Bioenergetics Evaluation of the Fish Community in the Western Arm of Lake Superior in 2004

Abstract

Lake Superior's fish community continues to change as a result of the recovery of populations of lake trout Salvelinus namaycush, the naturalization of introduced salmonids Oncorhynchus spp. and Salmo spp., declines in the populations of rainbow smelt Osmerus mordax, and the fluctuating recruitment of cisco Coregonus artedi. This study used bioenergetics modeling of the dominant predator fish in the western arm of Lake Superior in 2004 to provide a comprehensive picture of the relationship between predator demand and prey fish availability. The results, presented for nearshore and offshore areas and three geographically distinct ecoregions, indicate that the western arm is at or near its carrying capacity for predators. Estimated predator demand was about one-half the annual biomass plus production of coregonines but exceeded the biomass plus production of rainbow smelt, possibly because of underestimates of this species in shallow areas and recent shifts in predator diets. Lean lake trout were responsible for most of the consumption of these prey fish, while the deepwater siscowet form ranked second. The commercial harvest of prey fish biomass was trivial compared with the consumption by predator fish. Chinook salmon Oncorhynchus tshawytscha, along with other nearshore potamodromous species, played minor roles in total consumption. Chinook salmon are an indicator of forage status, as they have responded to declines in the rainbow smelt biomass with a dramatic diet shift to ciscoes and a decline in weight at age since the early 1990s. Slightly reduced lean lake trout growth and density-dependent survival in some areas also indicate that competition for prey is intense. Because most of the predators in the western arm are wild fish, stocked predators no longer have a great impact on prey populations. Bioenergetics modeling of predator demand combined with hydroacoustic estimates of prey supply can be valuable for managing the fisheries of large lakes, allocating resources to various interest groups, and increasing basic understanding of lake production and community dynamics.