Separating pygmy and Antarctic blue whales using ovarian corpora

• Main Author: Branch, Trevor A
• Format: Report
• Language: English
• Published: University of Cape Town 2006
• Subjects: ovarian corpora; Antarctic blue whales; Antarctic; The Antarctic; Antarc*; Blue whale
• Online Access: http://hdl.handle.net/11427/17414; https://open.uct.ac.za/bitstream/11427/17414/6/Branch_OvarianCorpora_2006.pdf

Two Southern Hemisphere subspecies of blue whales exist: pygmy blue whales are shorter (≤ 79 ft, 24.2 m) and generally found north of 54°S in summer, while Antarctic (true) blue whales exceed 100 ft (30.5 m) and are found in more southerly waters. Abundance estimates of Antarctic blue whales rely on sightings south of 60°S but at-sea identification is difficult and these sightings may include some proportion of pygmy blue whales. Ovarian corpora (corpora lutea plus corpora albicantia) are permanent ovulation records that can be used to estimate this proportion. Pregnant females of the two subspecies may overlap at 72–79 ft (21.9–24.1 m), but pygmy blue whales at these lengths have high (> 4) corpora counts, contrasting with immature or newly mature Antarctic blue whales (0–3 corpora). Published papers yielded pairs of length-corpora data for 104 pygmy and 2,064 Antarctic region blue whales. The relationship between length and ovarian corpora counts is well fitted by logistic models (with negative binomial variability). A mixture model estimates that 0.4% (95% confidence interval 0.0–1.1%) of Antarctic region blue whales were pygmy blue whales, much lower than the “less than 7%” currently accepted by the IWC. If later ovarian corpora data (1947–51) are separately analysed, the estimated proportion is zero (95% CI = 0.0–0.5%), suggesting that the pygmy proportion in the Antarctic did not increase when Antarctic blue whales were greatly depleted. No support is found for Ichihara’s suggestion that high (>7) ovarian corpora counts in 78–81 ft Antarctic region catches were pygmy blue whales. These whales are instead explained by natural variability in Antarctic blue whales. These methods could be applied to blue whale males through the analysis of testes weight, and may hold promise in separating catches of other species with diminutive forms such as fin and minke whales.