## 4 Discussion
Black Sea Water influx in the northern Aegean Sea: [66]). These conditions are typical for the northern Mediterranean coastline and create a northwestern-to-southeastern gradient in productivity. Indeed, biological productivity in the Mediterranean basin has been reported to decrease from north to south and from west to east and it is inversely related to the increase in temperature and salinity [67] indicating that the Mediterranean Sea is highly heterogeneous between its basins [68]. This gradient makes longitude appear vital in the distribution of pelagic species and even latitude seems significant, despite the narrow latitudinal axis of the Mediterranean due to riverine input that mostly affects the northern coastline. Changes in primary productivity, the composition of plankton community and the abundance of key plankton species, that may be climate-driven [69], directly affect the distribution of small pelagic fishes, which preferentially feed on zooplankton, such us anchovy and sardine [70], but they could also indirectly affect their somatic condition [10]. Enhanced primary and secondary productivity benefits these planktivorous species by increasing the availability of their prey (bottom-up control), but at the same time improves their somatic condition [71]. Although NW Mediterranean is richer in pelagic fishes, sharp regional gradients and gaps in the probability of occurrence exist, mostly in anchovy and round sardinella distribution (Figure 6a,c). Strong currents and meso-scale eddies in Alboran Sea, the Tyrrhenian Current and the Liguro-Provencal Current favor the presence of these species. In the Gulf of Lions the freshwater impact of Rhone River and the up- and downwelling events explain the sharp differences in fish species occurrence. According to the findings of the present work (Figure 5, Table 7), temperature and salinity are the main drivers of the distribution of most small and medium pelagic fishes in the Mediterranean. The spatial distribution and the abundance of small and medium pelagic species may be directly affected by sea surface temperature (SST) [72], but the effect of SST can be also indirect through changes in the planktonic components of the food webs [13] that constitute the main prey for small pelagic fishes [64]. The effect of SST, however, is not uniform across species and it depends on their thermal preferences [7], which may vary among the pelagic fishes [9]. Sardine, for example shows preference for colder waters compared to round sardinella and anchovy, and appears to confine its distribution and shrink its spawning grounds to colder waters when SST increases [46], a condition that affects its fisheries. This negative relationship between sardine landings and SST, indicates that the long-termtemperature changes in the Mediterranean could have a negative impact on sardine abundance [72]. In contrast, there is a positive relationship between sardine landings and chlorophyll concentration in the Alboran Sea [72] which has also been related to other areas of the Mediterranean Sea [73]. The distribution of large pelagic fishes may also be associated with various environmental conditions, despite their highly migratory activity. Several populations of swordfish have shifted latitudinally, whereas the Mediterranean population has shifted longitudinally towards the west, as a result of climate change [74]. Local conditions, such as clusters of higher density occurring near converging fronts and strong thermoclines may also affect swordfish distribution at a more local scale [75]. The regional distribution and abundance of Atlantic bluefin tuna has also been recently reported to be affected by climatic oscillation and water temperature [76]. Again, overfishing plays a crucial role in distribution and abundance patterns of large pelagic species, since the stocks of these two species are among the most valued and commercially exploited globally [76].