Trends in relative survival of hatchery-reared Chinook Salmon stocked in Lake Michigan and Lake Huron

ABSTRACT Objective Chinook Salmon Oncorhynchus tshawytscha and other salmonines were stocked in the Laurentian Great Lakes during the 1960s to develop and diversify sport fisheries and capitalize on abundant Alewife Alosa pseudoharengus populations that flourished from the collapse of native Lake Trout Salvelinus namaycush stocks. Stocking programs have continued to the present day, but declines in Alewife abundances in Lakes Michigan and Huron have raised concerns over the long-term sustainability of the Chinook Salmon fishery. We sought to describe spatial and temporal trends in relative survival of Chinook Salmon stocked in these lakes. Methods We examined open-water (April–July) recoveries of coded-wire-tagged Chinook Salmon of the 2011–2016 year-classes in sport fisheries to evaluate the effects of stocking location and year-class on relative survival in Lake Michigan and in the U.S. waters of Lake Huron. Relative survival was inferred from return rates measured as catch per effort per 100,000 fish stocked. Results Chinook Salmon stocked along the Wisconsin shoreline of Lake Michigan had the highest relative survival in the study area, while fish stocked in Green Bay and central Michigan waters of Lake Michigan had the lowest relative survival. Relative survival of fish stocked elsewhere along the Michigan shoreline of Lake Michigan, in northern Lake Michigan, and in Lake Huron was moderate and not statistically different from survival in other stocking regions. Relative survival generally decreased over time within the 2011–2016 year-classes but was positively correlated with published spatial patterns in Chinook Salmon growth rates and yearling and older Alewife availability. We speculate that factors such as hatchery effects, forage availability for age-0 fish, Alewife distribution patterns, water temperatures, predation, and competition between hatchery and wild-produced fish may have contributed to the regional patterns in Chinook Salmon relative survival. Year-class survival patterns in Chinook Salmon were likely a result of fluctuating Alewife abundances in Lake Michigan. Conclusions Our findings are consistent with previous works that highlighted the importance of Alewife in understanding Chinook Salmon population dynamics and postulated the role of environmental characteristics associated with stocking locations. These results will provide critical information to managers setting stocking levels and making other decisions and will enhance understanding of the mechanisms that shape the predator–prey balance in the Great Lakes.