Lake Trout Mortality and Abundance in Southern Lake Huron

Abstract

Populations of lake trout Salvelinus namaycush in the main basin of Lake Huron collapsed in the 1940s because of predation by sea lampreys Petromyzon marinus and commercial fishing. Efforts to rehabilitate lake trout have emphasized reduction of mortality and the stocking of hatchery-reared lake trout to reestablish populations. We fit a statistical catch-at-age model for lake trout in the southern main basin of Lake Huron using a maximum likelihood approach to estimate mortality rates and abundance during 1984–1993. This represents the first such analysis for lake trout in the Great Lakes, and a flexible application of the approach proved useful for integrating diverse information and assessing population and mortality trends. Sea lamprey-induced mortality and recruitment of lake trout to age 1 were calculated external to model fitting. Recruitment was based on numbers of lake trout stocked because natural recruitment is negligible. Sea lamprey-induced mortality rates were based on observed wounding data on lake trout. Other mortality rates were estimated during model fitting. Mortality rates have varied from year to year but do not show trends over time and are currently well below target maximum levels. Lake trout abundance has declined from a peak spawning biomass of 412,000 kg in 1988 to a low of 295,000 kg in 1993. This decline is a result of lower stocking levels since the mid-1980s. Sea lamprey predation was the dominant source of mortality for lake trout older than age 4. During 1984–1993, we estimated that sea lamprey predation caused 48% and fishing accounted for 16% of deaths of lake trout older than age 2. Although our estimates of sea lamprey-induced mortality are uncertain, current estimates of lethality of sea lamprey attacks would need to be greatly overestimated to overturn our conclusion that sea lamprey mortality has exceeded fishing mortality in southern Lake Huron.