Sauger and Walleye Year‐Class Formation in the Lower Wisconsin River, 1987–2016

Sauger Sander canadense and Walleye S. vitreus support important fisheries throughout much of northern North America, but their management is complicated by highly variable reproductive success from year to year. We analyzed patterns of juvenile abundance in the lower Wisconsin River, a large, lowland river in the upper Midwestern United States, to better understand the formation of strong and weak year‐classes. Annual standardized fall electrofishing catches of age‐0 Saugers and age‐0 Walleyes fluctuated greatly from 1987 to 2016. Few significant correlations existed between catches of age‐0 fish of either species and environmental and biological variables, although strong and weak year‐classes were often synchronous between Walleye and Sauger. Random forest models indicated that age‐0 Sauger catches were positively related to age‐0 Walleye catches and negatively related to the CV (100· SD /mean) of daily air temperatures during the growing season and catches of Largemouth Bass Micropterus salmoides . Age‐0 Walleye catches were positively related to age‐0 Sauger catches and negatively related to Largemouth Bass catches and the CV in daily river flows during the growing season. However, these models explained little of the variation in age‐0 abundance over the 30 years of data. Excluding the 1987 Walleye year‐class, catches of age‐0 Saugers and age‐0 Walleyes were significantly correlated with respective catches of age‐1 fish 1 year later and age‐2 fish 2 years later, indicating that year‐class strength was largely set in the first year of life and could be indexed by standardized fall electrofishing. Because a full understanding of the causes of year‐class formation remains elusive, continued annual monitoring of age‐0 Sauger and age‐0 Walleye abundance will benefit the management of the recreational fishery of the lower Wisconsin River.