Common loon (Gavia immer) catching a small fish to feed a chick. Adults are powerful divers that catch small fish in fast underwater chases. They mainly feed on fish, but will also take crayfish, shrimp, and other crustaceans; frogs; insect larvae; mollusks; and aquatic plants. Loons usually swallow their prey underwater, but will bring larger food items to the surface to feed the chicks. La Mauricie national park. Quebec. Canada