The American Merganser, Common Loon, Great Blue Heron, Belted Kingfisher, American Bittern, mink, otter, and water snake collected from good trout waters in northern lower Michigan contained high proportions of trout in their diet. These trout were generally larger than other species of fish eaten. Size of trout eaten also varied with species of predator. Estimates of daily ration indicated that Common Loons ate about 2.4 pounds of trout per day when feeding in good trout waters. Estimates for the Great Blue Heron were l.5 pounds and 1.0 pound per day depending on whether the birds were frequenting streams or lakes. Winter-feeding American Mergansers ate about 0. 9 pound per day. The otter consumed about 0.7 pound per day. Other predators ate lesser but substantial amounts.

On the North Branch of the Au Sable River (and probably most trout waters) the predators take more trout annually than fishermen. Predators are in direct competition with anglers, particularly for the larger trout. It is believed that benefits would accrue to anglers through predator reduction on waters containing primarily trout.