Assessing conditions associated with muskellunge and walleye proximity to reservoir spillways: Implications for escapement management

Reservoir escapement of sportfishes such as Muskellunge Esox masquinongy and Walleye Sander vitreus can be detrimental to fisheries. Predicting when fishes are present near water release structures and vulnerable to escapement is critical for effective management. To understand the environmental and behavioral drivers of escapement vulnerability, we used acoustic telemetry to continuously monitor Muskellunge and Walleye presence near two reservoir spillways in Iowa (Big Creek and Brushy Creek lakes) from 2021 to 2024. We used generalized additive mixed models (GAMMs) to evaluate how water temperature, stage height, photoperiod, and diel cycles influenced fish presence and depth near the spillways. At Big Creek Lake, Muskellunge presence peaked in spring, driven primarily by water temperature whereas Walleye presence remained consistently low (<10%) throughout the year. Both species exhibited higher spillway presence in Brushy Creek; Muskellunge presence was 25–50% across seasons and Walleye presence peaked in spring (~75%) and remained ~30% the rest of the year. Although escapement is commonly correlated with reservoir discharge, we found water temperature, photoperiod, and individual differences among fish were primarily responsible for driving fish presence. Parallel-bar fish barrier breakaway events during high flows did not occur during periods of high fish presence at Big Creek, indicating flood risk mitigation features may be incorporated without negating the efficacy of barriers. Our results can be used to prioritize maintenance and repair schedules on existing barriers by identifying critical time periods for escapement and can be used to assist in the design and construction of new barriers.