Characterizing the fish assemblage in a reservoir forebay & identifying the potential for nonnative fish escapement into a desert river

Authors: Barrett Friesen, Phaedra Budy, Casey Pennock

Year: 2022

Abstract

Native riverine fishes in arid regions face imperilment due to habitat fragmentation, water overallocation, and invasive species establishment. In the Colorado River basin, USA, these threats coalesce at Glen Canyon Dam, which impounds the Colorado River to create Lake Powell. Management agencies are concerned low water level in Lake Powell may increase the likelihood of nonnative fish entrainment through the dam, adversely affecting native fishes downstream in the Grand Canyon. Our ultimate goal in this study is to estimate the probability of nonnative fish entrainment and survival through Glen Canyon Dam to better inform invasion risk into the lower Colorado River. To this end, we are characterizing the fish community via sampling, conducting hydroacoustic surveys, and assessing fish movement and habitat use with acoustic telemetry near the dam. Preliminary data suggest entrainment risk varies by season and lake stratification. When the reservoir was cold (<10 °C) and well mixed in early March, the forebay was sparsely occupied by three species of nonnative coolwater fish (e.g., Walleye, Sander vitreus), which were restricted to the top 11 m of the mixed water column. Following reservoir warming (>20 °C surface temp.) and formation of the epilimnion, we collected an additional three species of nonnative warmwater fish (e.g., Smallmouth Bass, Micropterus dolomieu) down to 16 m depth. Gillnet catch per unit effort (CPUE) in the forebay increased by an order of magnitude between March and August, from 0.4 fish/net-night to 4.7 fish/net-night. Higher CPUE during the summer indicates greater fish abundance, movement, or likely both. As the dam penstocks are now drawing water from the epilimnion, increased nonnative fish abundance and activity in the forebay represents an increased entrainment risk. Our results will inform dam operations as managers seek to minimize the risk of entrainment while still meeting water delivery requirements and maintaining hydroelectric power generation.