Cooperative Fish and Wildlife Research Units Program: Montana Fishery
Education, Research and Technical Assistance for Managing Our Natural Resources

Non-native smallmouth bass were thought to be limited to the warm, downstream waters of the Yellowstone River, Montana, but adults were recently observed far upstream in coldwater reaches that support a major trout fishery. Along with our Montana Fish, Wildlife & Parks and USGS NoRock partners, we sought to identify the factors that determine their current and future establishment (i.e., successful recruitment) in the upper Yellowstone River. We hypothesized that reduced age-0 growth potential in upstream reaches limits recruitment through size-selective overwinter mortality, and tested this hypothesis by sampling age-0 individuals across a broad longitudinal extent from 2016 to 2019 to describe their distribution, size at the onset of winter, thermal experience, diet, and otolith-inferred hatch dates and growth rates. The upstream extent of age-0 fish was 111 to 145 RKM downstream of that of adults. Median size at the onset of winter was not significantly different across the study area. Comparative hatch dates, growth rates, and growth durations were also counter to our predictions. These results suggest that age-0 overwinter mortality is not currently preventing upstream establishment in the Yellowstone River.