Fish assemblages across a salinity gradient in the Zeeschelde estuary (Belgium)

Between 1991 and 2008 a total of 71 fish species was recorded within the Zeeschelde estuary.The results were obtained from fish surveys from the cooling water filter screens of the power plant at Doel (between 1991 and 2008) and fish surveys along the length of the estuary (collected with ‘paired-fyke’ nets between 1995 and 2008). Species abundance in the salinity zones was analysed using the fyke net data only. The ten most abundant species represent 90.8% of the total number of individuals caught. In decreasing order of abundance: flounder (Platichthys flesus), roach (Rutilus rutilus), herring (Clupea harengus), eel (Anguilla anguilla), pike-perch (Sander lucioperca), sole (Solea solea), common goby (Pomatoschistus microps), seabass (Dicentrarchus labrax), three-spined stickleback (Gasterosteus aculeatus) and white bream (Blicca bjoerkna). Species richness ranges from 33 species in the tidal freshwater zone, 43 species in the oligohaline zone to 59 species in the mesohaline zone. Each salinity zone is characterised by a typical fish assemblage, although some species are shared between all three salinity zones: e.g. three-spined stickleback (Gasterosteus aculeatus),Prussian carp (Carrasius gibelio), roach (Rutilus rutilus) and eel (Anguilla anguilla). Freshwater species comprise about 70% of the species in the freshwater zone. In the oligohaline zone the contribution of the freshwater species to the species richness is less while marine migrants become more abundant. As expected, the contribution of marine migrants and estuarine species is higher in the mesohaline zone. The recorded differences in fish assemblages between the different zones could be attributed to habitat differences and hence the definition of different estuarine fish guilds. The recent increase in species richness in the freshwater and oligohaline zone coincides with a remarkable increase in dissolved oxygen since 2007. It is argued that the fish community in the Zeeschelde would benefit from the creation of a diversified habitat with connected floodplains, mudflats and tidal marshes