Jan Breine

A crucial step in the development of a fish-based index for the ecological assessment of water bodies as provided by the European Water Framework Directive is to define a reference list of fish species occurring in pristine rivers. The aim of this study was to elaborate such a list. The reference corresponds to an ecological status that is referred to as Good or Maximal Ecological Potential (GEP/MEP). Based on historically-reported fish survey data of the Zeeschelde estuary (Belgian part of the Schelde estuary) and its tributaries, i.e. an affluent system, under tidal influence, presence/absence reference lists were compiled for different salinity zones and adjusted using information from recent catches. In addition, an MEP list of fishes occurring in the Westerschelde (Dutch part of the Schelde estuary), developed by JAGER & KRANENBARG (2004), is provided to present a complete overview of the Schelde estuary. Inclusion of fish species in the reference lists depended on their natural geographical distribution and ecological demands. These reference lists contain guild-specific information for the different zones within the estuary and its tidal tributaries.

Between 1991 and 2008 a total of 71 fish species was recorded in the brackish and fresh water zone of the Schelde estuary (Zeeschelde). The results were obtained from fish surveys from the cooling water filter screens of the power plant at Doel (between 1991 and 2008) and fyke net surveys along the length of the estuary between 1995 and 2008. Species abundance in the different 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). With fyke nets 33 species were caught in the tidal freshwater zone, 43 species in the oligohaline zone and 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). Diadromous species occur in all zones and make up, on average up 22% of the species richness. Freshwater species comprise about 70% of the species in the tidal 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 recent increase in species richness in the freshwater and oligohaline zone coincides with a remarkable increase in dissolved oxygen since 2007.