Effects of large lake trout (Salvelinus namaycush) on the dietary habits of small lake trout: a comparison of stable isotopes (δ15N and δ13C) and stomach content analyses

We examined the effect of large (potentially piscivorous) lake trout (Salvelinus namaycush) on the dietary habits of small lake trout in an arctic lake. We hypothesized that large lake trout constrain the foraging of small lake trout, thus, in the absence of large lake trout, small lake trout will shift their diet from littoral to more abundant prey offshore. We tested this hypothesis using samples from a removal experiment where all lake trout large enough to be susceptible to gill nets were removed from a small arctic Alaskan lake during 1988–1989. We examined size at age and conducted stomach content and stable isotope analyses of lake trout collected during removal (1988), early recovery (1990), and late recovery (1999) portions of the study. Lake trout grew more quickly following removal. All lake trout fed on a variety of prey, but stomach analyses provided little information on segregation of diet between size classes. δ15N and δ13C analyses showed that small lake trout shifted their diet after large lake trout were removed, apparently toward more reliance on offshore zooplankton, which also implies a habitat shift to open water. Thus, we conclude that large lake trout are restricting the dietary habits of small lake trout, a restriction that was removed in an exploited population.