Contrasts in nutrient metabolism and foraging strategies of surf and white-winged scoters in nearshore marine habitats

To contrast seasonal reliance on marine foraging resources for two closely related and declining species of sea duck, I coupled a synthesis of past studies with new observations of distributions, prey availability, diet, foraging behavior, and body condition in nearshore habitats along the Pacific Coast. Past studies of the marine diets of Surf Scoters (Melanitta perspicillata ) and White-winged Scoters (M. fusca) have reported that they eat mostly bivalves, but deviations from well-established methods by most studies suggest that the importance of soft-bodied prey has been underestimated for both species. Diets of Surf Scoters differed among three heavily used bays with distinct benthic habitats in northern Puget Sound, Washington, yet adherence to appropriate methods indicated that 67%--86% of the ash-free dry mass of esophagus contents from each bay was non-bivalve prey. A synthesis of previous and new diet data revealed differences between scoter species: relative to White-winged Scoters, Surf Scoters consume smaller bivalves, a smaller and more variable percentage of mollusk prey (including bivalves and gastropods), and a declining percentage of bivalves as winter progresses.;For Surf Scoters in these three Puget Sound sites, seasonal changes in abundance, body composition, and plasma metabolites indicated that foraging profitability was greater in a site with mussels (Mytilus trossulus ) in early winter, but shifted to become relatively greater during spring and summer in seagrass sites that either provided spawn of Pacific herring (Clupea pallasii) or epifaunal invertebrates for which body size had increased substantially since winter. Shifts in foraging sites during this period are common for Surf Scoters, and are likely driven by difficulty in meeting their seasonal energy demands; for Surf Scoters in this and past studies, overwinter changes in body mass were highly variable among sites and typically declined despite increased foraging effort.;In the Strait of Georgia, British Columbia, diets estimated from fatty acids and stable isotopes indicated that both scoter species gained mass by consuming herring spawn during late March to early April. However, compared to White-winged Scoters, Surf Scoters appear more reliant on spawn for recovering reserves depleted over winter and for preparing for migration and reproduction. At 11 spawning grounds in Puget Sound, Washington, the numerical response of scoters to spawn increased with increasing biomass of spawning herring; this response was four-fold greater for Surf Scoters than for White-winged Scoters after accounting for local differences in their abundances. The numerical response of Surf Scoters to spawn at these sites was even greater later in spring, when migrants appeared to use spawning sites as staging areas.;Contrary to the seasonally variable energetics and diet of Surf Scoters, White-winged Scoters display relatively stable distributions, diet, foraging rates, and body condition during winter. These patterns indicate that White-winged Scoters maintain a more constant energy balance regardless of changes in proximate environmental conditions. Greater inertia to seasonal environmental changes may relate to their 50% larger body size, which confers lower mass-specific energy costs and access to a wider size range of valuable bivalve prey. In marine conservation efforts for White-winged Scoters, it may be sufficient to establish site-level protections for nearshore habitats with high standing stocks of bivalve prey. However, preserving the suite of marine resources used by Surf Scoters will likely require a range of protection measures, and may be critical to sustaining their populations given the sensitivity of their body reserves to winter foraging conditions.