False killer whales around the main Hawaiian islands: an assessment of inter-island movements and population size using individual photo-identification

Citation

Baird, R.W., A.M. Gorgone, D.L. Webster, D.J. McSweeney, J.W. Durban, A.D. Ligon, D.R. Salden, and M.H. Deakos. 2005. False killer whales around the main Hawaiian islands: an assessment of inter-island movements and population size using individual photo-identification.Report prepared under Order No. JJ133F04SE0120 from the Pacific Islands Fisheries Science Center, National Marine Fisheries Service.

Summary

The current best estimate of population size for false killer whales within Hawaiian waters is only 268 individuals (Barlow 2003), though the estimate is not very precise (CV = 1.08). False killer whales are considered a “strategic” stock by the National Marine Fisheries Service, as “takes” in the Hawai‘i-based swordfish and tuna long-line fishery exceed the “Potential Biological Removal” (PBR) level. We studied false killer whales as part of small-boat based surveys for odontocetes around the main Hawaiian islands from 2000 through 2004, and in this report we assess inter-island movements, examine “mark” change over time on individual animals, estimate the proportion of marked individuals within the population, and provide a mark-recapture population estimate. Dedicated surveys for odontocetes were undertaken around all the main Hawaiian islands, and all groups of false killer whales encountered were approached and attempts made to photographically identify all individuals present. False killer whales were encountered on 14 occasions in directed surveys (2.9% of all odontocete sightings), in eight of the 10 months of the year surveyed, and in three of the four island-areas surveyed. Encounters were in a wide range of water depths (37 to 3,950 m). Photographs from seven opportunistic encounters were also available. Seventy-seven percent of individuals photographed were considered to have markings that could be recognized in the long-term (between-years). Seventysix individuals with such long-term markings were documented, 47 of which were seen on two or more occasions. Ten individuals were documented with mark changes, though the rate of mark change was low (approximately one change every six years). Re-sighting analysis suggest that there are considerable inter-island movements of individuals (for example, 19 of 21 individuals identified off O‘ahu have been recorded off the island of Hawai‘i or around the “4- islands”). A multi-site mark-recapture analysis, taking into the proportion of marked individuals in the population, resulted in an estimate of 123 individuals in the population (CV = 0.72). This estimate applies to a population of false killer whales that used the study area; however the geographic range of that population is not known. Also, we assumed population closure and homogenous capture probabilities among individuals. The degree to which these assumptions may have been violated and the resulting estimate biased remain unclear. 

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