Monitoring the abundances of the prey fish populations is critical to managing the lake ecosystem. Presently, salmonines are stocked into the lake to supplement and sustain their populations within the lake. The chinook salmon (Oncorhynchus tshawytscha) population in Lake Michigan supports a highly valuable recreational fishery. Management decisions on stocking rates of salmonines in Lake Michigan hinge on assessments of the abundance and condition of the prey fish populations. If abundance of the prey fish populations is low and the estimated biomass of salmon and trout is high, then stocking rates of salmon and trout should be reduced. Additionally, management decisions on opening commercial fisheries for the alewife (Alosa pseudoharengus) in Lake Michigan strongly depend on the long-term monitoring of this prey fish population. Also, the annual monitoring of the lake is used to detect introductions of exotics, sudden population surges or sharp declines of the prey fish populations, poor recruitment of prey fish, and changes in prey fish growth. Moreover, the U. S. Fish and Wildlife Service has recently identified the long-term monitoring of the prey fish community of Lake Michigan as the top-priority research task of the Great Lakes Science Center for its Lake Michigan program. No other agency investigates the prey fish dynamics of Lake Michigan as does the Great Lakes Science Center through its bottom trawling surveys in the fall. The results of these prey fish surveys are used extensively by management agencies to forge their salmonine stocking and harvest regulation policies.

Published outcomes of the research: One paper has shown the lake trout still select for alewives in their diet in Lake Michigan, despite the dominance of bloater in the prey fish community (Madenjian et al. 1999). A second paper illustrates the high variability in PCB concentrations of alewife and bloater in Lake Michigan; these measurements of PCB concentration in prey fish are, by far, the most detailed and extensive measurements made to date (Madenjian et al. 1999). A third paper provides estimates on the uncertainty in the estimates of biomass for the various prey species (Krause et al. in prep.). This is part of on-going work being done in cooperation with researchers at Michigan State University. A study on the effects of reducing the spatial coverage of the Lake Michigan bottom trawl survey was performed by Fabrizio et al. (1996). A 6-year study of integration and application of acoustics to compliment the trawl surveys was completed by Argyle et al. (1998). Argyle et al. showed that the integration of acoustics added to the understanding of the status and trends of the fish community in Lake Michigan.

The long-term trend data for the fish community collected by the GLSC is now being used as a major portion of a Grat Lakes Fishery Commission sponsored workshop on Salmonid Communities in Oligotrophic Lakes (SCOL-2). SCOL-2 will attempt to identify the separate and joint effects of the three major anthropogenic stressors of cultural eutrophication, exploitation, and exotic species introductions on fish communities, especially salmonid communities, in the five Laurentian Great Lakes. With this SCOL-2 effort, it will also be attempted to identify linkages between the various trophic levels.

Lake Report: Status and Trends of Prey Fish Populations in Lake Michigan, 2005