Larval Entrainment Through Fort Randall and GAVINS Point Dams, Lewis and Clark Lake, South Dakota

ABSTRACT Entrainment of Walleye Sander vitreus and Sauger Sander canadensis has been documented in all the Missouri River reservoirs in South Dakota and is a hypothesized mechanism through which source‐sink dynamics occur. However, an assessment of larval entrainment through Fort Randall Dam and Gavins Point Dam, which bound Lewis and Clark Lake, has not been conducted since 1971. Abundances of adult Walleye and Sauger, two economically important sport fish species, have remained low in the reservoir since 2011. Entrainment resulting from Lewis and Clark Lake's high turnover rate (mean 7.2 days) is hypothesized to be a primary factor driving declines in Walleye and Sauger, despite rapid growth rates, low exploitation rates, and high survival rates. We sought to provide a contemporary assessment of larval entrainment through Fort Randall and Gavins Point Dams, and to relate larval entrainment trends to environmental factors. Ichthyoplankton tows were used to measure entrainment through Fort Randall Dam and Gavins Point Dam, and factors driving entrainment were assessed using an information theoretic approach. We estimate between 9148 and 19,382 larval Walleye and Sauger and 67,824–1,753,760 larval fish of all species immigrated into Lewis and Clark Lake annually via entrainment through Fort Randall Dam; whereas, 13,680–67,135 larval Walleye and Sauger and 6,259,538–15,437,560 million fish of all species emigrated out of the reservoir via entrainment through Gavins Point Dam between May 25 and July 24 during 2021–2024. Water temperature and time of year (calendar date) were found to be the most supported models explaining variation in entrainment of all species for both dams. Flow characteristics (i.e., mean weekly discharge) were not important for explaining entrainment of all species through both dams. We believe the findings of our study inform management of Walleye and Sauger in Lewis and Clark and other reservoirs by enabling managers to account for source‐sink dynamics created by entrainment when making management decisions.