We compared density and morphology of wild mussels Mytilus trossulus growing naturally on shellfish farming structures to that of mussels from nearby intertidal areas, and considered the relevance to molluscivorous sea ducks. Mussel density on aquaculture structures greatly exceeded that of intertidal areas and farm mussels were larger, had lower shell mass, and had weaker byssal attachments. Differences in environmental conditions and predation regimes throughout the summer growing season were likely responsible for these differences. These morphological and density differences, in turn, have important implications for their primary winter predators, sea ducks, including surf scoters Melanitta perspicillata and Barrow's goldeneyes Bucephala islandica. Higher levels of mussel depletion in farm habitats and strong associations of ducks with shellfish farms indicate that sea ducks responded to this novel, profitable prey resource. Our results support recent research that indicates that artificial structures introduced into coastal areas create novel marine habitats that can support unique communities of epibiota. That sea ducks appear to profit from the introduction of aquaculture structures is evidence of a positive effect of anthropogenic modifications in a coastal environment.

Understanding interconnectivity among wintering, stopover, and breeding areas of migratory birds is pivotal to discerning how events occurring in each might have a cross-seasonal effect on another. Such information can guide the location and timing of conservation efforts. Thus, we examined spring migration routes, chronology, and stopover use of 85 surf scoters (Melanitta perspicillata (L., 1758)) marked with satellite transmitters at four Pacific Flyway wintering sites: San Quintin Bay, Baja California; San Francisco Bay, California; Puget Sound, Washington; and Strait of Georgia, British Columbia. Eighty-three percent of marked scoters followed two main routes to the breeding area: a Southern Inland route involving staging in Puget Sound and Strait of Georgia and protracted inland migration, or a Northern Coastal route characterized by short movements along the Pacific coast of British Columbia and southeast Alaska with inland migration initiating from Lynn Canal and surrounding areas. Route choice was related to nesting site latitude in the Canadian Northern Boreal Forest. Data from birds tracked over 2 years indicated strong migration route fidelity, but altered chronology and stopover locations between years. Departure date varied by wintering site, but arrival and apparent settling dates were synchronous, suggesting individuals adjusted migration timing to meet an optimized reproductive schedule.Résumé : La compréhension de l'interconnectivité entre les zones d'hivernage, d'arrêt et de reproduction chez les oiseaux migrateurs est essentielle si l'on veut voir comment les événements qui se passent dans chacune de ces zones peuvent avoir un effet lors d'une autre saison dans une autre zone. De telles informations pourraient orienter le choix des sites et du moment des interventions de conservation. Nous avons ainsi examiné les routes printanières de migration, leur chronologie et l'utilisation des points d'arrêt chez 85 macreuses à front blanc (Melanitta perspicillata (L., 1758)) munies d'émet-teurs satellites à quatre sites d'hivernage sur la route de migration du Pacifique, la baie de San Quintin en BasseCalifornie, la baie de San Francisco en Californie, Puget Sound au Washington et le détroit de Géorgie en ColombieBritannique. Quatre-vingt trois pourcent des macreuses marquées suivent deux voies principales vers la zone de reproduction, une route du sud dans l'intérieur des terres comportant des arrêts à Puget Sound et au détroit de Géorgie avec une migration prolongée à l'intérieur des terres et une route côtière du nord caractérisée par de courts déplacements le long de la côte de la Colombie-Britannique et du sud-est de l'Alaska et une migration à l'intérieur des terres débutant au canal de Lynn et les environs. Le choix de route dépend de la latitude du site de nidification dans la forêt boréale du nord du Canada. Des données obtenues d'oiseaux marqués au cours de deux années indiquent une forte fidélité à la route de migration, mais la chronologie de la migration et les sites d'arrêt peuvent changer...

Recent studies have documented strong, top-down predation effects of sea ducks on mussel populations in rocky intertidal communities. However, the impact of these gregarious predators in soft-bottom communities has been largely unexplored. We evaluated effects of predation by wintering surf scoters Melanitta perspicillata and white-winged scoters M. fusca on clam populations in soft-bottom intertidal habitats of the Strait of Georgia, British Columbia. Specifically, we documented spatial and temporal variation in clam density (clams m -2 ), scoter diet composition, and the consequences of scoter predation on clam abundance. Of the 3 most numerous clams, Manila clams Venerupis philippinarum and varnish clams Nuttallia obscurata were the primary prey items of both scoter species, while clams of the genus Macoma were rarely consumed by scoters. Between scoter arrival in the fall and departure in the spring, Manila clams decreased in density at most sample sites, while varnish clam densities did not change or declined slightly. Our estimates of numbers of clams consumed by scoters accounted for most of the observed declines in combined abundance of Manila and varnish clams, despite the presence of numerous other vertebrate and invertebrate species known to consume clams. For Macoma spp., we detected an over-winter increase in density, presumably due to growth of clams too small to be retained by our sieve (< 5 mm) during fall sampling, in addition to the lack of predation pressure by scoters. These results illustrate the strong predation potential of scoters in soft-bottom intertidal habitats, as well as their potentially important role in shaping community structure.

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We analysed variation in body mass of adult female Harlequin Ducks ( Histrionicus histrionicus (L., 1758)) on coastal wintering sites in southern British Columbia, Canada, to investigate nutrient acquisition prior to migration and reproduction. On average, female mass increased by 7% from late winter to premigration; however, the chronology of mass gain varied depending on prey type. Females feeding on superabundant roe from spawning Pacific herring ( Clupea pallasii Valenciennes, 1847) became considerably heavier than they had been before the herring spawning period (March) and appeared to be heavier than females eating marine invertebrates such as crabs, limpets, and snails during the herring spawning period. By mid-April, prior to migration, females at all sites had similar body masses, with birds at sites without spawn gaining mass and those at sites with spawn maintaining their earlier mass gain. Stable isotope analyses confirmed that birds at these different sites consumed very different diets. These results suggest that female Harlequin Ducks target an optimal premigratory body mass, regardless of whether they have access to a superabundant food source; this body mass is presumably shaped by the costs and benefits of nutrient storage for migration and reproduction.

Stable isotope analyses of animal tissues can be used to infer diet through application of mixing models. An important component in a mixing model is the incorporation of stable isotope discrimination factors so that isotopic shifts between diet and tissues built from the diet can be accounted for when comparing tissues to potential food sources. We determined the stable carbon and nitrogen isotopic discrimination factors between lipid-free diet and blood plasma, cellular blood, and adult chest contour feathers for captive female Spectacled Eiders ( Somateria fischeri (Brandt, 1847)). Mean discrimination factors for blood components and feathers were either similar or slightly larger compared with previously studied species. Additionally, we determined the stable carbon isotope discrimination factors between dietary lipids and adipose tissue fatty acids using three adipose tissue biopsies from captive male Spectacled Eiders that were fed three different diet treatments. Isotopic signatures of adipose tissue fatty acids closely reflected shifts in the diet and were either similar to or increased relative to diet. Our study provides a foundation for research using tissues as end-members in stable isotope nutrient allocation models and foraging ecology studies of Spectacled Eiders, and will provide the most applicable isotope data to date for sea ducks.

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