Using strain-specific genetic information to estimate the reproductive potential of lake trout spawning biomass in southern Lake Michigan

Lake trout reproduction has increased in Lake Michigan since the 2000 s. Previous genetic studies reported that the strains of stocked adults did not contribute equally to wild recruits. Consequently, reproductive potential of spawning biomass estimated in stock assessments will depend upon strain composition, complicating comparisons across time and space. We integrated data from a stock assessment with genetic data to estimate an effective lake trout spawning biomass that accounts for strain-specific reproductive efficiency. A reproductive power index (RPI) was developed for six strains of hatchery-reared lake trout using genetic data from lakes Michigan and Huron. The RPI is the ratio of the observed to expected genetic contribution of a strain to wild recruits. The Seneca Lake strain had the highest RPI, followed by Lake Manitou, Lewis Lake, Green Lake, Lake Superior, and Lake Huron strains. The RPI in southern Lake Michigan was 2.56 for Seneca Lake, 0.74 for Lake Superior, 0.50 for Lewis Lake, and 0.32 for Green Lake strains. Strain-specific effective spawning biomass in southern Lake Michigan was estimated using numbers stocked, population demographics from a stock assessment, and RPI to develop an annual effective spawning biomass index (ESBI) as a measure of reproductive potential. After 1996, ESBI increased faster than spawning biomass, and continued to increase when spawning biomass leveled off, reflecting the shift toward lake trout strains with higher RPI. The contribution to the ESBI after 2010 was 46 % Seneca Lake, 34 % wild adults, 12 % Lake Superior, and 4 % for the Lewis Lake and Green Lake strains.