Combining association and movement analyses of short-finned pilot whales in the Hawaiian Islands: evidence for multiple populations within the Hawaiian Archipelago

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Currently a single stock of short-finned pilot whales is recognized in Hawaiian waters, with an estimated abundance of over 12,000 individuals in the EEZ. However, preliminary genetic results suggest that pilot whales around the main Hawaiian Islands are reproductively isolated from those elsewhere in the tropical Pacific. Since the type and extent of anthropogenic interactions varies both among the main Hawaiian Islands and offshore, we test the hypothesis that there is only a single stock of this species in Hawai‘i using a combination of association analyses of photo-identification data and movement/habitat use analyses from satellite tags. Photographs over 13 years included over 6,500 identifications of 868 distinctive individuals. Satellite tag data from 64 deployments averaged 37 days (max=228 days). Combining tag and association analyses indicate individuals do not mix freely among the islands and suggest the existence of three socially isolated but partially overlapping insular communities and an offshore population. The ranges of the three insular communities are from Ni‘ihau to western O‘ahu, from O‘ahu to Lana‘i, and primarily off Hawai‘i Island. Tagged individuals from these insular communities (n=61 deployments) remained an average of 57.5 km from their tagging location, with a grand mean of depth used and distance from shore of 1,564 m and 15.2 km, respectively. Evidence for an offshore population comes from three individuals tagged in two isolated social clusters with mean distances from their tagging locations of 249 km, with a grand mean of depth used and distance from shore of 4,336 m and 81.1 km (max=381 km) respectively. Anthropogenic interactions vary among the population ranges, with long-line fishing in offshore waters, Navy activities primarily off Ni‘ihau, Kaua‘i and O‘ahu, and vessel-based whale watching and swim-with operations primarily off Hawai‘i Island. As each community is exposed to different anthropogenic threats they should be managed independently.