Dynamics of individual growth in a recovering population of lake trout (Salvelinus namaycush)

In 1976, the Wisconsin Department of Natural Resources established a refuge for a nearly depleted population of lake trout (Salvelinus namaycush) at G ull Island Shoal, Lake Superior. The refuge was intended to reduce fishing mortality by protecting adult lake trout. We examined the growth dynamics o f these lake trout during the period of recovery by comparing estimates of individual growth before and after the refuge was established. Our estimate s are based on an annual mark-recapture survey conducted at the spawning ar ea since 1969. We developed a model that allowed mean growth rates to diffe r among individuals of different sizes and that accommodated variation in g rowth rates of individuals of the same size. Likelihood ratio tests were us ed to determine if the mean growth increments of lake trout changed after t he refuge was established. Our results suggest that growth of mature lake t rout (particularly wild fish) decreased significantly in the postrefuge per iod. This decreased growth may have been associated with a reduction in foo d availability. We also observed reductions in growth as wild fish grew old er and larger, which suggests that the growth of these fish may be adequate ly approximated by a von Bertalanffy growth model if it becomes possible to obtain accurate ages.