full-year representation of mortality events (1624–2023) overlapping with the density of all 2019 vessels in the Mediterranean, while Figure 10b only represents the densities of vessel collision events.

The north-western Mediterranean is a high collision risk area (Figure 10b) [207]. In 2018, there were 58 cases of stranded whales, and 24 showed a high probability of having died from a collision [427]. Of these, 69% of the cases had occurred in fin whales. Of 383 whales that were photo identified in the Ligurian Sea between 1990 and 2001, 9 whales showed evidence of a collision on their bodies [22]. The most crucial factors contributing to the risk of collision are the overlap between areas of intense maritime traffic and the presence of cetaceans [428,429]. In addition, the risk increases in the summer, as both animal and vessel densities increase [136,430]. Eighty-two percent of lethal collisions occur in or near the waters of the Pelagos Sanctuary, showing the highest mortality rate (3.25) than in the rest of the Mediterranean [22]. David et al. [207] detected 2775 fin whales and 43 “near-collision events” between 2008 and 2009 in the Liguro–Corsico–Provençal area in the summer, coinciding with the highest abundance of fin whales. During this study, 63.4% of the whales appeared in front of the boats at less than 50 metres, and the avoidance behaviour developed by the animals was direct diving, non-reaction or change of direction. Tort et al. [11] photographed seven live animals on the Garraf coast (where fin whales feed) with collision-related injuries, indicating that the risks of this threat are increasing. They highlighted that between March and May (with the highest number of sightings in April), cargo ships are the type of vessel presenting the highest risk of collision for fin whales. These vessels travel at speeds between 10 and 20 knots, with a collision probability of 79% [431,432]. Although passenger vessels did not show a high risk of collision in the study area, they do constitute a high risk along the Catalan coast and the Balearic Sea, as these vessels travel at speeds of 20 knots during the summer (high tourist season) [11].

An average of 116,128 vessels transits the Strait of Gibraltar per year [433] according to data from Salvamento Marítimo’s VTS Traffic Tarifa, http://www.salvamentomaritimo.es (accessed on 23 April 2022), for the years 2018 and 2019. In addition, there is evidence of a negative correlation between ferry traffic and bottlenose dolphins (Tursiops truncatus), affecting annual apparent survival [434]. There is evidence of collision events reported in [15], in addition to those reported by the authors of this manuscript (Figure 1b,c).

Two collisions with fin whales have been recorded in the Strait of Gibraltar, one in July 2012 and one in May 2014 [435], on their way out into the Atlantic. In addition [436], two other collisions were reported in 2002 and 2004, but these have not been included in this review, because they could not be linked to the published data (no day or month given). In addition, three other strandings of fin whales with injuries compatible with collisions were reported in Andalusia (Spain) [435]. Scuderi et al. [433] recorded eight sightings of fin whales from the ferries Algeciras–Ceuta and Algeciras–Tangier and observed one collision Near-Miss Event (NME).

3.6. Acoustic Locations

Although they are not sightings per se, the detection of fin whales by acoustic signals should be considered in the monitoring, protection and conservation of the species where they may go unnoticed visually, but not to their acoustic signals. Maps of areas where these signals are recurrent will help to prevent collisions with these animals. For the Strait of Gibraltar, the authors of [23] obtained 89 records (5718 h of recording) between 2006 and 2009 and 37 records (8565 h of recording) between October 2008 and January 2009. Studies of fin whale acoustic signals in areas close to the SG concluded that during winter, Atlantic fin whales were concentrated in the Alboran Sea and the Strait of Gibraltar, while the resident Mediterranean population was absent from these areas during this time of year [148,150]. In addition, fin whale acoustic signals have been detected in the western Mediterranean in Provence, Columbretes Islands, the Ibiza Channel, Alboran Sea and Strait of Gibraltar during 2006 and 2009 [24], with special sampling efforts in eastern Sicily in 2012 and 2013, with 338 records in one day (21 February 2013) during seven hours of recording [437]. This technique is very useful to study the distribution and movements of cetaceans, which shows that fin whales are present in the Ionian Sea throughout the year [437].

