For the long haul: 34 years of seining with implications for Walleye management in Spirit Lake, Iowa

Abstract Objective Funding schedules, academic timelines, and the realities of scientific inquiry foster an environment in which multidecadal fisheries studies are less common than short-term inquiries. However, the Iowa Department of Natural Resources maintains a multidecadal (1989–2024) database on its summer seining program in Spirit Lake, Iowa. This database was used to identify the abiotic and biotic factors that have structured the Spirit Lake Walleye Sander vitreus population over multiple decades. The objectives were to (1) examine temporal trends in age-0 (65–127-mm), subadult (128–379-mm), and adult (≥380-mm) Walleye relative abundance (CPUE; catch per seine haul) over the span of 36 years in Spirit Lake (1989–2024) and (2) model how abiotic and biotic variables have shaped Walleye relative abundance over multiple decades. Methods A boat-deployed, lampara-style beach seine (measuring 152 × 4 m, with 7-mm bar mesh) was used to collect fish in Spirit Lake. Temporal trends in age-0, subadult, and adult Walleye CPUE were evaluated using Kendall’s tau correlation analysis. Generalized additive models were used to model age-0, subadult, and adult Walleye CPUE as a function of abiotic factors (e.g., chlorophyll a, mean summer water temperature, and turbidity) and biotic variables (i.e., CPUE of potential Walleye competitors and predators). Results The relative abundance of age-0 and adult Walleyes increased significantly from 1989 to 2024, but subadult abundance remained temporally stable. Specific conductance decreased significantly over time, whereas mean and minimum summer water temperature increased. Age-0 Walleye CPUE was positively related to the CPUE of age-0 Largemouth Bass Micropterus nigricans and negatively related to the CPUE of age-0 Smallmouth Bass M. dolomieu. Subadult Walleye CPUE was negatively related to orthophosphate concentration. Adult Walleye CPUE was positively related to total suspended solids, nitrate plus nitrite, and mean summer water temperature. Conclusions These findings have important implications for fisheries management. Results suggest that increased use of phosphorus reduction approaches (e.g., planting riparian buffers and controlling agricultural runoff) in recent decades has not harmed water quality or Walleye production in Spirit Lake. Hence, fisheries managers can partner with land managers and lake associations to frame nutrient management as a pathway to clean water and a high-quality Walleye fishery. In addition, results indicate that fisheries managers can use total suspended solids as an input variable for predicting trends in adult Walleye abundance, potentially providing opportunities to streamline Walleye monitoring programs, fine-tune Walleye stocking programs, and save resources (e.g., time, money, and personnel) that can be allocated to address other management priorities. Overall, this study furnishes a management-relevant, multidecadal perspective on the drivers of Walleye relative abundance and associated modeling tools that are applicable in one of Iowa’s most important lakes.