Approaches to the analysis of energetic costs of intraspecific competition for space by rainbow trout (<i>Salmo gairdneri</i>)

Abstract

Two approaches of estimating energy expended by rainbow trout ( Salmo gairdneri ) on density‐dependent activities were tested. Metabolic rates were obtained from growth experiments in laboratory systems. Increases in metabolic rates at higher population densities were attributed to three factors: (1) starvation, (2) increased levels of exercise by subdominant fish were forced to maintain stations in fast water due to saturation of territories of more dominant trout, (3) higher planes of excitation. It was observed that variance of energetic factors of routine metabolism, growth rate, consumption rate, and growth efficiency tended to increase with density. These responses seem to be a good indication of intraspecific competition. Little correlation was observed between density and average daily ration, or density and aggressive behaviour. Dominant trout, as a group grew faster, grew more efficiently, and contained greater lipid content than the rest of the population at each density. However, strictly defined, energetic advantages were not conferred on a ‘peck‐right’ basis. At higher densities, dominance conferred less benefits than was observed at lower population levels. Space limitation per se was not an influential factor affecting growth rates. Methods used to estimate density‐dependent energy costs are evaluated. Both methods give nearly identical estimates.