3.7. Breeding Season and Calves’ Location Areas

All dead fin whales measuring up to 7 metres in length were selected, resulting in 78 events. Two entries without month information were removed, and the remaining seventy-six were organised by month; the means, maxima and minima were calculated (

Table 1. Number of individuals, mean, maximum and minimum lengths of whales stranded in the Mediterranean, which were up to 7 metres in length, per month from 1624–2023.

	January	February	March	April	May	June	July	August	September	October	November	December
No.	15	7	3	1	3	2	3	1	5	8	19	11
Mean	6.034	5.88	5	2.75	5.33	4.50	5.23	7	6	5.5025	5.760	5.86
Min.	4.80	5.05	3	2.75	4	4	2.70	7	4.75	4	4	5.20
Max.	7	6.6	6	2.75	7	5	7	7	6	6.6	6.8	7

), and the results plotted (Figure 11).

Most strandings of fin whales smaller than 7 metres in length are distributed between October and February, which is consistent with what has been reported by [48] for Mediterranean fin whales, although according to [438], Mediterranean fin whales have adapted to the environmental conditions of the Mediterranean Sea by “extending and overlapping both their calving and feeding seasons”, as already noted Notarbartolo di Sicara et al. [19]. According to the data collected in this work for the entire Mediterranean Sea, a peak occurs in November, with up to seventeen immature stranded individuals ranging in length, from a minimum of 4 metres to a maximum of 6.8 metres. In order to visualise possible breeding areas, only animals that stranded alive, fresh or slightly decomposed during the months of maximum immature stranding (October–February) were selected, and a density map was drawn, also representing decomposed animals as pink dots (Figure 12).

The density map (Figure 12) represents the animals stranded below 7 m in length between October and February, which are classified as live, fresh or slightly decomposed, occurring in two specific areas of the Mediterranean Sea: the Ligurian Sea, such as the Gulf of Lions, part of the northern Tyrrhenian Sea and the Strait of Gibraltar. The length previously proposed by Notarbartolo di Sciara et al. [19] for a newborn in the Mediterranean Sea was 5.2 m, suggesting that they are smaller in Pacific congeners of 6.4 m [143]. In the present work, the mean length of the fin whales considered from October to February was 5.68 cm. This difference may be due to the inclusion of stranded fin whales in the Strait of Gibraltar from the northeast Atlantic population in the analysis.

3.8. Chemical Contamination

A small review of the concentrations of different pollutants found in the tissues of Mediterranean fin whales in recent decades has been carried out. These are polychlorinated biphenyls (PCBs), dichloro diphenyl trichloroethane (DDT), flame retardants such as polybrominated diphenyl ethers (PBDEs) (ng g⁻¹), microplastic elements (m⁻³), MEHP (mono-2-ethylhexyl phthalate) (ng/g) and some heavy metals such as cadmium, mercury, copper, lead, selenium and zinc (

Table 2. Mini review of the concentrations of ∑PCB and ∑DDT (mg/kg b.w.), microplastic elements, MEHP and PBDE found in tissues of fin whales in the Mediterranean Sea.

Year	No.	Location	Sex	∑PCB			∑DDT			Microplastic		PBDE (ng g−1)	Ref.
				Min	Mean (±SD)	Max	Min	Mean (±SD)	Max	Items /m3	MEHP
1976 1973	2	French coast	1 F 1 M		4.96			14.8					[439]
	1				0.3			0.8					[440]
1990–1993	68	Ligurian Sea		5.5		7.1	4.2		9.5				[92]
2008	12	Pelagos Sanctuary	6 F 6 M									210 15	[441]
2011–2013	30	Ligurian and Sardinian Seas			13.327 ± 8.548			10.477 ± 7.477		0.16 ± 0.31	55.14 ± 27.7		[29]
2006–2009	70	North-western Mediterranean Basin	35 F 35 M		1 3.776 ± 5.024 7.957 ± 4.613			3.239 ± 2.896 10.370 ± 6.246				119 ± 223 245 ± 179	[442]
1992–1999	63	Between Corsica and the French–Italian coast and in the Ionian Sea			6.597 ± 3.270			5.168 ± 3.844					[443]
1990–1992	1	Tyrrhenian Sea	1 F									3 3625	[444]
1998	1	Valencia (Spain)	1 F		2 20.8			2 9.8					[116]

