Abundance Estimates of False Killer Whales in Hawaiian Waters and the Broader Central Pacific

The abundance of two remote and broadly-distributed populations of false killer whales (Pseudorca crassidens) in Hawaiian waters was estimated using sighting data collected from ship-based, line-transect surveys conducted by NOAA Fisheries within the central Pacific between 1986−2017. Including the three Hawaiian Islands Cetacean and Ecosystem Assessment Survey (HICEAS) efforts in the summer and fall of 2002, 2010, and 2017.

The abundance of the Northwestern Hawaiian Islands (NWHI) population was estimated for 2010 and 2017 and of the pelagic population for each HICEAS year.

Both design- and model-based line-transect analysis methods were used in the abundance estimation of the transboundary pelagic population, with the design-based approach estimating density uniformly within the U.S. Hawaiian Islands Exclusive Economic Zone (EEZ), and the model-based approach estimating density as a function of habitat covariates within the broader central Pacific and, from that, the EEZ.

The design-based estimates of NWHI abundance in 2010 and 2017 are 878 (CV, 1.15; 95% CI, 145 to 5,329) and 477 (CV, 1.71; 95% CI, 48 to 4,712) individuals, respectively. The design-based estimates of pelagic false killer whale abundance in the EEZ in 2002, 2010, and 2017 are highly variable across years compared to the model-based estimates, although the average estimates over the three survey years are similar at 2,736 (CV, 0.53; 95% CI, 1,030 to 7,269) and 2,115 (CV, 0.34; 95% CI, 1,111 to 4,026) individuals for the design- and model-based approaches, respectively.

Random variation in encounter rate and shifts in distribution within and around the EEZ are likely contributors to the variation in the design-based estimates. The combined use of these approaches strengthens the assessment and management of pelagic false killer whales by providing consistent abundance estimates for context-specific management applications.

Ultimately, the model-based approach offers a more stable basis for managing pelagic false killer whales over time and allows for the estimation of abundance outside the EEZ, which is especially important for mitigating population-level impacts from longline fisheries.

Bradford AL, Becker EA, Oleson EM, Forney KA, Moore JE, Barlow J. 2020. Abundance Estimates of False Killer Whales in Hawaiian Waters and the Broader Central Pacific. U.S. Dept. of Commerce, NOAA Technical Memorandum NOAA-TM-NMFS-PIFSC-104, 78 p. https://doi.org/10.25923/2jjg-p807.