Within the context of invasion ecology and native trout conservation, there is a critical need to identify mechanisms limiting the altitudinal species zonation of brown trout (Salmo trutta), as their expansion into higher elevations due to climate change or anthropogenic habitat alterations could be deleterious to conservation of native fishes. Unlike many native species in the western United States, brown trout spawn in the fall, and their fry emerge during the spring snowmelt period. Recent research suggests that the embryos and emerging fry may be susceptible to streambed scour during this crucial life stage. The Logan River, Utah is a coarse bed high-mountain stream in which brown trout are nearly absent from the highest elevations despite barriers to passage or other physiological limits. Partial entrainment of sediment, whereby smaller than average (D₅₀) sediments move during flood events, is a likely phenomenon. We hypothesized that higher entrainment of gravels could be predicted in higher-elevation sections, which contain steeper slopes and smaller gravel sizes, and that entrainment would coincide with predicted emergence of brown trout fry. We used geomorphic surveys in conjunction with grain size estimates from pebble counts to predict entrainment potential of gravels at sites along an altitudinal gradient during the spring flood. We compared the temporal window of entrainment potential with the predicted emergence period for brown trout fry in each reach based on temperature.

Results/Conclusions

Potential entrainment potential varied spatially and temporally and coincided with the window of predicted fry emergence. High entrainment potential in high-elevation reaches suggests that streambed scour could be a mechanism limiting the altitudinal distribution of brown trout in mountain streams. However, further research is needed to determine the influence of within-reach variation in geomorphic characteristics on entrainment of spawning gravels. This research demonstrates the strong link between limits to exotic fish invasion and geomorphic processes, a frontier area in ecological research.