Abundance estimates of cetaceans from a line-transect survey within the U.S. Hawaiian Islands Exclusive Economic Zone


Amanda L. Bradford, Karin A. Forney, Erin M. Oleson, and Jay Barlow
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A ship-based line-transect survey was conducted during the summer and fall of 2010 to obtain abundance estimates of cetaceans in the U.S. Hawaiian Islands Exclusive Economic Zone (EEZ). Given the low sighting rates for cetaceans in the study area, sightings from 2010 were pooled with sightings made during previous line-transect surveys within the central Pacific for calculating detection functions, which were estimated by using a multiple-covariate approach. The trackline detection probabilities used in this study are the first to reflect the effect of sighting conditions in the central Pacific and are markedly lower than estimates used in previous studies. During the survey, 23 cetacean species (17 odontocetes and 6 mysticetes) were seen, and abundance was estimated for 19 of them (15 odontocetes and 4 mysticetes). Group size and Beaufort sea state were the most important factors affecting the detectability of cetacean groups. Across all species, abundance estimates and coefficients of variation range from 133 to 72,528 and from 0.29 to 1.13, respectively. Estimated abundance is highest for delphinid species and lowest for the killer whale (Orcinus orca) and rorqual species. Overall, cetacean density in the Hawaiian Islands EEZ is low in comparison with highly productive oceanic regions.