Ranking Predatory Threats by Nonnative Fishes in the Yampa River, Colorado, via Bioenergetics Modeling

Abstract

AbstractBecause of its relatively natural hydrograph, the Yampa River, Colorado, is considered the crown jewel of native fish habitat in the upper basin of the Colorado River and has supported a relatively intact native fish assemblage. Nonnative fishes are thought to pose the greatest threat to native fishes in this system. Removal programs for nonnative northern pike Esox lucius and channel catfish Ictalurus punctatus have highlighted managers' perception of the threat posed by each species. Recent expansion of nonnative smallmouth bass Micropterus dolomieu in the Yampa River attracted an avid angling clientele but also coincided with a precipitous decline in native fishes, necessitating a rigorous assessment of the relative impact of all three nonnative predators on the native fishes. We used abundance, growth, and diet estimates for each predator species to quantify consumptive demand using bioenergetics models. Despite a low abundance of small‐bodied fishes and thus a low number of fish in the smallmouth bass diet, total fish consumption by smallmouth bass (mean = 15.2 kg·km−1·year−1; 95% confidence interval [CI] = 13.3–17.1 kg·km−1·year−1) was similar to that estimated for northern pike (mean = 13.7 kg·km−1·year−1, 95% CI = 11.4–16.0 kg·km−1·year−1) and was about 65 times higher than the estimate for channel catfish (mean = 0.22 kg·km−1·year−1; 95% CI = 0.05–0.40 kg·km−1·year−1). Diet data from the upper Colorado River, where small‐bodied fish were plentiful, suggested that piscivory by smallmouth bass in the Yampa River could be 10 times the piscivory by northern pike and channel catfish, or about 168.5 kg·km−1·year−1 (95% CI = 147.0–189.9 kg·km−1·year−1), if prey fish were more available. This level of piscivory suggested that smallmouth bass presented the greatest predatory threat to native fishes of the Yampa River. As environmental conditions change, use of field monitoring together with bioenergetics modeling will be an effective framework to assist managers in adapting their nonnative fish control efforts to maximize the likelihood of native fish recovery.