Filling the Gap of DataLimited Fish Species in the Eastern Mediterranean Sea: A Contribution by Citizen Science

: The biodiversity of the Mediterranean Sea is rapidly changing due to anthropogenic activity and the recent increase of seawater temperature. Citizen science is escalating as an important contributor in the inventory of rare and data-limited species. In this study, we present several records of five data-limited native fish species from the eastern Mediterranean Sea: Alectis alexandrina (Geoffroy Saint-Hilaire, 1817) , Ranzania laevis (Pennant, 1776) , Dalatias licha (Bonnaterre, 1788) , Lophotus lacepede (Giorna, 1809), and Sudis hyalina (Rafinesque, 1810). All of the records were collected by a participatory process involving fishers and validated by associated taxonomic experts of the citizen science programme “Is it Alien to you? Share it!!!” . This study fills an important gap for the distribution of the reported species and signifies the important role of citizen participation as a tool for extending marine biodiversity knowledge and fisheries management in an area with several gaps of knowledge on targeted and non-targeted species.


Introduction
The Mediterranean Sea is facing several unprecedented anthropic pressures (e.g. pollution, habitat destruction, and geographical reshuffling of species) [1,2]. Along with climate change, species community shifts are regularly observed, leading to the tropicalization of the Mediterranean Sea [3,4]. Projections have indicated that at least 25% of the Mediterranean continental shelf might experience a total modification of species assemblages by the end of the 21 st century [5]. Major gaps exist regarding deep species assemblages [6], particularly rare and data-limited species. This is mainly attributed to fragmented research, scarcity of observations, less fishing pressure, and practical difficulties in monitoring deeper waters.
Historically, the low economic value of non-targeted species has led to less data and a lack of vulnerability assessments. Some progress has been made in recent years to develop methods on status and risk assessments [7], such as mixed fisheries [8] and policy requirements [9,10]. Thus, the methodology has been developed, including time-series catch data [11], life history aspects [12], and size structure [13]. Another aspect is the willingness by citizens to pay for climate adaptation and fisheries resources, as shown by Tulone et al. [14].
Citizen science is emerging as a key component for the exploration of marine biodiversity, being widely acknowledged by scientists, policy-makers, and conservationists, due to its capacity to address conservation issues that are related to rare species, climate change, and coastal systems [15][16][17]. Currently, there is an increasing number of citizen science efforts in the Mediterranean Sea t focused on various topics that are related to the marine environment [18][19][20]. This provides high potential for addressing data gaps related to least studied species. However, citizen science data should be carefully treated, since they often incorporate taxonomic uncertainties and misidentification due to photographic identification [21,22].
Here, we report several records of five data-limited fish species in the eastern Mediterranean Sea and highlight the use of citizen science as an emerging scientific tool for increasing our understanding on species distributions and contributing to fisheries management in the Mediterranean Sea. In addition, we used current published literature to highlight and give a quantitative aspect of current and historical knowledge of the studied species distribution in the eastern Mediterranean Sea (Supplementary Table S1).

Materials and Methods
The citizen-science programme "Is it Alien to you? Share it!!!" was initially launched in May 2016 by the Environmental Organisation iSea, with the aim of recording information on the occurrence, distribution, and expansion of marine non-native and rare species in Greece. An online easy-to-upload data repository was generated for citizens that allowed for records with pictures to be tracked. In addition, a Facebook group was created and all of the uploaded pictures to the group were automatically uploaded to the same data repository. At the time, this manuscript was written, this project's Facebook group numbers > 10,000 members, among which approximately 5000 are actively engaged. Each observer is requested to provide for each photo information on species size in total length (TL in cm.) and/or wet weight (WW; in g.). It is also requested to provide more information regarding the depth (D; in meters), the number of individuals (N), the location in latitude and longitude (LAT: LON), the date (DD: MM: YYYY), and the type of observation, as: underwater observation (UW), stranded (S) (i.e. found deceased on shore). Trained observers with silhouettes measured all of the individuals that were categorized as UW. If fishing gear/practice was provided, each observation was categorized following the FAO classification: longline (LL), bottom longline (bLL), surface longline (sLL), harpoons (HAR), and handlines and pole-lines that are hand-operated (LHP) ( Figure 1, Table 1). All of the records are pre-scanned for quality and sent out for external evaluation to taxonomic experts prior to the inclusion in the final dataset of iSea. Only confirmed observations are recorded in the dataset, either through photo-identification or in-situ validation.  All of the data used in this study has been uploaded to an electronic repository (https://wp.me/P94Vaj-1Mm).

