The fish community of the Loosdrecht Lakes and the perspective for biomanipulation

In a two-year-study, the fish community of the Loosdrecht Lakes area was characterized with regard to the professional fishery and the forage base. The lakes are shallow (mean depth 2 m) and eutrophic to hypertrophic. The bream,Abramis brama, dominates the fish biomass. The pikeperch,Stizostedion lucioperca, is the main predator.Bream up to 30 cm have a slow growth rate and are in a bad condition. A faster growth rate and a better condition are shown by bream of 30 cm and more. The small bream feeds on chironomid larvae, benthic cladocerans and zooplankton. The better condition of bream over 30 cm is explained by the more efficient feeding of larger bream onChironomus plumosus larvae. Pikeperch show a fast growth rate and a good condition. Recruitment is limited by the low densities of smelt,Osmerus eperlanus, leaving cannibalism as the most important way for the 0+ pikeperch to become piscivorous and to manifest a fast growth.The impact of the professional gillnet fishery on bream and pikeperch is small because the mesh sizes in use are as large as 75–100 mm bar mesh.The planktivorous 0+ pikeperch consumes mainly the carnivorous zooplanktersLeptodora kindtii and cyclopoids. The zooplankton community lacks large herbivorous species likeDaphnia hyalina, capable of consuming bluegreens. A possible experiment in biomanipulation with a view to find out whether the development ofD.hyalina is depressed by the small planktivorous cyprinids, is predator enhancement with the aid of a stocking programme for indoor-raised 0+ pikeperch in early summer.