Dietary and non-dietary contributions to among-individual variation in carbon and nitrogen isotopic composition of lake trout

Abstract

Variation among individuals in stable isotope composition is increasingly being used as an ecological index of trophic niche size. This is based on the assumption that most of the observed variation arises from differences in diet. We tested this assumption by comparing variability in C and N isotopic compositions of adult female lake trout in five wild populations with those of their captive counterparts reared on a common diet, using mature ova as the tissue of comparison. Variation in δ13C and δ15N were related to female size and age, and to a lesser extent growth rate, in both wild and captive fish but relationships were generally stronger for wild fish. Variation among strains in mean δ13C and δ15N was much lower in captive than wild fish, but δ13C and δ15N still varied significantly among strains for the captive fish. As expected, the magnitude and orientation of individual dispersion in δ15N – δ13C space (ID) was much less variable among captive populations than among wild populations. The ID of captive populations expressed as a percentage of the ID of corresponding wild populations ranged from 3 to 16% when δ13C and δ15N were adjusted for body size covariation, and from 3 to 28% when data were not adjusted. Relationships between ID and growth rate at the population level were positive for captive lake trout and negative for wild lake trout but neither were significant. Our results suggest that non-dietary variation in δ13C and δ15N is usually a small component of the δ13C and δ15N variation seen in wild lake trout populations and current isotopic niche metrics primarily capture dietary variation.