Changes in the structure of fish assemblages in a bathyal site of the Ionian Sea (eastern Mediterranean)

Abstract

The spatial structure and temporal changes of the demersal fish assemblages on the upper slope (473–603 m) in the eastern Ionian Sea (Greece) were analysed. Fish were obtained from four 24 h day–night sampling cycles covering the four annual seasons. 8136 individuals representing 35 species were collected from 29 hauls, with dominant species being Nezumia sclerorhynchus, Hoplostethus mediterraneus, Hymenocephalus italicus, Chlorophthalmus agassizi, Helicolenus dactylopterus, Phycis blennoides and Coelorhynchus coelorhynchus (accumulated abundance=84.63%). Season was the main identified factor influencing both fish assemblage composition and biomass, while no differences were found with depth. Fish biomass was maximal in August (212 kg/h) and minimal in November (30 kg/h). Day/night effects were secondary and restricted to changes in biomass without any change affecting species composition. Fish biomass was significantly higher by day in August. The following explanatory causes of the observed temporal changes are discussed: (i) day–night upward/downward movements performed by species to feed, (ii) changes related to spatio-temporal patterns in their reproductive biology, and (iii) changes related to seasonal fluctuations in food availability. Based on simultaneous sampling of suprabenthos/zooplankton, food sources of fish, we suggest that food availability is the main factor affecting the demersal fish assemblages in the Ionian Sea. Specifically, we found maximum dominance of pelagic feeders (e.g. H. mediterraneus and C. agassizi) in August coinciding with a higher biomass of benthopelagic zooplankton taxa preyed by fish, with significant correlations between both trophic levels.