Assessing Potential Impacts of Double-Crested Cormorants on Sportfish Populations in Saginaw Bay, Lake Huron

Stakeholders and fisheries managers expressed concern that Double-crested Cormorant predation in Saginaw Bay may be a factor in the recent low Yellow Perch abundance. Therefore, the objective of this study was to quantify cormorant diets from the two nesting colonies in Saginaw Bay.  Cormorants (n=303) were collected when returning to colonies after foraging from April-September. Diet items were identified, enumerated, and measured (n=9,554). Round Goby had the highest frequency of occurrence, followed by Yellow Perch, Emerald Shiner, Walleye, Gizzard Shad, and Freshwater Drum.  The highest consumption by biomass was Round Goby, followed by Yellow Perch, Walleye, Freshwater Drum, Gizzard Shad, Emerald Shiner, White Sucker, Pumpkinseed, and White Perch. At Little Charity Island, Yellow Perch consumption was highest in April and August, Walleye in May and September, Freshwater Drum in August, and Round Goby from May-September. Spoils Island cormorant diet biomass had a large proportion of Freshwater Drum in April and May, White Perch in May, Walleye in June, and Round Goby from July to September. Analyses relating cormorant consumption to Yellow Perch and Walleye population metrics provide context and assist in management decision making.