Factors affecting growth of lake whitefish in the upper Laurentian Great Lakes

Lake whitefish support the single largest and most valuable commercial fishery in the Laurentian Great Lakes. Recently, fishery managers have reported declining growth and productivity of lake whitefish in the upper Great Lakes. Several causes for the declines noted in individual growth rates have been proposed. These include changes in: 1) lake whitefish density, 2) food quality and abundance, 3) population genetics, and 4) climatic conditions. We evaluated the relationships between each of these factors and lake whitefish growth in selected regions of the upper Great Lakes. Specifically, we examined the timing of the changes in the environment with lake whitefish growth to determine causal relationships. Lake whitefish growth declines began with the development of a very strong 1991 year class due to favorable climatic conditions, leading to density dependent growth dynamics, which were exacerbated by a significant decline in the high-energy, benthic prey item (Diporeia spp.) toward the latter part of the 1990's. It appears that declines in Diporeia density, which are related to the introduction of two invasive species (Dreissena spp.) have resulted in a lower carrying capacity for lake whitefish in the upper Great Lakes. As such, managers need to implement conservative harvest strategies that protect the viability of these stocks under lower productivity conditions.