Summary
Blue whale populations each produce distinctive repeated songs that are stable over periods of decades, and can be used to identify where each population resides and moves. In the Southern Hemisphere and northern Indian Ocean, there are at least seven distinct populations based on their songs, some of which inhabit distinct areas (south-east Pacific, and the Antarctic), while five populations in the "pygmy blue whale" grouping overlap in their ranges. Here we use contemporary song detections from a broad compilation of data to predict where each pygmy population resides, and use these fitted spatial surfaces to separate historical catches of these populations, as a first step required to conduct population assessments. The five populations are in the north-west Indian Ocean (NWIO, Oman song type), central Indian Ocean (CIO, Sri Lanka), south-west Indian Ocean (SWIO, Madagascar to Kerguelen), south-east Indian Ocean (SEIO, Australia to Indonesia), and south-west Pacific Ocean (SWPO, New Zealand). Almost all pygmy blue whale catches (97% of 12,043) were taken by Japanese and Soviet pelagic whalers during 1959/60 to 1971/72, with a few taken in earlier years from opportunistic pelagic whaling or shore-based whaling in temperate regions. Generalized additive models based on latitude and longitude, plus a factor for month, were fitted to the song data for each population, using beta-binomial likelihoods to account for overdispersion, except for the NWIO where binomial likelihood produced better predictions. These fitted surfaces were used to estimate the probability of individual catches belonging to each population, and these probabilities were applied to annual pygmy blue whale catch totals for each whaling expedition and season. Estimated catches were highest from the SWIO and SEIO populations and lowest from the SWPO population, with total catches of 827 (NWIO), 1,729 (CIO), 6,723 (SWIO), 2,651 (SEIO), and 561 (SWPO).