Comparison of the Feeding Ecology of Great Lakes and North Pacific Adult Chinook Salmon: Implications of Dramatic Prey Declines and Nutritive Quality on Sustaining Introduced Populations

Chinook salmon (Oncorhynchus tshawytscha) diets in Lake Huron (Laurentian Great Lakes) have undergone dramatic changes in prey consumption over time, both with prey species composition and nutritive quality. Chinook salmon were introduced into the Great Lakes in the 1960s, in concert with efforts to reduce invasive alewife (Alosa pseudoharengus) populations.  Our three-year study (2009-2011) of angler-caught piscivores in Lake Huron, compared to results of a study 25 years prior, revealed how Chinook diets have responded to an unprecedented decline of forage fish, particularly alewife. The decline of pelagic prey fish has markedly reduced the number and nutritive quality of prey. Our results indicate that alewife declines have not been offset by consumption of other prey fish or invertebrates. We then compared these results to studies of Chinook diets in their native areas of the North Pacific. While pelagic vertebrate prey form an important part of the Chinook diet in both regions, Lake Huron lacks important large pelagic invertebrates that can act as a buffer when vertebrate prey are scarce. In Lake Huron, we suspect that changes in energy pathways as well as physical factors affecting prey fish recruitment have contributed to declines in Chinook condition and abundance.