Ecology of lacustrine-adfluvial bull trout populations in an interconnected system of natural lakes

Abstract

Loss of connectivity among populations and interactions with nonnative species can negatively influence abundance of bull trout Salvelinus confluentus. Connectivity among bull trout populations and trophic relationships among native and nonnative fishes in Glacier National Park (GNP), Montana, were examined. Competition between juvenile (< or equal to 80 mm) bull trout and lake trout S. namaycush for cover habitat was examined in a laboratory environment. Connectivity among bull trout populations was inferred from genetic data. Barriers (i.e., waterfalls > or equal to 1.8 m) reduced genetic diversity and increased genetic differentiation among populations. Genetic differentiation was positively related to the length of tributary stream sections between populations and populations within the same drainage were more similar than populations in different drainages. Competition between bull trout and nonnative lake trout for prey is a potential mechanism for declines in bull trout abundance. Stable isotopes analyses were used to examine trophic relationships among fishes in GNP lakes. Bull trout and lake trout were top-level predators among lakes (Delta15N analysis), lake trout occupied a higher trophic position than bull trout (Delta15N analysis), and bull trout and lake trout likely used different foraging habitats (Delta13C analysis). These data do not support the prediction that these species are complete competitors for prey resources in GNP. Cover habitat protects fish from predators and is competed for if limiting. Habitat use by juvenile bull trout and lake trout was experimentally evaluated. Bull trout and lake trout differed in habitat use. Lake trout avoided bottom habitat, bull trout avoided water column habitat when lake trout were present, and neither species selected cover habitat. The hypothesis that bull trout and lake trout compete for cover habitat was not supported. The landscape in GNP allows connectivity among bull trout populations that are not isolated by barriers and one-way dispersal past waterfalls is likely. This connectivity allows dispersal and colonization by nonnative fishes into GNP lakes. Bull trout and nonnative lake trout are not complete competitors for prey resources in GNP or cover habitat; however, future studies should examine trophic shifts by these species associated with prey limitation and diel variability in habitat use by these species.