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

Many aquatic consumers have flexible feeding habits, and the diet and troph ic position of a species can be expected to vary both within and among popu lations. In this study, we quantify the importance of both within- and amon g-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 lak e trout trophic position. Analysis using both stable isotopes and published dietary data demonstrated that the trophic position of lake trout failed t o increase appreciably as a function of animal body size. This finding was attributed to weak predator size - prey size relationships as well as to th ere being no relationship between prey fish trophic position and body size. The variance in trophic position of a population reflects the extent to wh ich individuals forage as trophic specialists; however, we did not identify any one factor that was correlated with within-population trophic variatio n. Our finding that much of the total variation in trophic position represe nts among-population differences indicates that considering the average tro phic position of a population does not mask substantial within-population t rophic variation.