Food Web Effects of Lake Trout (Salvelinus namaycush) Invasion in Northwestern Montana

Around the turn of the 20th century, lake trout (Salvelinus namaycush) were widely introduced in several lakes and reservoirs outside their native range in western North America. Since then, lake trout have become problematic in many lakes where they were introduced, causing significant declines in popular sport fishes and native species, most notably federally protected bull trout (Salvelinus confluentus). Despite evidence that invasive fish can cause cascading trophic effects in aquatic communities, the impacts of lake trout introduction / invasion on aquatic food webs remain poorly understood. Moreover, native fish restoration programs tend to focus on suppression of invasive fish and rarely examine the broader food web effects of remediation efforts. In this study, I used stable isotope analysis to examine the food web effects of lake trout invasion and remediation (e.g., gillnetting suppression) in 12 lakes (four uninvaded, five invaded, and three remediated) to which bull trout are native in northwestern Montana. Although bull trout and lake trout had higher δ15N than other fishes, lake trout had higher δ15N than bull trout in all invasion categories, indicating bull trout may both compete with and be preyed upon by lake trout. Analyses of bull trout diets revealed bull trout consumed low proportions of pelagic fish in remediated lakes. In contrast, bull trout consumed relatively high proportions of pelagic prey in uninvaded or invaded lakes. Bayesian standard isotope ellipse area indicated that remediated lakes had uniquely disorganized food web structures compared to invaded and uninvaded lakes, suggesting that remediated lakes may be at an intermediate stage of food web succession. Isotope niche overlap between bull trout and lake trout was symmetric in remediated lakes and asymmetric in invaded lakes, suggesting suppression may diminish lake trout impacts on bull trout. Finally, space-for-time substitution revealed that it takes about 70 years for lake trout to displace bull trout in the study region, ...