Diel changes in the fish assemblage in a coastal surf-zone area in the eastern Baltic Sea

The composition of the fish assemblage in the surf zone in Eru Bay, Gulf of Finland, was highly variable in relation to the time of day (dawn, noon, dusk and midnight) during the ice-free season in 2008. The diel variation in the surf-zone fish assemblage composition was also associated with seasonal changes. Species that exhibited the most variation during the diel cycle were the European smelt (Osmerus eperlanus), gudgeon (Gobio gobio), bleak (Alburnus alburnus), three-spined stickleback (Gasterosteus aculeatus), ninespine stickleback (Pungitius pungitius), small sandeel (Ammodytes tobianus), and gobies (Pomatoschistus spp.). To our knowledge, this is the first study that describes diel variations in the fish assemblage composition of a non-tidal brackish surf-zone environment using an annual data set. The results imply that time of day effects species abundances, and this should be taken into account in future studies where the aim is to evaluate littoral fish assemblages of the Baltic Sea.