Monthly movements and historical catches of pygmy blue whale populations inferred from song detections
Trevor A. Branch, Cole C. Monnahan,, Ana ?irovi?, Suaad Al Harthi, Cherry Allison, Naysa E. Balcazar, Dawn R. Barlow, Susannah Calderan, Salvatore Cerchio, Michael C. Double, Richard Dr?o, Alexander N. Gavrilov, Jason Gedamke, Kristin B. Hodge, K. Cu
Multiple populations of blue whales inhabit the Southern Hemisphere and northern Indian Ocean. Each population produces repeated songs that are stable over decades and has been used to identify which regions they inhabit. While Antarctic blue whales and south-east Pacific Ocean (SEPO, Chile/Peru) blue whales inhabit discrete regions and are easier to assess, the remaining five populations of “pygmy blue whales” inhabit overlapping regions and are harder to separate and assess. These five populations are roughly 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-eastern Indian Ocean (SEIO, Australia to Indonesia), and south-western Pacific Ocean (SWPO, New Zealand). Here, we collate all available acoustic song recording location data, and fit spatial models to predict where each population occurs by month. We then used these surfaces to separate historical catches among each population, under the assumption that current distribution is similar to historical distribution. Almost all pygmy blue whale catches (97% of the total of 12,043) were taken by Japanese and Soviet pelagic whalers during 1959/60 to 1971/72. A pygmy blue whale region was defined using catch length frequencies and the knowledge that pygmy blue whales are considerably shorter than Antarctic blue whales. Catches within this region were assumed to belong to one of the five pygmy populations if they were shorter than 24.5 m. For each catch, the relevant monthly fitted surfaces are used to estimate the probability of that catch belonging to a pygmy population, and these estimates for individual catches were applied to the annual totals for each whaling expedition and pelagic season. Estimated total catches for each of the five pygmy blue whale populations were 1,118 (NWIO), 822 (CIO), 5,677 (SWIO), 3,953 (SEIO), and 473 (SWPO).