Stable Isotope Analysis of Lake Michigan Salmonines
BY MATTHEW KORNIS, GREEN BAY FWCO
Fisheries managers in the Great Lakes stock between 20 and 30 million salmonines (salmon and trout) each year to control invasive fishes, restore native populations, and support sport fisheries. In Lake Michigan, five different salmonine species (i.e., lake trout, Chinook salmon, coho salmon, steelhead/rainbow trout, and brown trout) are managed through stocking and harvest regulations. Although these species appear similar, they likely occupy different niches within the Lake Michigan food web, meaning that they may rely on different prey items or forage in different areas.
Researchers from the U.S. Fish and Wildlife Service (Service) Green Bay Fish and Wildlife Conservation Office, Great Lakes Fish Tag and Recovery Lab, and the United States Geological Survey (USGS) Great Lakes Science Center recently received funding from a competitive grant to use stable isotope analysis (SIA) to better understand foraging and movement patterns of Lake Michigan salmonines as well as their potential for competition. The two-year cooperative project will be led by Dr. Matthew Kornis and Charles Bronte from the Service’s Great Lakes Fish Tag and Recovery Lab and by Dr. David Bunnell from the USGS Great Lakes Science Center.
Unlike radioactive isotopes, which are harmful, stable isotopes of carbon and nitrogen are totally safe and naturally occurring in the environment. Carbon and nitrogen isotopes taken from small (grain of rice size) samples of fish muscle tissue provide unique signatures that can reveal a species’ feeding ecology and habitat use. In aquatic systems, carbon stable isotope values are depleted from offshore energy sources and enriched from nearshore energy sources, thereby providing information on foraging locations. By contrast, nitrogen stable isotope values increase up the food chain, and thus can be used to determine a species’ place within a food web. Combined, carbon and nitrogen isotopes can provide a clear picture of what a species eats and where a species feeds. Stable isotopes also offer a time-integrated picture of an animal’s diet: conventional analysis of stomach contents provides a snapshot of what an animal has eaten over the past several hours, while stable isotopes describe the diet over the past several months.
In this study, muscle tissue samples are being collected from fish captured by anglers willing to volunteer their catch, in conjunction with an ongoing effort coordinated by the Service’s Great Lakes Fish Tag and Recovery Lab to tag and recapture stocked salmonines. The research team hopes to identify size-specific patterns by collecting isotopes from small and large fish, and to pinpoint possible differences between stocked and wild lake trout and Chinook salmon by collecting isotopes from coded wire tagged and untagged fish. Using Lake Michigan as an example, the study will also compare salmonine foraging ecology from the heavily altered lower Great Lakes with the relatively intact Lake Superior food web (documented isotopically in a 2009 University of Wisconsin study). Finally, the research team will examine overlap in stable isotope signatures to determine the potential for competition among salmonines. The overlap between lake trout and other stocked salmonines will be emphasized in order to inform lake trout restoration efforts. Lake trout may occupy a relatively unique place within Lake Michigan's food web because of their reliance on bottom-oriented prey resources and their ability to consume a diverse diet throughout their native range. If this hypothesis is correct, this study’s results could ease perceived conflict between the restoration of a native Service trust species, lake trout, and non-native salmonine species commonly targeted by anglers.