The Adriatic Sea and the Gulf of Antalya are the poorest areas in Elasmobranches’ species diversity which decreased from the westernmost to easternmost of the Mediterranean Sea (Coll et al., 2010). The FAO's geographical classification divides the Mediterranean Sea into three sub-areas (western, central and eastern), while the GFCM divides into four sub-regions (plus the Adriatic Sea) and englobes 27 Geographical Sub-Areas (GSAs). Together with these, the Mediterranean Sea can be roughly divided into two main sub-basins, the western and the eastern Mediterranean, separated by the Sicily-Tunisia Ridge, also according to oceanographic variability. The eastern basin is characterised by a higher variability of SSTs (sea surface temperatures), ranging from 17.8°C in winter to 29.7°C in the warmest months (Poulos et al. 1997; Abboud-Abi Saab et al. 2013), and also by a higher average salinity (39‰). In contrast, the western basin is generally cooler and relatively less saline, with an average SST of 12°C in winter and 23°C in summer, and an average salinity of about 36‰ (Mansuci et al., 2020).
Presented study provides information on the composition of the chondrichthyan assemblage on the slope of Antalya Bay (Northern Levant Sea, GSA24), and contributes to a deeper understanding of its structure and spatial distribution. A total of 17 species were recorded on the slope, including 10 sharks, 6 batoids and 1 chimaera. Together with 7 batoid (Aetomylaeus bovinus, D. centroura, Gymnura altavela, R. asterias, R. radula, Rhinobatos rhinobatos and T. marmorata and 2 shark species (Squatina squatina and Carcharhinus plumbeus found on the continental shelf (Mutlu et al. 2022a, b), the total number of demersal cartilaginous species distributed in the Antalya Bay reaches 26 which was more than threshold of < 20 cartilaginous species previously recognized for the Antalya Bay by Coll et al. (2010). In the Antalya Bay, most of the batoids were found on the continental shelf (Mutlu et al. 2022a, b). In contrast, most sharks preferred the slope. These results are consistent with the results of GSAs in the central and western Mediterranean (Table 7).
Demersal chondrichthyan species show distinct patterns of distribution and abundance patterns in different GSAs of the Mediterranean Sea. These patterns may be influenced by several factors, including habitat preferences, environmental conditions, fishing pressure and ecological interactions. Table 7 provides an overview of the distribution and density of demersal chondrichthyan assemblages on the slope with 24 sites relating for 17 different GSAs. While 14 of these results belong to the western, 7 to the central, there are only 3 results for the eastern Mediterranean (including our study). Our study provides standardised information which is lacking for the easternmost sub-area (GSA24), and contributes to a better assessment of the demersal chondrichthyan assemblages on the slopes of the entire northern Mediterranean basin. 31 chondrichthyans were recorded from the slopes in 17 different GSAs, including 15 sharks, 15 batoids and 1 chimaera. The number of species reported in the GSAs; (i) between 6 in GSA6 and 19 in GSA11 with a total of 27 in the western Mediterranean, (ii), between 7 in GSA17 and 20 in GSAs 16 and 20 with a total of 23 in the central Mediterranean, and (iii) between 7 in GSA23 and 17 in GSA24 (this study) with a total of 22 in the eastern Mediterranean. The correlation between GSAs and species numbers from west to east was not significant (r = 0.105, p = 0.624) and sub-areas had no statistically effect on species numbers (F = 0.346, p = 0.712). Rare species with mean densities ≤ 1 n/km2 accounted for about 70% of the chondrichthyan species found on the slopes of these three sub-areas. In our study, the number of species with mean densities above 1 n/km2 and 5 n/km2 was 8 and 7, respectively.
Few species showed a wide distribution on the slopes in all GSAs: the sharks S. canicula and G. melastomus occurred in all studies. Another shark, E. spinax was found in all GSAs excepting GSA 23, and two important batoids, R. clavata and R. oxyrinchus, were found in all GSAs excepting GSA19 and GSA17, respectively. Although the densities of G. melastumus (r = -0.346, p = 0.098) and S. canicula (r = -0.393, p = 0.057) decreased from the western to the eastern basin, these decreases did not show a statistically significant trend. In contrast to these shark species, there was a positive increase for batoids of R. clavata (r = 0.219, p = 0.304) and R. oxyrinchus (r = 0.285, p = 0.178), but also these increases were not statistically significant. However, there was significant trend increasing from west to east basin for both E. spinax (r = 0.410, p = 0.047) and S. blainvillei (r = 0.599, p = 0.002).
