Any species of the diving duck genus Mergus. Essentially freshwater birds, they are classified as a sea duck (tribe Mergini). Mergansers have a long body and a narrow, serrated, hooked bill for catching fish. The males of all but the common merganser, or goosander (M. merganser), are crested. The common merganser, the hooded merganser (M. cucullatus), the red-breasted merganser (M. serrator), and the smew (M. albellus), a small, compact merganser with a short bill, live in northern regions; the only southern species is the Brazilian merganser (M. octosetaceus). Mergansers are called trash ducks because their flesh is rank. See alsoshelduck.