Graduate Thesis Or Dissertation


Marine habitat use of black-footed and Laysan albatrosses during the post-breeding season and their spatial and temporal overlap with commercial fisheries Public Deposited

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  • Incidental bycatch in commercial fisheries has been identified as a threat to black-footed albatrosses (Phoebastria nigripes) and Laysan albatrosses (P. immutabilis). Effective long-term conservation of these albatrosses necessitates a thorough understanding of their marine distribution, which marine habitats are selected during foraging, and where they are most likely to interact with fisheries throughout the annual cycle. During August 2005 and July 2006 we captured and satellite-tagged black-footed and Laysan albatrosses at-sea near Seguam Pass, central Aleutian Islands, Alaska (52.08o N, 172.95o W). To quantify habitat use and assess potential risk from bycatch we integrated satellite-tracking data from seven black-footed albatrosses (all in 2005) and 18 Laysan albatrosses (2005: n = 9; 2006: n = 9) with remotely-sensed habitat variables and data on fishing effort and distribution from commercial fisheries in the North Pacific Ocean. We used first-passage time analysis and model selection to identify locations of area-restricted search (ARS), quantify the relationship between area-restricted search and marine habitat variables, and compare and contrast habitats used by the two albatross species. For both species ARS was inversely related to wind speed and depth and positively related to gradients in depth, sea surface temperature, and chlorophyll a concentration. Black-footed albatross ARS was inversely related to sea surface temperature and increased in continental margin habitats (shelf, shelf break, and slope) relative to oceanic habitats; Laysan albatrosses did not exhibit such strong relationships with sea surface temperature and seafloor depth. Area-restricted search for both albatrosses was greater in vertically mixed waters (e.g., coastal upwelling and frontal zones) compared to vertically stratified waters (e.g., oceanic waters, weak frontal boundaries). We found that despite these similarities in habitat-specific area-restricted search in relation to habitat variables, black-footed albatrosses were more varied in their habitat use compared to Laysan albatrosses. Black-footed albatrosses spent similar proportions of time among domains of the continental margin and oceanic waters; Laysan albatrosses were strongly associated with oceanic waters. Both albatross species spent the greatest proportion of time in subarctic and eutrophic waters, but when black-footed albatrosses dispersed from the Alaska region they entered subtropical and tropical waters with relatively low chlorophyll a concentrations (south of 45ºN). Laysan albatrosses, however, remained almost entirely within more nutrient rich and cooler subarctic and North Pacific Transition Domain waters. Spatial overlap with fisheries differed between albatross species and among fisheries. Black-footed albatrosses overlapped with fisheries throughout their range of tracked movements and there was little evidence that they spent time within regions devoid of fisheries. In the Alaskan Exclusive Economic Zone, fishing effort occurred almost entirely within the continental margin, thus overlapping with habitats more commonly used by black-footed albatrosses than by Laysan albatrosses. Potential fishery interaction for black-footed albatrosses was greatest with the sablefish (Anoplopoma fimbria) longline and pot fisheries and with the Pacific halibut (Hippoglossus stenolepsis) longline fishery. Black-footed albatrosses also overlapped with Canadian west coast fisheries and pelagic tuna (Thunnus spp.) and billfish (Xiphias gladius, Makaira spp., Tetrapturus spp.) longline fisheries in the North Pacific Transition Domain. Laysan albatrosses tracked in this study remained largely over oceanic waters, away from the concentration of fisheries within the continental margin, and north of pelagic longline fisheries in the transition domain. Regionally, the Laysan albatrosses tracked as part of this study had the greatest overlap with the Atka mackerel (Pleurogrammus monopterygius) trawl fishery in the western Aleutian Islands and the sablefish pot fishery in the central Aleutian Islands. Due to inter-specific differences in distribution and habitat use, the overlap of fisheries with the post-breeding distribution of black-footed albatrosses was greater than that for Laysan albatrosses. Consequently, the potential vulnerability to bycatch and population-level impacts from fisheries is greater for black-footed albatrosses than for Laysan albatrosses.
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