HABITAT PREFERENCE, ACCESSIBILITY AND COMPETITON LIMIT THE GLOBAL DISTRIBUTION OF BREEDING BLACK-BROWED ALBATROSSES
Résumé
Telemetry methods and remote sensing now make it possible to record the spatial usage of wide-ranging marine animals and the biophysical characteristics of their pelagic habitats. Furthermore, recent statistical advances mean such data can be used to test ecological hypotheses and estimate species' distributions. Black-browed albatrosses Thalassarche melanophrys are highly mobile marine predators with a circumpolar breeding and foraging distribution in the southern hemisphere. Although they remain relatively abundant, increased fisheries bycatch has led to their listing as endangered by conservation bodies. We satellite tracked 163 breeding black-browed albatrosses and eight closely-related Campbell albatrosses T. impavida from nine colonies. We then quantified habitat usage, and modeled population level spatial distribution at spatiotemporal scales >50 km and 1 month, as a function of habitat accessibility, habitat preference and intra-specific competition, using mixed-effects Generalized Additive Models. During incubation, birds foraged over a wider area than in the post-brood chick-rearing period, when they are more time-constrained. Throughout breeding, the order of habitat preference of black-browed albatrosses was for neritic (0-500 m), shelf break and upper shelf-slope (500 - 1000 m), and then oceanic (>1000 m) waters. Black browed albatrosses also preferred areas with steeper (>3°) bathymetric relief and in addition, during incubation, warmer sea surface temperatures (peak preference ~16°). Although this suggests specialization in neritic habitats, incubation stage black-browed albatrosses from South Georgia also foraged extensively in oceanic waters, preferring areas with high Eddy Kinetic Energy (>250 cm2/s2), especially the Brazil-Malvinas Confluence, a region of intense mesoscale turbulence. During chick-rearing this species, had a more southerly distribution, and following the seasonal retreat of sea ice birds from some populations utilized neritic polar waters. Campbell albatrosses showed similar bathymetric preferences but also preferred positive sea level anomalies. Black-browed albatross foraging areas were partially spatially segregated with respect to colony and region, with birds preferring locations distant from neighboring colonies, presumably in order to reduce competition between parapatric conspecifics. At the global scale, the greatest concentrations of breeding black-browed albatrosses are in southern South American neritic, shelf-break and shelf-slope waters. These regions also hold large fisheries and should therefore be a priority for introduction of bycatch mitigation measures.