Are Brown Trout Replacing or Displacing Bull Trout in Montana? Inference from Longterm Monitoring Data

Recently, brown trout have increased in distribution and abundance within Montana, rendering concerns about potential influence on bull trout populations.  We synthesize data from longterm monitoring sites to evaluate factors potentially driving patterns of bull trout and brown trout population abundance and trends.   We investigate how thermal and hydrologic regimes are influencing changes in bull trout and brown trout abundance.  We delineate our analyses based on bull trout life-history patterns and habitat considered by the USFWS as spawning and rearing (SR) or foraging, migrating, and overwintering (FMO) habitat.  We find the abundance of bull trout to be strongly negatively associated with increasing August stream temperatures; for sites within SR habitat, the number of bull trout decrease dramatically at sites where August stream temperatures are above 11-12°C.  Concomitantly, brown trout abundance is highest at sites with temperatures between 12-15°C.  Across both SR and FMO sites, bull trout trends include a mix of stable and decreasing trends with little evidence of increasing trends, while brown trout trends exhibit contrasting patterns with stable and increasing trends.   Declining trends in bull trout abundance in the absence of brown trout populations suggest brown trout replacement of bull trout, particularly in areas where temperatures are increasing.