Female Sex Ratio Bias in Extended Growth Hatchery Walleye Fingerlings Produced in Wisconsin

Abstract

Abstract Sex differentiation in teleost fish is complex and often dictated by genetics, environmental conditions, and population density during critical embryonic/larval development periods. Walleye Sander vitreus have a long propagation and stocking history, and sex ratios of hatchery-reared Walleye have been variable, typically skewed towards males, and potentially influenced by temperature. In contrast to previous fry and small fingerling Walleye stocking in Wisconsin water bodies, the Wisconsin Walleye Initiative of 2013 focused on the production of extended growth Walleye fingerlings (EGW) stocked out in fall under the assumption of greater survivorship and in response to observed declines in Walleye natural recruitment. Observed sex ratios of adult Walleye in a stocked northern Wisconsin lake chain were suggestive of EGW sex ratios skewed towards females. This lake chain has been under rehabilitation and an aggressive EGW stocking regime due to prolonged Walleye natural recruitment failures. To test whether imbalances in EGW were occurring, we collected 30 EGW from one Wisconsin hatchery in 2019 and about 50 EGW from each of three Wisconsin hatcheries in 2020 to histologically determine sex ratios. In 2019, EGW were about 97% female. In 2020, the percentage of females in hatchery samples ranged from 60% to 100%. Nonfemales in 2019 and 2020 were all undifferentiated. Our results suggest that EGW raised in Wisconsin under similar incubation and rearing protocols may be skewed towards females. If sex ratios of EGW are skewed towards females, the use of EGW may hinder efforts to rehabilitate natural recruitment. A deductive experimental and observational framework will be used to test for mechanism(s) influencing the high percentage of females in the hatchery product to inform protocols to better balance sex ratios.