¹ Sum of NDL congeners no. 8, 18, 28, 52, 44, 66, 101, 87, 153, 138, 187, 128, 180, 170, 195, 206, 209 and DL congeners no. 118, 105, 77, 81, 126, 169, 144, 123, 156, 157, 167,189. ² μg/g/g⁻¹ wet weight. ³ μg/kg lw.

and

Table 3. Mini review of the concentrations of heavy metals found in fin whales in the Mediterranean.

Date	Location	No.	Source	Cd	Hg	Cu	Pb	See	Zn	Ref.
1991–2001	Gulf of Genoa (Italy)	2	Muscle	0.04 μg g−1 d.w.	1.645 μg g−1 d.w.	2.4 μg g−1 d.w.	0.094 μg g−1 d.w.	1.03 μg g−1 d.w.	106.5 μg g−1 d.w.	[445]
1991–2001	Gulf of Genoa (Italy)	1	Kidney	1.56 μg g−1 d.w.	0.87 μg g−1 d.w.	11.6 μg g−1 d.w.	0.172 μg g−1 d.w.	8.68 μg g−1 d.w.	122 μg g−1 d.w.	[445]
1991–2001	Gulf of Genoa (Italy)	1	Liver	0.04 μg g−1 d.w.	0.11 μg g−1 d.w.	4.7 μg g−1 d.w.	0.041 μg g−1 d.w.	3.20 μg g−1 d.w.	29 μg g−1 d.w.	[445]
1998	Valencia (Spain)	1 F	Liver	0.31 mg kg−1 w.w	4.29 mg kg−1 w.w					[116]
1998	Valencia (Spain)	1 F	Kidney	0.97 mg kg−1 w.w	3.16 mg kg−1 w.w					[116]
2000–2002	Croatia	2 M	Muscle		0.079 ± 0.111 mg kg−1 w.w			0.046 ± 0.063 mg kg−1 w.w		[446]
2007–2008	Haifa Port (Israel)	2 M	Muscle	0.002 mg kg−1 w.w.	0.09 mg kg−1 w.w	0.77 mg kg−1 w.w	0.05 mg kg−1 w.w	0.315 mg kg−1 w.w	18 mg kg−1 w.w	[447]
2007–2008	Haifa Port (Israel)	2 M	Blubber	0.001 mg kg−1 w.w	0.06 mg kg−1 w.w	0.425 mg kg−1 w.w	0.06 mg kg−1 w.w	0.515 mg kg−1 w.w	5.545 mg kg−1 w.w	[447]
2007–2008	Haifa Port (Israel)	1 M	Kidney	0.08 mg kg−1 w.w	0.13 mg kg−1 w.w	2.36 mg kg−1 w.w	0.01 mg kg−1 w.w	0.99 mg kg−1 w.w	33.1 mg kg−1 w.w	[447]
2018	15 miles south of San Remo (Italy)	2	Faecal	0.037 mg kg−1		61.32 mg kg−1	0.0655 mg kg−1	1.06 mg kg−1	52.175 mg kg−1	[257]

).

3.9. Diseases

Dolphin morbillivirus (DMV) is the culprit of epizootics in odontocetes and mysticetes [49], including fin whales (Balaenoptera physalus) in the Mediterranean [50,51,52]. In the last 25 years, different strains of DMV have caused neurological and pulmonary diseases in cetaceans, triggering single- or mass-stranding events [53]. Sequences of herpesviruses of the Alphaherpesvirinae family have also been detected using phylogenetic analysis in Mediterranean whales [335], which also have immunosuppressive effects in cetaceans [448]. On the other hand, the ectoparasitic copepod Penella balaenopterae (Koren and Danielssen, 1877) [325] is also described, which is considered to be common on the skin of fin whales in the Pelagos Sanctuary [19] along the Italian, Tyrrhenian and Turkish coasts [325,338,449]. In addition, other metazoan parasites, such as Ogmogaster antarcticus Johnston, 1931, Bolbosoma sp. and Tetrabothius ruudi, were also found in seven fin whales analysed in [338]. This author also detected Crassicauda boopis infections of previously reported nematodes in an animal that also presented nephropathologies and severe lesions in the mesentery [450]. In the Strait of Gibraltar, specifically in 2014, a specimen stranded in Huelva (Andalusia, Spain) presented severe parasitic vascular obstruction compatible with C. boopis [435].