Ranzania Laevis
On 21 st December 2014, an individual of R. laevis was captured with a surface longline near Psathoura Island in North Aegean Sea (39°26′46.0″ N, 24°04′46.1″ E) at a depth of approximately 200 m and landed at Skopelos Island ( Figure 1G, H). The specimen was 53 cm in TL. The fisherman and a local journalist contacted the local authorities, which identified the species as "Propela", the common Greek name for Mola mola. On 29 th of July 2017 the journalist contacted iSea, in the context of the citizen science project "Is it Alien to you? Share it!!!" for sharing a log of rare species observations from Skopelos Island. Among the observations, the record of R. laevis was found and all of the available information was collected.    (Fig. 4).

Discussion
The present work provides additional information on five uncommon fish species for the Aegean and Levantine Sea. Rare species are often considered to be data-limited. However, spatial and temporal variations in the distribution of rare species might provide relevant signs of climate and environmental change [23,24]. Until recently, the scarce communication between researchers and citizens entailed the loss of an important part of available information on these species, while the recent increase in the use of social networks allowed for a closer communication that can result in an increase of records of rare and non-native species [20,[25][26][27][28]. To date, the project "Is it Alien to you? Share it!!!" has gathered a vast amount of information regarding the distribution and establishment of species, including several first records, as well as expansion evidences [20].
Alectis alexandrina is a thermophilic species with a rather sporadic occurrence in the Mediterranean Sea. Fishers state that the species is increasingly caught the last years, being very common in Cyprus and rather sporadic in southern Aegean Sea, despite the few observations from the northeast Mediterranean Sea.
Ranzania laevis commonly known as slender sunfish (Fam. Molidae) is a pelagic-oceanic cosmopolitan species found in tropical and temperate seas and feeding on small fish, planktonic crustaceans, and jellyfish. It is one of the two Molidae species occurring in the Mediterranean Sea; however, reports of R. laevis from the eastern basin are scarce. Occasional records from Greece [29], Cyprus and Israel [30], Turkey [31], and Lybia [32] have been reported. Current distribution of R. laevis covers the whole Mediterranean Sea while quantitative information has revealed a rather sporadic occurrence and local peaks of abundance with increased zooplankton biomass [23]. The reported species is the second record from the north Aegean Sea [29].
The remaining three species reported are typical rare deep-water species in the eastern Mediterranean Sea [6]. Recent studies demonstrated the occurrence of these deep-water species, as previously overlooked in the eastern Mediterranean Sea [33,34].
Dalatias licha, commonly known as the kitefin shark, is one of the largest deep-sea sharks occurring in the Mediterranean Sea, being distributed at depths between 200 and 900 m, where it can be considered a top predator. It is although an uncommon species in the western and central part of the basin [35,36], while its presence in the eastern Mediterranean Sea is considered as very scarce [37,38], signifying any reports as highly valuable, also given the conservation status of the Mediterranean population of the species as vulnerable [39].
Lophotus lacepede, commonly known as the crested oarfish, is a large bathypelagic species that is remarkably sporadic in the whole Mediterranean Sea. Similarly, to the majority of deep-sea fish species it is more regularly observed in the western Mediterranean Sea, with only a few records in the eastern part of the basin. Currently, only four published records are known, all being from the Aegean Sea [40]. The observations that are reported in the current work highlight the deep waters between Chalkidiki Peninsula and Evoia Islands as an important area for the species.
Sudis hyalina, which is a rare bathypelagic cosmopolitan fish species, is considered to be regular in the western part of the Mediterranean basin, while only five documented records of the species exist from the eastern Mediterranean basin [41]. In the Aegean Sea only two records of the species have been published [29]. Consequently, the record presented in this study is the third record of the species from the Aegean Sea. Interestingly, the report from Cyprus is the first record of the species from the Cypriot territorial waters, most possibly an overlooked species.

Conclusions
This study reveals that the contribution of citizen-science accounts for 45.4% of the total number of the studied species published records and 58.5% of the total number of published records during the last decade in the eastern Mediterranean Sea. This signifies the important role of citizen-science efforts, both for scientific and public awareness. Nine more records of the reported species have been published since "Is it Alien to you? Share it!!!" was launched [38,[42][43][44][45][46], whereas, in this article, we report the occurrence of 26 individuals contributing with 74.2% during the same period. Of course, it can be hard to determine whether the species density in the region has increased or if the reports were overlooked before pictures of the species were circulated on social media as something rare. Thus, post-hoc interviews with citizens and fishermen who report catches can be used to reconstruct species distributions and densities. Funding: This research was funded by iSea, Environmental Organisation for the preservation of the aquatic ecosystems-"Is it Alien to you? Share it!!!".