Within- and among-population variation in the trophic position of a pelagic predator, lake trout (Salvelinus namaycush)

Abstract

Many aquatic consumers have flexible feeding habits, and the diet and trophic position of a species can be expected to vary both within and among populations. In this study, we quantify the importance of both within- and among-population trophic variation for lake trout (Salvelinus namaycush) using stable isotope designations of trophic position from 13 Ontario and Quebec lakes. Lake-to-lake differences explained 78% of the total variation in lake trout trophic position. Analysis using both stable isotopes and published dietary data demonstrated that the trophic position of lake trout failed to increase appreciably as a function of animal body size. This finding was attributed to weak predator size - prey size relationships as well as to there being no relationship between prey fish trophic position and body size. The variance in trophic position of a population reflects the extent to which individuals forage as trophic specialists; however, we did not identify any one factor that was correlated with within-population trophic variation. Our finding that much of the total variation in trophic position represents among-population differences indicates that considering the average trophic position of a population does not mask substantial within-population trophic variation.