4. Discussion

Conservation Strategies

Collisions are the main threat and must be addressed in conservation management. Unknown causes of strandings are the most common mortality event in the region, and although they may include natural or human-related causes, it is noted that in many cases, necropsies are not carried out, the results of necropsies are not published or are inconclusive. They also highlight the problem of noise pollution and the disorientation of whales, as well as other possible causes of various origins (including collisions). Although Sciacca et al. [437] have confirmed that noise levels from human activities are high in the Mediterranean, Castellote et al. [23] argue that noise levels in the Mediterranean are higher than in any other ocean basin and are mainly caused by ships and reflection seismology.

It is important to note the relationship between the threats to fin whales and the areas of greatest importance for the species in the Mediterranean Sea, as this is where conservation efforts should be focused. For example, given that collisions account for a significant percentage of mortality, the control of maritime traffic in areas such as the Ligurian Sea, the Western Basin and the Strait of Gibraltar are fundamental. From the information obtained here, it is possible to identify the western Mediterranean area that is shown on the map in Figure 13 as the most vulnerable (including the Pelagos Sanctuary, where the highest number of collisions has been recorded), as it integrates a high concentration of sightings and mortalities (with a record number of collisions in the Mediterranean), as well as maritime traffic. For this reason, the competent authorities should carry out strict environmental monitoring, as well as impose legal requirements for speed reduction to avoid collisions and the introduction of early warning systems for the ability to detect animals on likely collision routes early.

To this end, it is essential to have good knowledge of the migratory routes of fin whales to be able to predict the periods and specific areas where they will be detected. This would imply, as indicated by Panigada et al. [22], the implementation of proposals to modify and regulate maritime routes according to the presence or absence of the species. In addition to the measures described above, in order to protect these areas, other measures to reduce the level of noise pollution should be implemented, regulating or limiting the activities that generate underwater noise [42].

However, global solutions are not effective. Detailed knowledge of the local areas and time periods when fin whales are feeding or migrating is essential to manage the collision risk in the Mediterranean. The most effective measures to date have been “re-routing”, which is defined as keeping whales away from major shipping lanes through traffic separation schemes, and reducing vessel speeds, although these measures are not feasible in the north-west Mediterranean due to the wide distribution of fin whales. Instead, permanent or seasonal speed restriction zones throughout the risk area, based on the northwestern IMMA, the new Critical Cetacean Habitat and the high-risk exposure maps from this and other studies may be the best option [451]. It has been shown that a speed of 8.8 knots would reduce collisions with vessels by 50% [431,432] and that even below 13 knots, a reduction in “near-collision” events would be feasible, improving sighting distances and minimising mortalities [452,453,454,455].

The Spanish government (Royal Decree 699/2018) declared a new MPA, the CMC between the Catalan–Valencian coast and the Balearic archipelago, as these waters constitute a Cetacean Migration Corridor for cetaceans. In 2019, and within the framework of the Barcelona Convention (BOE no. 158, 30 June 2018), this area has been included in the list of Specially Protected Areas of Mediterranean Importance (SPAMI list) [456]. Recently, it has been published that the fin whales use this area as a seasonal feeding ground, rather than as a transit area, resulting in poor and ill-adapted protection measures, creating the need to adapt management and protection measures for fin whales in this area during the spring and summer months [38]. In the same work, it was found that the recent NW Mediterranean PSSA covers most of the feeding grounds of fin whales, but management measures are voluntary, so it is important that more restrictive protection measures are implemented [38].

