Declining growth, condition of Lake Michigan lake whitefish creates concern for fishery

Study finds lack of specific food source just one factor in complex issue.

Great Lakes lake whitefish is an important species in the Great Lakes commercial fisheries. Photo: Ron Kinnunen, Michigan Sea Grant

Great Lakes lake whitefish is an important species in the Great Lakes commercial fisheries. Photo: Ron Kinnunen, Michigan Sea Grant

Lake whitefish is the most important fish species harvested in the commercial fisheries in the upper Great Lakes. Diporeia spp., a small, shrimp-like crustacean, was an important energy-rich prey resource for lake whitefish before their decline in lakes Michigan and Huron after the arrival of zebra and quagga mussels. Both of these native species play an important role in the benthic food web, connecting pelagic primary productivity to harvestable fish destined for human consumption. In the early 2000s the growth and condition of lake whitefish from Lake Michigan declined thus creating a concern for the fishery.

A concurrent decrease in lake whitefish condition and Diporeia spp. abundance in Lake Michigan recently spurred investigations into possible links between the two phenomena.

A team of fisheries researchers from the University of Waterloo, Fisheries and Oceans Canada, Environment and Climate Change Canada, University of Alaska Fairbanks, Michigan Sea Grant, and the Great Lakes Fishery Commission examined female lake whitefish δ13C and δ15N stable isotopes, growth, reproductive investment, dorsal muscle total lipid and docosahexaenoic acid (DHA) contents from lakes Erie, Michigan and Superior to determine whether differences in food source were correlated with measures of stock success. Results were published in the Journal of Great Lakes Research in a paper “Lake whitefish energy and nutrient partitioning in lakes Michigan, Erie, and Superior.”

The study found that lake whitefish stocks with higher growth rates and mean reproductive potential had higher energy stores in terms of percent total lipid. Stocks with low muscle lipid concentration had smaller egg sizes as egg number increased. Diet varied among stocks as evidenced by δ13C and δ15N stable isotope analyses; however, muscle total lipid and DHA were not correlated to apparent Diporeia spp. prey use

When compared to stocks from lakes Erie and Superior, Lake Michigan stocks had lower growth, reproduction, and lipid stores. While stocks in Lake Michigan with access to declining Diporeia spp. populations may still feed on the amphipod, it appears that they are unable to consume the quantities necessary to maintain historical growth and reproduction. Stable isotope analyses of lakes Erie and Superior stocks, with higher growth rates and lipid values, indicated different feeding strategies with no indication of reliance on Diporeia spp.

While differences in prey resources may have an effect on lake whitefish stocks, differences in Diporeia spp. abundance alone cannot explain differences in lake whitefish condition observed among the Great Lakes included in this study. Given previous evidence of varied diets, the linkages between diet and declining condition in lake whitefish may be more complex than the lack of availability of a single prey resource.

Michigan Sea Grant helps to foster economic growth and protect Michigan’s coastal, Great Lakes resources through education, research and outreach. A collaborative effort of the University of Michigan and Michigan State University and its MSU Extension, Michigan Sea Grant is part of the NOAA-National Sea Grant network of 33 university-based programs.