Diets of Lake Michigan Salmon and Maximum Size of Alewife Prey

ABSTRACT

The diets of Lake Michigan salmon were determined by examining stomach contents of sport-caught salmon. The study was conducted from 2001 through 2008 when the population of principal forage (i.e., the alewife) was declining and there was a decrease in salmon stocking. The largest alewives were measured to determine upper size limits utilized by salmon. Sixty-four charter boat trips were taken with a total catch of 713 salmon (430 coho salmon, 210 chinook salmon, 34 rainbow trout, 27 lake trout, 11 brown trout, and one pink salmon). Small alewives were the predominant prey of salmon. Of the coho salmon stomachs with prey, 97.7% contained alewives, and for chinook salmon, 99.5 % contained alewives. Other occasional prey items were smelt, sticklebacks, yellow perch, and blackflies. No round gobies were found in the diet. The largest alewives in the diets were 178 mm and only 9.6% of the alewives were greater than 152 mm. Coho salmon had fewer empty stomachs (13%) than chinook salmon (51.8%), and coho salmon had a more diverse diet than chinook salmon.