To reduce the risk of ship collisions with marine mammals and unidentified floating objects (UFOs), automated detection systems (e.g., SEADETECT) are being developed to detect and identify cetaceans up to 1 km from a ship and alert the crew in time to take appropriate action [457].

Awareness programmes or crew training as part of an overall proactive avoidance system [458] could improve the effectiveness of such mitigation measures. A survey of ferry passengers crossing the Pelagos Sanctuary showed that more than 75% of passengers would prefer to choose a company that reduced the speed of the ferry if it reduced the risk of collision with vessels [459].

Activities related to tourism, fishing, shipping and marine industries pose a risk of accumulation of marine litter pollution [460]. The Mediterranean Sea is a major accumulation area for this waste, particularly in the Ligurian Sea, and regulation or elimination of sources is necessary for the effective conservation of the species concerned.

By-catch does not represent a major threat. According to [461], fin whales can sometimes escape from nets when entangled, but there are reports of deaths in pelagic driftnets [409,462], so this threat needs to be addressed. Innovative designs and projects are being developed to prevent the entanglement of cetaceans in fishing gear. A particular case in point is the North Atlantic right whale (Eubalaena glacialis), which is critically endangered by mussel traps. Fin whales become entangled in the lines connecting these traps to buoys floating on the surface. To avoid this problem, the use of GPS-locatable ropeless traps has already been proposed [463]. This raises the possibility of conducting studies on fishing gear in the Mediterranean and its impact on fin whales, so that new nets or traps can be designed to reduce their impact in an analogous way to the North Atlantic fin whale.

Since the Mediterranean is a semi-enclosed sea in which the outflow occurs at depth, combined with that of the current circulation system, ocean eddies, wind drift, high human pressure and river discharge, among others [464,465], it is considered one of the most polluted areas by marine litter, mainly small plastics [466,467,468], with a total surface load of floating marine litter that is estimated to be between a few thousand tons to 30,000 tons [466,469]. In places such as in the Sardinian–Balearic sub-basin, the Catalan coast, the Gulf of Lions and the NW Mediterranean basin, marine litter accumulates mainly in the spring and summer [468,470,471,472]. These plastic convergence zones overlap with important feeding areas for fin whales, which, at least in the NW Mediterranean Sea, present fidelity to these feeding grounds [41], making plastic a great threat to these animals due to the potential ingestion of these particles during feeding activity [468]. Knowing this, solutions are proposed, such as locating and categorizing potential sources of marine litter through model simulations, in addition to reducing single-use plastics, among other measures [468].

5. Conclusions

The conclusions reached on the current state of knowledge of the fin whale (B. physalus) in the Mediterranean Sea based on an extensive review of public information over the last four centuries are as follows:

(1) There are two distinct populations (resident and migratory). The migratory population enters the Mediterranean Sea between November and April and returns to the Atlantic Ocean between May and October, often with their young. (2) Collisions with vessels are the cause of most mortalities, although the largest number of recorded mortalities are strandings without the identification of the cause of death (although it is possible that most of these are due to seismic reflection noise and noise generated by vessels). (3) The highest number of strandings were recorded between November and April and the lowest number between May and October, in contrast to the lowest number of sightings between November and April and the highest number between May and October. (4) Bycatch mortalities are residual. (5) There is a high overlap between sighting locations and feeding and primary (chlorophyll) production areas. This is particularly relevant in the winter far from the coast when fin whales are found in the Mediterranean Sea, which seems to be explained by synergistic reasons of reproduction and food resources. (6) A map of overlaps between high-density sea routes in 2019 and detected areas with different causes of B. physalus mortality is produced. (7) An integrated representation map of the results obtained was produced. It integrates sightings and mortalities (strandings, collisions, deaths of animals that appear floating for unknown reasons and accidental captures). (8) A large area with a maximum risk of collisions has been identified, which can assist the authorities in the management and implementation of an environmental monitoring plan to help prevent and mitigate this threat (Figure 12). (9) The review and counting of small and low-stranding decomposition events of fin whales provides new information for the detection of breeding areas of this species.