Respiration Rates of Eurasian Perch Perca fluviatilis and Ruffe: Lower Energy Costs in Groups

—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. Individual 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 considering group size the results of bioenergetic modeling may be seriously biased. Environmental factors strongly impact the metabolic costs of animals (Keddy 2001). In fish, these impacts are widely studied through respiration measurements, and the effects of abiotic factors such as temperature, salinity, and oxygen depletion on metabolism have been well described (e.g., Rao 1968; Hölker 2003; Ishibashi et al. 2005; Peck et al. 2005). Among biological factors, the allometry of consumption and respiration are also well analyzed (e.g., Herrmann and Enders 2000; Hunt von Herbing and White 2002; Hölker 2003; Peck et al. 2005); the influence on its metabolism of biotic factors relating to the fish’s ecology, however, has rarely been considered. Stress situations such as competition, inadequate habitat, or predation risk can increase metabolic costs (Huuskonen and Karjalainen 1997; Fischer 2000; Keddy 2001; Woodley and Peterson 2003). Shoaling, on the other hand, can improve foraging success and reduce predation risk (Pitcher and Magurran 1983; Pitcher 1986; Magurran 1990) and thus may act as a calming, cost-reducing factor (Parker 1973). An influence of group size on metabolic costs has been found in several fish species (Schuett 1933; Parker 1973; Itazawa et al. 1978; Smatresk and Herreid 1980; Klyashtorin and Salikzyanov 1981; Ross et al. 1992) but seems to be absent in others (Konchin 1981; Hölker 2003, 2006). Irrespective of these findings, however, growth and respiration rates are often determined in laboratory experiments on single, isolated individuals, regardless of the species’ social behavior. The results obtained in these experiments may therefore not be representative of the species’ performance in the wild. Bioenergetic modeling has become an increasingly important tool with which to estimate fish growth or consumption, especially for the management of wild stocks, and the Wisconsin model (Hanson et al. 1997) has been parameterized for an increasing number of species. Sensitivity analyses have demonstrated that for most species, including perch, consumption and respiration parameters are decisive for the model’s output (Kitchell et al. 1977; Bartell et al. 1986; Horppila and Peltonen 1997). In a recent evaluation of bioenergetic models for yellow perch Perca flavescens, Bajer et al. (2003) found strong evidence of deficiencies in estimates of the metabolic rate. Irrespective of the sensitivity of bioenergetic model outputs to respiration parameters and the potentially strong group effect on respiration, of 32 parameter sets listed in Hanson et al. (1997) for different fish species and ontogenetic stages, only 25% are based on respiratory data gained from group experiments (Table 1). For solitary species such as northern pike Esox lucius respiratory experiments on a single fish are adequate: experiments with grouped fish would overestimate metabolic rates as a result of aggressive interactions (Wirtz and Davenport 1976). For shoaling fish species such as Coregonus spp. or Eurasian perch P. fluviatilis, by contrast, respiration rates might be overestimated in experiments with isolated fish. Social * Corresponding author: [email protected] Received May 24, 2006; accepted August 14, 2006 Published online January 15, 2007 43 Transactions of the American Fisheries Society 136:43–55, 2007 Copyright by the American Fisheries Society 2007 DOI: 10.1577/T06-123.1 [Article]