Trophic relationships between a native and a nonnative predator in a system of natural lakes

Bull trout, a species of char listed as threatened under the US Endangered Species Act, have been displaced from portions of their historic range following the introduction of nonnative lake trout. It has been suggested that competitive exclusion as a result of trophic overlap between bull trout and lake trout may be the causal mechanism associated with displacement of bull trout. This study used stable isotope data to evaluate trophic relationships among native bull trout, nonnative lake trout and other fishes in seven lakes in Glacier National Park (GNP), Montana. Bull trout and lake trout had greater δ¹⁵N values relative to other fishes among lakes (δ¹⁵N ≥ 3.0‰). Lake trout had greater δ¹⁵N values relative to bull trout (δ¹⁵N = +1.0‰). Bull trout had greater δ¹³C values relative to lake trout in six of the seven lakes examined. Although both bull trout and lake trout had greater δ¹⁵N values relative to other fishes within lakes in GNP, differences in δ¹⁵N and δ¹³C between bull trout and lake trout suggest that they are consuming different prey species or similar prey species in different proportions. Therefore, displacement of bull trout as a direct result of complete overlap in food resource use is not anticipated unless diet shifts occur or food resources become limiting. Additionally, future studies should evaluate food habits to identify important prey species and sources of partial dietary overlap between bull trout and lake trout.