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The effect of group size on the routine metabolic rate and activity of the two shoaling percids, Eurasian perch Perca fluviatilis and ruffe Gymnocephalus cernuus, was studied by using twin-flow intermittent respirometry and time-lapse video techniques. In both species, we found a clear group effect. In isolated fish, oxygen consumption was as much as twice that in groups of eight fish, with intermediate values in groups of four fish. The routine metabolic rate was highest during twilight in both species, irrespective of group size. Eurasian perch consumed more oxygen and were more active during the day than during the night, whereas the oxygen consumption and activity of ruffe were higher during the night than during the day. With increasing group size, the differences between day and night decreased and the diel cycle was less pronounced. Indi- vidual fish may benefit from the presence of conspecifics through a calming effect that reduces their energetic costs. We advise that the social behavior of a species be more thoroughly considered when planning behavioral, growth, and respiration experiments. Because bioenergetic model parameters for many species are based on data gained from isolated fish, we conclude that without consider- ing group size the results of bioenergetic modeling may be seriously biased.

The central issue of this doctoral thesis is the understanding of the mechanisms underlying the competitive relationship and the general performances of the two percids perch (Perca fluviatilis L.) and ruffe (Gymnocephalus cernuus (L.)) in oligotrophic lakes. Both species are potential competitors for benthic food resources. The visual predator perch is supposed to flourish best under mesotrophic conditions, whereas ruffe with its very sensitive lateral line organ performs best under meso- to eutrophic conditions. In oligotrophic lakes ruffe is far less abundant than perch or even absent. While a mechanistic explanation for higher ruffe abundances in lakes of high productivity is well established, it is less clear why ruffe are less abundant or even absent in lakes of low productivity.This thesis summarises laboratory, enclosure and field experiments on the competitive relationship between perch and ruffe in Lake Constance, a large, pre-alpine lake that has recently returned to oligotrophic conditions.