The Energetic Equivalence Hypothesis and the Relation between Population Density and Body Size in Stream-Living Salmonids

Abstract

We studied density and mean body size in populations of stream-living resident brown trout Salmo trutta, anadromous populations of S. trutta, and Atlantic salmon Salmo salar in southern Sweden using annual quantitative electric fishing during time periods ranging from 2 to 16 yr in 54 sites in nine streams. With a total of 236 observations, ANCOVA of log(population density) versus log(mean body size) showed a negative dependence with an estimated slope of -0.984 ± 0.148 (95% confidence interval) and intercepts significantly varying among stream sites. As previously published data indicate the metabolic rate of fishes proportional to (body weight)0.9, the result supports the hypothesis of energetic equivalence; that is, the total energy flow through a population tends to be constant within but varying among sites and populations. In a review of the theory of self-thinning in food-regulated animals, we emphasize the effects of a negatively size-dependent growth and mortality; each will modify the thinning line, but combined they will tend to compensate for each other.