Invasive Walleye demographics and predation on rare native fish in the Upper Colorado River basin

Abstract

Invasive Walleye, Sander vitreus, pose a threat to endangered native fishes in the upper Colorado River basin, including Colorado Pikeminnow Ptychocheilus lucius. We evaluated Walleye demographic characteristics and used bioenergetics modeling to quantify their predation on native fishes in the lower Green and Colorado Rivers, Utah. Walleye were sampled via electrofishing in 2021 and 2022. Age and growth were assessed from sagittal otoliths, and diet composition was determined from stomach contents of 643 individuals. Demographic data and estimated Walleye abundance informed bioenergetics simulations under four scenarios: baseline conditions (Baseline Scenario), a 2°C temperature increase (Climate Change Scenario), increased Colorado Pikeminnow abundance (Recovery Scenario), and abundance reductions of Walleye (Removal Scenario). Walleye grew rapidly, reaching an average total length (TL) of 250 mm at age-1 and approximately 347 mm by age-2. Females exhibited a maximum predicted size of 611 mm TL and males were 507 mm TL. Most fish sampled were 3-5 years old and 412 – 501 mm TL. Diets were 87% nonnative fish by weight, but native species (12%) included Colorado Pikeminnow, Bonytail Gila elegans, other Gila, native suckers, and Speckled Dace Rhinichthys osculus. Bioenergetics modeling estimated that Walleye consumed 7875.8 kg of prey over two years under Baseline Scenario conditions, including 14,872 age-0 Colorado Pikeminnow. A 2°C increase in river temperature raised Walleye consumption of native fish by 141 kg, including 3,333 more age-0 Colorado Pikeminnow. If Colorado Pikeminnow abundance returned to historical levels and represented 5% of fish consumed by Walleye, modeled consumption would exceed 497949 age-0 individuals over the two-year simulation. Reducing Walleye abundance by 75% lowered age-0 Pikeminnow consumption by 11,282 individuals. Walleye in the lower Green and Colorado Rivers present a significant predation risk to threatened and endangered native fishes. Management actions such as population suppression and immigration control are necessary to conserve Colorado Pikeminnow and other imperiled Upper Colorado River Basin species.