Factors Influencing Rainbow Trout Annual Growth in a Large Regulated River

Abstract

AbstractThe construction and operation of dams represents one of the most significant anthropogenic impacts to the aquatic environment of freshwater ecosystems and includes changes in flow, temperature, water chemistry, sedimentation, and nutrient delivery. Despite the substantial changes caused by dams, we have a limited understanding of how dams influence important rate functions of fish, including growth rates. This study measured the growth rates of Rainbow Trout Oncorhynchus mykiss from successive captures of individually marked fish over seven annual increments within four river sections downstream of Libby Dam on the Kootenai River, Montana. We modeled the influence of hydropower‐related environmental variables on Rainbow Trout length and weight growth rates using linear mixed‐effects models. The top models predicting annual length and weight growth rates contained measures of water chemistry (ratio of total N to total P [N:P]) during the growing season, winter substrate coverage by the diatom Didymosphenia geminata, and an interactive term between winter D. geminata coverage and fish size at tagging. Winter D. geminata coverage and N:P were negatively correlated with annual growth rates, but the interactive term indicates that the influence of winter D. geminata coverage disproportionally affects smaller fish more than larger fish. We hypothesize that N:P and D. geminata are influencing Rainbow Trout growth rates through lower‐trophic‐level impacts. Top Rainbow Trout length and weight growth models explained 94.6% and 92.2%, respectively, of the annual variability in growth rates, of which 87.7% and 76.2%, respectively, were attributable to fixed effects. An experimental nutrient addition study and robust trophic monitoring efforts in the Kootenai River downstream of Libby Dam would be an effective means of independent corroboration of these study results. If successful, nutrient addition may be an effective management strategy to improve annual Rainbow Trout growth rates, mitigating for the nutrient retention occurring in the large reservoir upstream of Libby Dam.