A review of predation impact by 0+fish on zooplankton in fresh and brackish waters of the temperate northern hemisphere

To assess potential differences in predation impact on zooplankton communit ies by small (larva) and larger 0+ juvenile fish, 18 studies were reviewed from fresh water and the brackish Baltic Sea of the northern hemisphere tem perate region. These case studies were performed either in the field or in mesocosm experiments. Larva stocks were found to exert only minor impact on small zooplankton species such as rotifers, copepodids and small cladocera ns. In contrast, stocks of 0+ juveniles were found to have the potential to depress populations of large cladocerans and copepods, especially during l ate summer and autumn. However, studies where both 0+ juvenile fish consump tion and zooplankton dynamics and production were exactly quantified are st ill very rare, and therefore final evaluation of this interaction cannot be made. In addition, papers were summarized that describe differences in mor phological and physiological performance between larva and 0+ juvenile fish . The greater impact of 0+ juvenile fish on large zooplankton may be explai ned by their larger mouth gape and by their better developed abilities to d etect and consume their prey items. However, this partly is lessened by the lower energy requirements of juvenile fish compared with identical biomass es of fish larvae, although larva bioenergetics remains only fragmentarily understood. Consequently, selective predation by fish larvae on particular small zooplankton prey may be more important than has been detected so far.