Recruitment and Growth Dynamics of Lake Trout in Western Lake Superior

Abstract

Lake trout Salvelinus namaycush stocks in Lake Superior are recovering from historic collapses through stocking, sea lamprey Petromyzon marinus control, and fishery regulation. My objectives were to determine the relative contribution of stocked and wild lake trout to recruitment in Minnesota and Wisconsin waters of Lake Superior during 1980-1995, and to determine if growth of wild lake trout changed significantly during 1980-2003 in Minnesota and Wisconsin waters of Lake Superior. To model recruitment, I used variants of the Ricker stock-recruitment model to evaluate production of age-7 lake trout (recruits) by age-8 and older wild and stocked parental lake trout (spawners). Density of wild lake trout spawners and recruits generally increased, while density of stocked lake trout spawners generally decreased in all management areas. Three competing models best described recruit density, two in which wild and stocked parents each contributed to recruitment, but stocked parents contributed negatively, and a third in which only wild parents contributed to recruitment. Recruitment rates declined significantly with increasing spawner density in four of five management areas, which suggests that recruitment is strongly density dependent in much of western Lake Superior. I conclude that stocked lake trout do not contribute to lake trout restoration and that stocking should be discontinued in three of five management areas in western Lake Superior. To evaluate changes in growth, I used von Bertalanffy length-age models and likelihood-ratio tests to determine if growth of adult lake trout varied temporally and spatially in Minnesota and Wisconsin waters during 1980-2003. Growth changed temporally in all areas, and differed between Minnesota and Wisconsin. Growth differed significantly between western and eastern Wisconsin waters in 7 of 15 years. Mean length-at-age-7 decreased significantly in eastern Wisconsin, but did not change significantly in western Wisconsin or Minnesota. I conclude that increasing abundance of wild lake trout affected growth dynamics of wild lake trout in western Lake Superior during 1980-2003. Spatial differences in growth indicate that management units should be treated as separate entities for purposes of fishery management.