Further Evidence that Lake Trout Displace Bull Trout in Mountain Lake

Abstract

I surveyed five large mountain lakes in Glacier National Park, Montana, with gill nets in 2000 to assess the status of bull trout (Salvelinus confluentus) populations. I compared results to previous surveys, conducted in 1969 and 1977, at which time numbers of native bull trout were higher than recently established populations of lake trout (Salvelinus namaycush). The data indicate a broad decline in bull trout numbers and corresponding increases in lake trout population size in Kintla, Bowman, Logging, and McDonald lakes. In Quartz Lake, where lake trout are not known to occur, bull trout catch was stable across years. These data suggest that lake trout expansion has had a substantial detrimental impact on Glacier National Park bull trout populations especially because variables commonly implicated in bull trout population decline elsewhere across the species' range are not significant factors in Park lakes. I contend that effective recovery actions for adfluvial bull trout populations, in mountain lakes where nonnative lake trout have become established, must be directed at reducing species interaction through directed control actions on lake trout. I suggest that the rate and magnitude of the transition from native bull trout to introduced lake trout may depend on multiple factors, including migration of either species, the extent and quality of bull trout spawning and rearing habitat, and the structure of the lacustrine food chain. Four of the five bull trout populations I studied in Glacier National Park lakes are currently at high risk of extirpation, due primarily to incompatibility with introduced lake trout populations.