Patterns of change in zooplankton community structures and the selective feeding of bleak, Alburnus alburnus (L.) in the Serre Poncon dam between 1980 and 1996

The Serre Poncon reservoir, in the southern French alps, filled when the da m was completed in 1960. It has meso-eutrophic characteristics. Two hydrobi ological studies carried out in 1980 and in 1996 showed interspecific varia tion in zooplankton and fish communities. A strong development of B. longir ostris, D. brachyurum and C. pulchella, characteristic of eutrophic environ ments, was apparent in 1996, while the population of Daphnia longispina, wh ich was dominant in 1980, had declined. At the same time, strong growth of the bleak Alburnus alburnus, was observed. Research into prey selection ove r several seasons showed that prey size and density were often a determinin g factor for capture, e.g., in C. pulchella, D. longispina and C.v. vicinus . However, some small sized rare species such as B. longirostris were also much sought after.