Relative roles of eelgrass vs. hard-mixed substrates as habitatfor scoters (Melanitta spp.) in Puget Sound

Author(s): 
Eric Anderson
University of Wyoming, Department of Zoology and Physiology
National Oceanic and Atmospheric Administration (NOAA)
Padilla Bay National Research Reserve
Publication Date: 
2011

Scoters ( Melanitta spp. ) are sea ducks that breed in freshwater wetlands of Canada and Alaska and winter in estuaries and bays on the Pacific and Atlantic coasts (Fig. 1). As with most sea ducks, scoter populations have declined in many parts of their Pacific Coast ranges. Three species of scoter can be found in wintering in Puget Sound, the common scoter ( Melanitta nigra ), the white-winged scoter ( Melanitta deglandi ) and the surf scoter ( Melanitta perspicillata ). The combined population of all three scoters declined by 57% between 1978-79 and 1992-99. Scoter habitat-use patterns, particularly in soft-bottom eelgrass ( Zostera spp.) beds are insufficiently characterized to identify if and how changes in habitat are affecting scoters.

Scoters are diving ducks and depend on availability of food sources on the sediment surface for food. Several factors suggest that scoter foraging profitability is greater in hard-mixed substrates than in soft-bottom eelgrass beds in early to mid-winter, but shifts in favor of eelgrass habitat in later winter. This conjecture is reflected by the location of the scoters throughout the winter. During early to mid-winter, scoter densities are greater in areas of hard-mixed substrate than in eelgrass beds. In late winter, substantial scoter populations arrive in Padilla Bay, which has one of the largest eelgrass beds on the Pacific Coast (Fig. 2). While the exact reason for this move is not known, large flocks of wintering sea ducks are known to deplete mussels seasonally in hard-mixed substrates. Thus, large eelgrass beds, such in Padilla Bay, may be important to sustaining regional scoter populations through the late winter and early spring.

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Author: Eric Anderson