The top 3 GSAs with the highest density of cartilaginous fish are GSA19 (Western Ionian Sea) with 3739 n/km2, GSA1 (Northern Alboran Sea) with 3114 n/km2 and GSA8 (Corsica) with 3025 n/km2. The Antalya Bay in GSA24 is the 5th highest sub-area with a total value of 1532 n/km2. A negative insignificant correlation (r=-0.247, p = 0.245) was found between GSAs and D values from east to west and the sub-areas also had no significant effect on total density (F = 1.032, p = 0.377).
One of the most important factors stressing the Chondrichthyes assemblage is the bottom trawl fishery (Tiralongo et al. 2018; Mutlu et al. 2022b). The bottom trawl fishery in Antalya Bay was concentrated in two different groups of fishing operations targeting a specific assemblage of species and these two métiers do not overlap bathymetrically: i) The "first métier" targets Mullus spp, porgies, Farfantepnaus aztecus and Penaus japonicus and occurred mainly on the continental shelf (approximately 50–150 m depth) (Deval 2020b; Deval and Koçancı 2021), ii) the "second métier" targets two red shrimp species, with the main by-catch species Merluccius merluccius, Helicolenus dactylopterus and Parapenaus longirostris occurring in the slope (approximately 350–650 m deep) (Deval et al. 2016; Deval 2020a) (Supplementary F1). Although there is only one trawler registered in Antalya port, the number of bottom trawl vessels signalled in the Gulf of Antalya between 2018 and 2022 was 55. Strength of the operations varied significantly with years, months and days (unpublished data). Considering that only one vessel was actively trawling for 460 hours in 2019 (in February, March, October and December) (unpublished data), and an average CPEU value of 120 n/h was estimated during our study, it can be concluded that the discarded cartilaginous species in the Antalya Bay fishery is very high. Areas of high fishing pressure, such as the Adriatic Sea and the Spanish coast (excluding the Balearic Islands), show a low abundance of chondrichthyans, but other areas with a high level of fishing pressure, such as southwestern Sicily, show a high abundance, suggesting that other environmental drivers work together with fishing pressure to shape their distribution (Follesa et al. 2019).
Depthwise distribution of Chondrichthyes assemblage in the present study area suggested that a bottom depth of around 500 m which was highly variable in terms of Chondrichthyes diversity and abundance in year was considered a critical depth which discriminated the study area from the upper slope to lower slope of the bathyal. The depth, seafloor morphology, and water temperature were the main factors for elasmobranchs choosing suitable habitats, which occurred in late spring–early summer in the central Mediterranean (Lauria et al. 2015). Besides seasonal and ontogenic migration of the species along the bottom depth gradient as abruptly observed for E. spinax (Fig. 3), deep-water species were impacted by the North Atlantic Oscillation index (Ligas et al. 2010). E. spinax was responded unimodally to the bottom depth but the modal depth was shallower in Algerian water than the Balearic waters (Ordines et al., 2011). Therefore, the species had unimodal distribution (deep water) in the eastern Mediterraenan Sea as occurred along the depth gradient in Antalya Gulf during the present study.
In conclusion, the slope in Antalya Bay serves as an important feeding and reproductive grounds for chondrichthyan species. It's important to note that the demersal chondrichthyan assemblages on the slope are influenced by a combination of factors, including depth, substrate type, prey availability, and anthropogenic pressures. However, as in other areas of the Mediterranean, overfishing remains a concern for the conservation of chondrichthyan populations. Conservation measures, such as the establishment of marine protected areas and sustainable fishing practices, are crucial for the preservation of these important habitats and the long-term sustainability of demersal chondrichthyan populations in the Mediterranean. Totally 22 chondrichthyan species with 4 (Oxynotus centrina, Squalus blainville, Squatina aculeate and Raja clavata) obtained from our study have been included to the list of prohibited species since 2018. Although fishing for these species has been banned, it is not possible to prevent them from entering in bottom trawl nets and dying during the hauls. Therefore, in order to reduce incidental catching of batoid species, and increase survival as a result, trials of a modified turtle excluder device (TED) net should be conducted in bottom trawl fisheries.