Water temperature gradient shapes the structure and composition of nearshore marine fish communities in southern Europe

The nearshore ecosystem is an important nursery ground for many fish species. Considering that these areas have a considerable economic value because they support a wide variety of highly valuable species, it is essential to study the structure of their fish communities and the factors that may inlfuence them. The nearshore fish assemblages were studied from March to November of 2015 at 15 sandy beaches, arranged in 6 seaports, along 700 Km in the coast of Portugal. This Atlantic coastline is an important geographical transition area between cold-temperate and warm-temperate regions. Sampling was performed by the beach seine fishery, an artisenal fishery that operates in these areas. The fish assemblage was studied according to its species richness, biogeographical origin, habitat use and resource exploitation, in both biomass and number of individuals. The nearshore fish assemblage was diverse with 70 species belonging to 32 families, though a few species dominated the community: Engraulis encrasicolus, Trachurus trachurus, Sardina pilchardus and Scomber colias. Results indicated that the structure and composition of the fish assemblage from northern and southern seaports are different. Overall, species richness was higher in the southern seaports. Planktivorous species dominated the community in the northernmost seaport, and piscivorous species dominated the community in the remaining seaports. As expected, cold-temperate species presented higher abundance in the northern seaports, and both warm-temperate and sub-tropical species increased in number towards the southern limits. Tropical species were only present in the southernmost area. The main driver for variations in the fish community structure and composition was the water temperature gradient imposed by latitude, highlighting the environmental control in shaping marine biological communities.