Competition for food between perch (Perca fluviatilis L.) and invasive ruffe (Gymnocephalus cernuus (L.)) in re-oligotrophic Lake Constance

The effect of group size on routine metabolic rate (RMR) and activity of the two shoaling percids Eurasian perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus) was studied 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 up to two times as high as in groups of eight fish, with intermediate values in the groups of four fish. RMR was highest during twilight in both species, irrespective of group size. Perch consumed more oxygen and were more active during the day than during the night, whereas ruffe’s oxygen consumption and activity 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. Individual fish may benefit from the presence of conspecifics through a calming effect that reduces their energetic costs. We advise that the social behaviour of a species should be more thoroughly considered when planning behavioural, growth and respiration experiments. As bioenergetic model parameters for many species are based on data gained from isolated fish, we conclude that without considering group size the results of bioenergetic modelling may be severely biased.