Evidence for Natural Reproduction by Stocked Walleyes in the Saginaw River Tributary System, Michigan

Abstract

A program of stocking walleyes Stizostedion vitreum into Saginaw Bay, Lake Huron, begun in the late 1970s by the Michigan Department of Natural Resources and sportsmen’s groups, resulted in large spawning runs in tributary streams in the early 1980s as water quality in the bay improved. Natural reproduction was documented by a 2‐year sampling effort for larval walleyes that revealed the presence of substantial numbers of larvae over a 15–18‐d period, with maximum densities (350‐3,500/1,000 m3) recorded on 21 April 1987 and 6 May 1988 in the Saginaw River and on 6 May 1988 on the Tittabawassee River. More larval walleyes were collected at night and near bottom (3 m) than during the day and near the surface (1 m). The Tittabawassee River, one of four major rivers that form the Saginaw, was a major source of walleye larvae collected in the Saginaw River, but some walleye larvae were also collected in the Shiawassee River. Over 500,000 walleye larvae passed by the study area on the Saginaw River in 24 h on 21 April 1987. Using discharge and average walleye density data, 1 calculated that approximately 3 million walleye larvae passed by the study area during the 20‐d period of occurrence in 1987 and approximately 28 million during a comparable period in 1988. I was able to confirm entry of some walleye larvae into Saginaw Bay with four tows at the river mouth during peak densities at the upriver study site. Adult walleye stocks are now abundant enough to produce substantial numbers of offspring, but environmental factors (flow reversals, poor water quality) or biological factors (predation, starvation) may be preventing successful recruitment to the Saginaw Bay sport fishery.