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Abstract:
The purpose of this study was to estimate the number of walruses in Svalbard during the ice-free summer period by (1) counting walruses on land at all known haul-out sites using aerial digital-photographic images and 2) developing a model based on the analysis of behavioural data collected from satellite relay data loggers to account for walruses in the water.All known terrestrial haul-out sites for walruses in Svalbard (n = 79) were surveyed during the period 1–3 August 2006, and sites that were in use (n = 17) were documented using digital photography. If walruses were present at a haul-out site, they were easily observed from the helicopter flying at about 300 m altitude, and digital photographs were taken of the hauled-out groups of animals. A total of 657 walruses were counted on land in the resultant images. To create a correction factor that would account for walruses in the water during the survey, an extensive behavioural data set (covering August 2002 to August 2005) was used, obtained from walruses equipped with satellite relay data loggers on their left tusk. The data loggers continually monitor the data their sensors are collecting (surface sensor, depth sensor, and time interactions) and group the animals’ activities into three different states: diving, hauled out, and at surface. A walrus is identified as diving when it is below a preset depth, in this case 6 m. Behaviour is classified as hauled out when the tag has been dry for at least 10 minutes, and this state ends when wet-dry sensors indicate wet for more than 40 seconds. A walrus at surface is neither diving nor hauled out, i.e., the animal enters this state when it remains above the depth threshold for diving but is not dry long enough to be classified as hauled out. The proportion of walruses at sea during the survey was calculated to be 0.750 on the basis of 28 thirty-day periods from 23 male walruses. The estimated total number of walruses in Svalbard in August 2006 was 2629 (95% CI: 2318– 2998).