The Harlequins main predator is man. Over hunting, poaching, oil and hydro developments have drastically reduced the populations. Population Decline Most of the decline in the eastern North America population of Harlequins occurred before 1920. Newfoundland had healthy wintering populations until the 1950's, however, these also have declined dramatically since then. The Harlequin Duck is known for its tameness, inshore habitat use, and hence vulnerability to hunters compared to other sea ducks.
Oil is disastrous to any bird which uses marine habitats.
Although few oiled Harlequins have been observed, this source of mortality could be a factor in the decline of the species.