ENVIRONMENTAL-FACTORS AS FORCES STRUCTURING THE FISH COMMUNITY OF THEELBE ESTUARY

Smelt, flounder, eel, three-spined stickleback, twaite shad, ruffe and herring were the major species among 62 forming the fish community of the Elbe estuary. Species richness, species diversity, evenness and t otal fish biomass decreased in the upstream direction. Total abundance was greatest during summer. Salinity was the most significant physico -chemical factor affecting species richness and total fish biomass, wh ereas water temperature was the best predictor of total abundance. Mar ine fish species decreased with decreasing salinity: while the freshwa ter species roach and ide were absent at salinities >15%. Bream, blue bream, while bream, perch and pikeperch occurred more frequently in sh allow marginal regions with lower current velocities, while herring an d other marine species occurred closer to the deep central regions wit h high current velocities. Annual and seasonal variations of the commu nity structure mainly reflected in population dynamics of smelt, floun der, twaite shad, three-spined stickleback and eel, especially those o f 0-age smelt. Populations of smelt and ruffe and their importance in the fish community increased between 1989 and 1992, while those of twa ite shad and eel decreased. At oxygen concentrations <3 mg l(-1), smel t and flounder were rare, but eels were at their maximum at 3.5 mg l(- 1). Changes of species composition and fish abundance were closely rel ated to daytime and tidal cycle.