Estimate of Lamprey-Induced Mortality in a Lake Trout (Salvelinus namaycush) Population

Abstract

A competing risk model was applied to mortality occurring in a lake trout population subjected to fishing and lamprey parasitism. Two indices are proposed as a measure of lamprey-induced mortality in the lake trout (Salvelinus namaycush) population: the average number of wounds per fish (W/F) and the average number of scars per fish (S/F) accumulated by a cohort over a period of 1 yr. These indices were used as a measure of the force of lamprey-induced mortality. Fishing mortality was estimated from angler returns of tags on lake trout. Tag returns were also used in a maximum likelihood method to estimate survival of lake trout. Fishing and lamprey-induced mortality components of the model were analyzed by linear regression methods with the force of total mortality (i) as the independent variable. The component for lamprey-induced mortality (either W/F or S/F) did not contribute significantly to the reduction in variation of the force of total mortality, but the component for fishing mortality was significant. A negative coefficient for some analyses associated with the component for lamprey-induced mortality suggested a possible conditioning influence by other factors. Growth of lake trout was examined as a possible conditioning factor but analyses suggested that growth did not influence survival. Lamprey did not apparently cause any appreciable mortality in the lake trout population, age 4 and older, in Cayuga Lake during the years studied. It was not possible to consider younger age-groups of lake trout. Keywords: lake trout, lamprey, mortality