The Role of Environmental Factors in Determining Early Survival and Invasion Success of Exotic Brown Trout

Due to significant threats to native species posed by normative fishes, it is important to understand how species life history strategies interact with environmental conditions to explain the outcome of normative fish invasions. Brown trout Salmo trutta are prolific invaders but often exhibit upstream distributional limits in streams of the intermountain western United States. We used redd counts, embryo survival experiments, and temperature modeling to identify limits to brown trout invasion. Brown trout spawned later than previously reported and established spawning areas in high-elevation stream reaches (1,983-nm elevation), where adult recruitment is typically very low. While embryo survival was lower in high-elevation, cooler-water areas, these harsh overwinter conditions did not necessarily preclude hatching success (>= 36%). However, model predictions based on winter temperature data indicate that during most years, brown trout fry probably would fail to emerge from the gravel before the onset of peak spring flooding in these high-elevation reaches, suggesting that high spring flows could limit invasion success. A better understanding of mechanistic limits to invasion success across multiple life stages is crucial to predicting the future expansion of exotic fish species.