Influence of body size and food ration on the energy budget of rainbow trout Salmo gairdneri Richardson

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Abstract
Three size classes (3 g, 200 g, 1–3 kg) of rainbow trout, Salmo gairdneri Richardson, were each fed five rations ranging from zero to a ‘maximum’ ration and the relationships among body size, food ration, metabolism and growth examined. Water temperature was constant at 11–4°C. The gross growth efficiency in wet weight, K, increased asymptotically with increasing ration but decreased with increasing body size. Changes in fat and moisture content of the body, however, were such that Kin terms of energy utilization was independent of body size when the fish were feeding at the same level. This result was not apparent when wet weight values only were considered and use of a constant to convert wet weight offish to calorific values can lead to errors in interpretation of results. Assimilation efficiency averaged 71 ‐7 % over all feeding groups and was independent of body size and food ration. Total metabolic expenditure, however, increased with increasing body size and feeding level, but at any given feeding level a similar proportion of the energy of the assimilated ration was used for metabolism. At a single feeding level, therefore, the percentage utilization of energy in each category of the energy budget remained similar over all body sizes. Such a result can only be expected under laboratory conditions but it provides a base‐line for comparison with the performance of other more complicated systems.