Estimate of 2011 Abundance of the Bering-Chukchi-Beaufort Seas Bowhead Whale Population

We estimate the total abundance of the Bering-Chukchi-Beaufort Seas population of bowhead whales (Balaena mysticetus) in 2011 from large datasets of visual sightings and acoustic locations. A Horvitz-Thompson type estimator is used, based on the numbers of whales counted at ice-based visual observation stations. It divides sightings counts by three correction factors. The first adjusts for detectability using the results of Givens et al. (2012), who estimate detection probabilities and their dependence on offshore distance, lead condition, and whale group size. The second correction adjusts for availability using the acoustic location data to estimate a time-varying smooth function of the probability that animals pass within visual range of the observation stations. The third correction accounts for missed visual watch effort. The mean correction factors are estimated to be 0.501 (detection), 0.619 (availability) and 0.520 (effort). The resulting 2011 abundance estimate is 16,892 with a 95% confidence interval of (15,704, 18,928). We also insert this estimate into a time series of past visual abundance estimates to estimate the rate of increase of the population. The annual increase rate is estimated to be 3.7% with a 95% confidence interval of (2.8%, 4.7%). These abundance and trend estimates are consistent with previous findings and are indicative of very low conservation risk for this population under the current indigenous whaling management scheme.