The Energetics of Feeding, Metabolism and Growth of Brown Trout (Salmo trutta L.) in Relation to Body Weight, Water Temperature and Ration Size

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Abstract
Energy budgets were calculated for brown trout (live wt 5-281 g) fed on zero to maximum rations at different water temperatures (range 3.8-21.7.degree. C). Both the weight of the trout and the water temperature affected the 5 components of the energy budget: daily intake (C cal day-1), losses in feces (F cal day-1) and excretory products (U cal day-1), change in total energy content of the fish (.DELTA.B cal day-1), metabolism (R cal day-1). Equations were developed to estimate these components for trout on maximum rations. Of the 3 components of metabolism, the energy required for activity (Ra cal day-1) and for the apparent specific dynamic action (Rd cal day-1) could not be separated, but the energy required for standard metabolism (Rs cal day-1) was estimated from equations developed from the data for trout on zero rations. Energy budgets were also constructed for trout on reduced rations. As energy intake decreased, the optimum temperature for maximum values of .DELTA.B also decreased, and the temperature range over which growth occurred (i.e., .DELTA.B was positive) also decreased. Equations were developed to estimate the maintenance energy intake (Cmain cal day-1) and the energy required for maintenance metabolism Rmain cal day-1). Estimates of Rmain provided close estimates of the metabolic requirements of trout on reduced rations at temperatures below 13.degree. C. All those equations were applicable to trout feeding on a variety of food organisms. The gross efficiency of energy utilization exceeded 33% in a narrow zone of energy intake and a temperature range of 8-11.degree. C. It then decreased with both increasing and decreasing temperature, energy intake and change in the total energy content of the trout. Equations were developed to estimate the optimum energy intake, i.e., the most efficient energy intake which produces the greatest increase in .DELTA.B for the least energy intake, and the scope for growth between the maintenance and maximum energy intakes.