Resale or republication not permitted without written consent of the publisherAdult female and young Pacific walruses Odobenus rosmarus divergens resting on sea ice during summer in the Chukchi Sea. Walrus at front left has a radio tag attached.

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Pacific walruses (Odobenus rosmarus divergens) make trips from ice or land haul‐out sites to forage for benthic prey. We describe dive and trip characteristics from time‐depth‐recorder data collected over a one‐month period during summer from four male Pacific walruses in Bristol Bay, Alaska. Dives were classified into four types. Shallow (4 m), short (2.7 min), square‐shaped dives accounted for 11% of trip time, and many were probably associated with traveling. Shallow (2 m) and very short (0.5 min) dives composed only 1% of trip time. Deep (41 m), long (7.2 min), square‐shaped dives accounted for 46% of trip time and were undoubtedly associated with benthic foraging. V‐shaped dives ranged widely in depth, were of moderate duration (4.7 min), and composed 3% of trip time. These dives may have been associated with navigation or exploration of the seafloor for potential prey habitat. Surface intervals between dives were similar among dive types, and generally lasted 1–2 min. Total foraging time was strongly correlated with trip duration and there was no apparent diel pattern of diving in any dive type among animals. We found no correlation between dive duration and postdive surface interval within dive types, suggesting that diving occurred within aerobic dive limits. Trip duration varied considerably within and among walruses (0.3–9.4 d), and there was evidence that some of the very short trips were unrelated to foraging. Overall, walruses were in the water for 76.6% of the time, of which 60.3% was spent diving.

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Understanding haul-out behavior of ice-associated pinnipeds is essential for designing and interpreting population surveys and for assessing effects of potential changes in their ice environments. We used satellite-linked transmitters to obtain sequential information about location and haul-out state for Pacific walruses, Odobenus rosmarus divergens (Illiger, 1815), in the Bering Sea during April of 2004April of , 2005April of , and 2006. We used these data in a generalized mixed model of haul-out bout durations and a hierarchical Bayesian model of haul-out probabilities to assess factors related to walrus haulout behavior, and provide the first predictive model of walrus haul-out behavior in sea ice habitat. Average haul-out bout duration was 9 h, but durations of haul-out bouts tended to increase with durations of preceding in-water bouts. On average, tagged walruses spent only about 17% of their time hauled out on sea ice. Probability of being hauled out decreased with wind speed, increased with temperature, and followed a diurnal cycle with the highest values in the evening. Our haul-out probability model can be used to estimate the proportion of the population that is unavailable for detection in spring surveys of Pacific walruses on sea ice.Résumé : Il est essentiel de comprendre le comportement d'échouerie chez les pinnipèdes associés aux glaces pour planifier et interpréter les inventaires démographiques et pour évaluer les effets des changements potentiels dans leurs environnements glacials. Nous avons utilisé des émetteurs reliés aux satellites pour obtenir des informations séquentielles sur la position et l'état d'échouerie de morses du Pacifique (Odobenus rosmarus divergens (Illiger, 1815)) dans la mer de Béring en avril 2004de Béring en avril , 2005de Béring en avril et 2006. Ces données utilisées dans un modèle de mélange généralisé de durée des épisodes d'échouerie et un modèle hiérarchique bayésien des probabilités d'échouerie nous servent à évaluer les facteurs reliés aux comportement d'échouerie chez les morses et à mettre au point le premier modèle prédictif sur le comportement d'échouerie chez les morses dans un habitat de glace de mer. Les morses marqués passent en moyenne 9 h par épisode d'échouerie, mais les durées des périodes d'échouerie ont tendance à s'allonger en fonction de la durée des périodes précé-dentes passées en mer. Globalement, les morses marqués passent en moyenne seulement 17 % de leur temps en échouerie sur la glace de mer. La probabilité d'échouerie diminue en fonction de la vitesse du vent, augmente avec la température et suit un cycle journalier avec un maximum en soirée. Notre modèle de probabilité des échoueries peut servir à estimer la proportion de la population qui est à l'abri de la détection lors des inventaires printaniers des morses du Pacifique sur la glace de mer.[Traduit par la Rédaction]