Evaluating the Relationship between Flow & Smallmouth Bass Abundance, Growth & Diet on the White River, Utah

Authors: Justin Furby, Matt Breen, Casey Pennock

Year: 2022

Abstract

Establishment and expansion of invasive Smallmouth Bass, Micropterus dolomieu, in the upper Colorado River basin coincides with declining native fish populations. An enhanced understanding of Smallmouth Bass ecology in invaded rivers is crucial to inform management efforts directed at reducing bass abundance to aid recovery of native fishes. The White River in Utah maintains a relatively natural flow regime in comparison to other tributaries to the middle Green River. Our objective is to quantify relationships between flow and Smallmouth Bass catch-per-unit-effort, first-year growth, and diet in the White River, Utah. To achieve this objective, we will use a multifaceted approach including active sampling via raft electrofishing, growth estimation using sagittal otoliths, and stomach content analysis. Catch rates of Smallmouth Bass in a 76-kilometer river reach (UT state line to Enron) of the White River varied over time, and increased between 2012 (0.49 bass/hr) and 2021 (23.53 bass/hr). However, catch rates in 2022 (3.09 bass/hr), a relatively low-flow year, were 86% lower than those in 2021. Juvenile fish consistently make up the majority of fish captured in this reach (2012-2022: 73-98% of individuals <200 mm TL). In June 2022, we preserved 58 fish for age, growth, and diet analysis. Analysis of otoliths confirmed 67% of captured fish were age-1 (mean TL = 122 mm, range= 99 mm-144 mm), and the oldest fish analyzed were age-4 (range = 308 mm-368 mm). Aquatic macroinvertebrates were the most frequently encountered prey group in bass stomachs and were found in 63% of fish (17% of total biomass). Bass demonstrated moderate rates of piscivory as cyprinids were encountered in 31% of stomachs (62% total biomass). Crayfish were the least encountered prey item, found in 6% of stomachs (21% biomass). Preliminary results suggest a potential negative correlation (r = -0.59, P < 0.001, n = 75) between mean daily spring flow (May-June) and first-year growth. We plan to continue collecting Smallmouth Bass in 2023 to bolster our sample size, and potentially include fish captured in reaches closer to Taylor Draw Dam, Colorado, where adult fish are more prevalent. Ultimately, this study will inform management of nonnative Smallmouth Bass by providing a more complete understanding of lotic Smallmouth Bass ecology in the Upper Colorado River Basin.