A long-term view of Largemouth Bass recruitment variation in Illinois reservoirs

Abstract Objectives Long-term studies can provide insight into complex dynamics such as fish recruitment. We leveraged more than a decade of monitoring data on seven reservoirs in central Illinois to test for trends in Largemouth Bass Micropterus nigricans recruitment and to reassess an earlier study’s findings on the relative importance of the nesting period, prey fish availability, and first-winter mortality for recruitment variability by expanding the previous time series by 20 additional year-classes. Methods We revisited the question of whether recruitment strength was set by the first fall or over the first winter by testing whether fall boat-electrofishing catch per unit effort (CPUE) of age-0 Largemouth Bass or winter severity was a more informative index of boat-electrofishing CPUE of age-1 Largemouth Bass the subsequent spring. Because of a change in fish sampling methodology from AC to pulsed DC boat-electrofishing during the final 11 years of the monitoring period, we split the data into two time series to assess whether trends in recruitment were present. Using a subset of four reservoirs with more complete information on prey fish, we compared the relative support of annual nest productivity, prey fish phenology, prey fish reproductive productivity, and CPUE of vulnerable-sized prey fish as sources of recruitment variation. Results Largemouth Bass recruitment strength was found to be set prior to a cohort’s first winter, and therefore, fall CPUE of age 0-fish was an informative index of first-year recruitment. Only a single reservoir exhibited a significant trend in recruitment, a decline over time. Based on the earlier longer time series, recruitment variation was more closely associated with nest productivity than with prey fish availability, whereas none of the tested hypotheses were supported as shaping recruitment during the latter 11 years of the study period. Nest productivity declined over time in half of the four study populations, with Largemouth Bass recruitment also becoming more similar among the reservoirs during the final 11-year period. Conclusions The findings of this and other long-term studies of Micropterus recruitment highlight the importance of understanding and managing for the conditions that can promote the resilience of Largemouth Bass nest success.