Surf scoter (Melanitta perspicillata) and prey size

This paper examines the importance of prey size to shifting scoter populations in two bays in north Puget Sound.

Surf scoters in Padilla bay, seen through a spotting scope. Photo from the Washington Department of Ecology.
Surf scoters in Padilla bay, seen through a spotting scope. Photo from the Washington Department of Ecology.

The surf scoter (Melanitta perspicillata) is a sea duck common to bays and estuaries on the Pacific Northwest coast. Anderson and Lovvern observed populations in Padilla Bay and Penn Cove, both areas heavily frequented by scoters. Penn Cove is mostly unvegetated, while Padilla Bay contains one of the largest contiguous seagrass beds in the region. The researchers predicted that seasonal changes in prey biomass at each site would explain shifts in the number of surf scoters, which have been observed leaving Penn Cove in midwinter, when they do not ordinarily migrate. The prey available at Penn Cove is mostly mussels, while Padilla Bay hosts a greater diversity of crustaceans. Bivalves are a valuable prey source for scoters, but they do consume a wide range of other macroinvertebrates.

The study found that scoter population decreased at Penn Cove as the supply of smaller mussels was exhausted. Scoters consume the entire mussel, so smaller specimens reduce the cost to the birds of processing the shells. During the spring and early summer, scoters concentrated at Padilla Bay, where they consumed mostly epifaunal crustaceans and gastropods. The researchers observed that total prey biomass and distribution were less important to shifting scoter populations than prey size, and that seagrass beds provide an important supplement when the size fraction of mussels that are most profitable to scoters decline.

Read the full paper:

Anderson, E.M. and J. R. Lovvorn. 2012. Seasonal dynamics of prey size mediate complementary functions of mussel bed and seagrass habitats for an avian predator. Marine Ecology Progress Series 467: 219-232. doi:10.3354/meps09943.

 

About the Author: 
Both authors are affiliated with the Department of Zoology and Physiology at the University of Wyoming.