Chinook Salmon Foraging Patterns in a Changing Lake Michigan

Abstract

Since Pacific salmon stocking began in Lake Michigan, managers have attempted to maintain salmon abundance at high levels within what can be sustained by available prey fishes, primarily Alewife Alosa pseudoharengus. Chinook Salmon Oncorhynchus tshawytscha are the primary apex predators in pelagic Lake Michigan and patterns in their prey selection (by species and size) may strongly influence pelagic prey fish communities in any given year. In 1994–1996, there were larger Alewives, relatively more abundant alternative prey species, fewer Chinook Salmon, and fewer invasive species in Lake Michigan than in 2009–2010. The years 2009–2010 were instead characterized by smaller, leaner Alewives, fewer alternative prey species, higher abundance of Chinook Salmon, a firmly established nonnative benthic community, and reduced abundance of Diporeia, an important food of Lake Michigan prey fish. We characterized Chinook Salmon diets, prey species selectivity, and prey size selectivity between 1994–1996 and 2009–2010 time periods. In 1994–1996, Alewife as prey represented a smaller percentage of Chinook Salmon diets than in 2009–2010, when alewife comprised over 90% of Chinook Salmon diets, possibly due to declines in alternative prey fish populations. The size of Alewives eaten by Chinook Salmon also decreased between these two time periods. For the largest Chinook Salmon in 2009–2010, the average size of Alewife prey was nearly 50 mm total length shorter than in 1994–1996. We suggest that changes in the Lake Michigan food web, such as the decline in Diporeia, may have contributed to the relatively low abundance of large Alewives during the late 2000s by heightening the effect of predation from top predators like Chinook Salmon, which have retained a preference for Alewife and now forage with greater frequency on smaller Alewives.

Received March 24, 2012; accepted October 1, 2012

ACKNOWLEDGMENTS

We thank Scott Hansen, Brad Eggold, and the Wisconsin Department of Natural Resources (DNR); Brian Breidert and the Indiana DNR; Steve Robillard and the Illinois DNR; and Jay Wesley and the MDNR for providing Chinook Salmon stomachs from fishing tournaments and fishery-independent gill-net surveys. We thank Marty Williams for helping coordinate Chinook Salmon collections and for biological data management. Jeff Holuszko and Emily Haug helped conduct diet analyses. We appreciate the contribution of the MDNR vessel crew members and researchers including Jerry Ranville, James Harris, Eric Crissman, Bryce Kucharek, and Brian Flood. Jean Adams assisted with statistical analyses. Mention of specific products does not constitute endorsement by the U.S. Government or the MDNR. This article is Contribution 1712 of the U.S. Geological Survey Great Lakes Science Center.