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Article Excerpt
ABSTRACT. Ivory gulls (Pagophila eburnea, Phipps, 1774), one of the world's least-known species, have declined throughout their range in recent years. This study describes the patterns of ivory gull use of the North Water polynya, a large polynya that occurs every year near ivory gull breeding sites on Ellesmere Island. Nunavut, Canada. We conducted at-sea surveys from Canadian icebreakers during the summers of 1997-99. In 1998, stomach contents of five ivory gulls were analyzed. We measured stable isotope ratios ([[delta].sup.13]C, [[delta].sup.15]N) of liver, muscle, feather and bone to determine how ivory gull diets vary during the year. We observed a total of 275 individuals, most of which were seen on the western side of the polynya. Flying was the predominant behavior (76% of individuals); other behaviors included sitting on ice, sitting on water, and feeding. Four juveniles were seen in August and September. Birds collected had arctic cod (Boreogadus saida) in their stomachs. Other food items included an otolith from an unidentified species of Liparid fish and two bones from a mole, presumed to be a star-nosed mole (Condylura cristata). Stable isotope results indicated that ivory gull trophic levels (TLs) were high (around TL 4 based on [[delta].sup.15]N) but varied over the season. From the three individuals whose feathers were analyzed, we conclude that differences among individuals in their winter diets or foraging locations are possible.
Key words: ivory gull, Pagophila eburnea, arctic cod, Boreogadus saida, North Water polynya, star-nosed mole, Condylura cristata, stable isotope analysis, carbon-13, nitrogen-15.
RESUME. Les mouettes blanches (Pagophila eburnea), soit I'une des especes les moins connues dans le monde, ont enregistre un declin a travers l'ensemble de leurs habitats ces dernieres annees. Cette etude decrit les habitudes des mouettes blanches dans la polynie des eaux du Nord, une grande polynie qui se developpe chaque annee pres des sites de reproduction de mouettes blanches sur I'ile Ellesmere, au Nunavut, Canada. Nous avons effectue des releves depuis des brise-glace canadiens au cours des etes 1997 a 1999. En 1998, le contenu des estomacs de cinq mouettes blanches a ete analyse. Nous avons egalement mesure les ratios d'isotopes stables ([[delta].sup.13]C, [[delta].sup.15]N) dans les foies, les muscles, les plumages et les os afin de determiner comment le regime alimentaire des mouettes blanches variait au cours de I'annee. Nous avons observe 275 individus, provenant principalement du cote ouest de la Polynie. La majorite (76 %) etaient en vol. Parmi les autres comportements observes, notons la presence d'oiseaux sur la glace, d'oiseaux sur l'eau et d'oiseaux en train de se nourrir. Quatre oiseaux juveniles ont ete reperes en aout et en septembre. Les oiseaux captures avaient de la morue polaire (Boreogadus saida) dans l'estomac. Les autres aliments trouves comprenaient un otolithe provenant d'une espece non identifiee de poisson Liparide, et deux os de taupe, probablement un condylure etoile (Condylura cristata). L'analyse des isotopes stables indique que les niveaux trophiques (NT) des mouettes blanches etaient eleves (NT 4 environ en fonction de [[delta].sup.15]N), mais variaient au cours de la saison. A partir des trois individus dont les plumes ont ete analysees, nous en avons conclu qu'il est possible qu'il existe des differences entre les regimes d'hiver ou les lieux de nourriture des individus.
Mots cles: mouette blanche, Pagophila eburnea, morue polaire, Boreogadus saida, polynie des eaux du Nord, condylure etoile, Condylura cristata, analyse des isotopes stables, carbone 13, azote 15
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INTRODUCTION
Recent at-sea surveys suggest that Canadian ivory gull (Pagophila eburnea, Phipps, 1774) populations are declining (Chardine et al., 2004). Local residents in Arctic Canada have also observed declines in this species (Mallory et al., 2003). Gilchrist and Mallory (2005) conducted aerial surveys of colony sites and reported an 80% decline in numbers since the 1980s. Since 2006, ivory gulls have been classified as Endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC, 2006). Mechanisms driving the decline of this species are unknown; hypotheses include contamination from anthropogenic sources (e.g., Braune et al., 2006, 2007), hunting {Stenhouse et al, 2004; Gilchrist and Mallory, 2005), relocation to new breeding sites (deemed unlikely because of the large extent of aerial surveys; Gilchrist and Mallory, 2005), increased predation with the loss of protective ice around nunatak nesting sites, and reduction of ice edges associated with feeding during winter (Krajick, 2003). As the ivory gull is associated with ice for much of its life history, climate change and sea ice declines (e.g., Comiso, 2002) may be major threats.
The North Water polynya in northern Baffin Bay attracts millions of seabirds every year (Fig. 1; Karnovsky and Hunt, 2002). The North Water is the largest (~ 80000 km2) recurrent polynya in the Arctic and one of the most important for seabirds and other upper-trophic level predators (Fig. 1; Stirling, 1980). As the most northerly polynya of its size, the North Water may be a crucial feeding ground for ivory gulls during their breeding season, when they are at the northernmost extent of their range. Breeding colonies have been reported in southeastern Ellesmere Island--the western edge of the North Water (Robertson et al., 2007). This study documents the distribution, abundance, and foraging ecology of ivory gulls in the North Water. We determined the at-sea distribution of ivory gulls in the North Water throughout their breeding season (over three years) and assessed the diets of five individuals using both stomach-content analysis and stable isotope analysis of different tissues. Stomach-content analysis provides an indication of the taxa and size of recently consumed prey. By measuring 5I3C values in different tissues, we determined the relative use of coastal (more enriched) or pelagic (more depleted) prey (Hobson et al., 1995). Stable nitrogen isotope values ([[delta].sup.15]N) show stepwise enrichment with trophic level and so reflect trophic position (Hobson and Welch, 1992; Michener and Schell, 1994). The period of temporal integration of diets using stable isotope tracers depends on the metabolic rate of tissues; the rate is high for tissues like liver and...
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