| Summary: | Female blue whale ovarian corpora data were translated and encoded from the USSR’s Slava (1961/62–1965/66) and Sovietskaya Ukraina (1961/62–1971/72) expeditions. Complete ovarian data were available for 1,425 blue whales (1,272 pygmy, 153 Antarctic). Catches north of 52°S were assumed to be pygmy blue whales (Balaenoptera musculus brevicauda), while those south of 56°S were assumed to be Antarctic (true) blue whales (B. m. intermedia), although there was some evidence for a small proportion (<1%) of both Antarctic blue whales north of 52°S and pygmy blue whales south of 56°S. A small proportion of lengths were rounded to the nearest metre, and many whales shorter than 18.0 were recorded as 18.0 m or greater (whale stretching). A Bayesian logistic model fitted to the data provided estimates of L50 and L95 (the lengths at which 50% and 95% of females are sexually mature). For pygmy blue whales L50 was 19.2 m (95% interval 19.1–19.3 m) and L95 was 20.5 m (95% interval 20.4–20.7 m). These estimates are more precise than those from Japanese data because the Soviet vessels recorded 32 times more pygmy blue whales shorter than the legal minimum length (21.3 m). Among small areas, L50 varied from 18.4 to 19.9 m for pygmy blue whales; all estimates were much shorter than the 23.4 m from the Antarctic. The status of northern Indian Ocean pygmy blue whales is unclear: L50 for these blue whales was statistically significantly shorter than L50 for both the southern Indian Ocean and around Australia, but the magnitude of the differences was small: 0.5–0.6 m.
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