Revised assessment for Oceania humpbacks: survey design considerations and power analysis

The Oceania population of humpback whales, comprising sub-stocks E2, E3, and F, was heavily depleted by whaling operations in the 20th century and is now classed as Endangered by the IUCN. A mark-recapture study of individuals identified by genotypes and photo-identification found the Oceania population had a superpopulation size of 4,329 whales (95% CL 3345, 5315) from 1999 to 2005 and an annual population growth (λ) for this period of 1.03 (95% CL 0.90-1.18). This low and imprecise estimate of growth contrasts with the high rate of growth for east Australia (10.9% per annum, corresponding to λ = 1.109; 95% CL 10.5%-11.3%) but the wide confidence intervals do not allow a firm trend to be detected. At a recent meeting of the South Pacific Whale Research Consortium (SPWRC), it was recommended that a future assessment of the Oceania humpback stock through mark-recapture methodology should be able to meet 3 objectives: (1) determine population size with a coefficient of variation (CV) of 1 if the true λ=1.05. The simulations also suggest that the power to meet the objectives on a regional basis varies with the survey design and simulated scenario. However, in general, the biennial survey design was able to detect with ≥90% power if the growth rate is significantly lower than that of east Australia if the true λ ≤1.03 for both NC and TG. Therefore, it seems the proposed surveys would allow us to test whether population growth rates in these different regions are significantly different: this appears to be important given the recent rapid increase in the New Caledonian calving ground.

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