Unique perspectives on the influence of size and age on consumer δ 15 N from a rainbow smelt complex

Analysis of covariance indicated size had a greater influence than age on δ 15 N in three rainbow smelt Osmerus mordax morphotypes of Lake Utopia, New Brunswick, Canada. The distinction in size between the three forms allowed a unique analysis and separation of size and age effects, which is not commonly possible because of the strong correlation between size and age. These results provide support for the continued use of δ 15 N as a trophic level indicator in food web studies. While a slight increase in δ 15 N with age ( P ≤ 0·05) in intermediate‐sized smelt raises questions about a potential accumulation of 15 N with age, the effect of size was far stronger in governing δ 15 N values. The possible confounding influence of age when interpreting trophodynamics of short‐lived consumers appears to be small.