Assignment of South Georgia catches between southeast Pacific blue whales and Antarctic blue whales
Trevor A. Branch
International Whaling Commission
Mixture models of sexually mature female blue whales are used to assess the proportion of historical catches at South Georgia that may have been Southeast Pacific (“Chilean”) blue whales rather than Antarctic blue whales. Earlier data before 1923/24 were considered unreliable due to large proportions of very small mature females (14% were ≤ 70 ft) and high proportions of rounding to the nearest 5 ft interval in the length data (14%), therefore analyses focused on data from 1923/24 onwards. An estimated 3.3% (95% CI 1.6–5.1%) of catches during this later period were Southeast Pacific blue whales, but when the analysis is restricted to lengths with no rounding, the estimates were much lower and not statistically different from zero (mean 0.6%, 95% CI 0.0–2.6%). When this information is combined with the evidence from acoustics (a few distant calls in one month) and genetics (one Southeast Pacific individual in the Antarctic), it is most likely that any Southeast Pacific blue whales encountered at South Georgia are rare vagrants and not a common occurrence.