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Separating pygmy blue whale catches by population

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Branch, T A, Monnahan, C C, Sirovic, A

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International Whaling Commission


Based largely on song type, there are at least four clearly distinct populations of pygmy blue whales found respectively in the northern Indian Ocean (NIO, Oman to Sri Lanka), south-western Indian Ocean (SWIO, Madagascar), south-eastern Indian Ocean (SEIO, Australia-Indonesia), and south-western Pacific Ocean (SWPO, New Zealand). For population assessments, catches need to be separated by these populations, but this has not previously been attempted. Scattered pygmy blue whale catches were taken at Durban (n = 4, SWIO); the west coast of Australia (n = 33, SEIO); east coast of Australia (n = 1, 1954, SWPO); in New Zealand and eastern Australia (n = 127, 1912-13, SWPO); and in pelagic expeditions off Kerguelen Island (n = 125, largely 1929/30), and in the southern Indian Ocean in 1934/35 (n = 13), 1935/36 (n = 1), 1937/38 (n = 1) and 1961/62 (n = 2). However, the vast majority of pygmy blue whale catches were caught by Japanese expeditions (2578, nearly all in 1959/60–1963/64) in the southern Indian Ocean; and Soviet expeditions (9299, nearly all in 1962/63–1971/72) that whaled in the northern and southern Indian Oceans and eastward into New Zealand waters. Here, we separate blue whale catches among the four populations using song types, satellite tag data, and fetal lengths. Fetal lengths in Soviet catches demonstrate that blue whales off southern Somalia (south of 2°N) have synchronized conception dates in the austral summer, while those north of 9°N and off India (north of 4°N) have aseasonal reproduction, suggesting SWIO blue whales extend to 2°N, further north than previously assumed. A surface fitted to the locations of different blue whale calls was used to estimate the proportions of each population throughout the range of pygmy blue whales, and to split the pelagic catches by population. Estimated total catches for each of the four pygmy blue whale populations were 1228 (NIO), 6889 (SWIO), 3646 (SEIO), and 421 (SWPO), with 97.6% of the overall total of 12,184 coming during 1959/60 to 1971/72.

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