Spatial gradients of food consumption and production of juvenile fish in the lower River Elbe
Food consumption and net production of juveniles of the dominant fish species of the lower River Elbe were estimated based on quantitative values of abundance and biomass of fish. Juvenile fishes were caught with a framed gape stow net in the main channel and with a demersal otter trawl in the marginal regions. Fish larvae were sampled at each station using ring-nets. Diet analysis was carried out on 6 dominant fish species which were smelt Osmerus eperlanus, twaite shad Alosa fallax, herring Clupea harengus, flounder Pleuronectes flesus, ruffe Gymnocephalus cemuus and three-spined stickleback Gasterosteus aculeatus. The Winberg (1956) bioenergetic model was used and verified through the models of Elliott & Persson (1978) and Eggers (1979) to calculate daily rations of juvenile fish. Net production of fish was calculated according to the equation of Ricker (1975). A total of 56 different food components were identified in the diet of the dominant fish species. Their main food items were copepods and mysids and to a lesser extent cladocerans, amphipods, shrimps and insects. The fish species were classified into two feeding guilds based on their food utilization. The juveniles of herring, twaite shad and smelt consumed higher amounts of fishes, whereas detritus was an important prey item of three-spined stickleback, ruffe and flounder. Significant spatial differences in food composition of juvenile fish occurred along the lower River Elbe. The importance of the main prey organism, the calanoid copepod Eurytemora affinis, increased upstream. Total annual consumption and production of fish increased upstream. Highest annual production values of more than 200 kg ha -1 were estimated for age group 0 fish in the sub- and intertidal freshwater habitats of the southern marginal areas. In the main channel, consumption and production values of age group 1 were higher compared to the marginal areas.