Chinook Salmon, Alewife, and a Changing Lake Michigan

Since Pacific salmon stocking began in Lake Michigan, managers have attempted to maintain salmon abundance at levels near the maximum that can be sustained by available orage fishes, primarily alewife (Alosa pseudoharengus).  The trophic link between Chinook salmon (Oncorhynchus tshawytscha) and alewife is among the strongest and most exclusive of such links in the Great Lakes.  Because alewives are a short-lived, fast-growing, forage species subject to variable year-class strength and Chinook salmon are short-lived, fast-growing, predators that feed predominantly on alewives, patterns in Chinook prey selection (by species and size) may interact with alewife year-class strength to strongly influence available forage in any given year.  In 1994-1996, there were larger alewife, relatively abundant alternative prey species, and fewer dreissenid mussels in Lake Michigan than in 2009-2010, which was characterized by fewer and smaller alewives, fewer alternative prey species, a firmly established non-native benthic community, and the conspicuous absence of an important prey-fish forage species, Diporeia. We characterized Chinook salmon diets, prey species selectivity, and prey size selectivity between 1994-1996 and 2009-2010 time periods. In 1994-1996, alewife prey represented a smaller percentage of Chinook diets than in 2009-2010, when alewife composed over 90% of Chinook diets. Most of this increase in importance may be due to declines in alternative prey-fish species. The size of adult alewife eaten by Chinook salmon also decreased between 1994-1996 and 2009-2010, likely due to a decrease in alewife growth between the two time periods. Our analysis suggests that, in addition to declines in the alewife population in Lake Michigan, the abundance of large adult alewife has decreased, truncating the range of available forage for large Chinook and increasing predation pressure on smaller-sized alewife. We suggest that changes in the Lake Michigan food web, such as the decline in Diporeia, between 1994-1996 and 2009-2010 may have contributed to a decrease in alewife abundance.