UW Aquatic & Fishery Sciences Quantitative Seminar

Trevor Branch

UW Aquatic & Fishery Sciences

Separating blue whale subspecies using mixture models

Abstract

Three subspecies of blue whales are currently recognized: northern, Antarctic and pygmy. The last two occur in the Southern Hemisphere, and are the focus of this seminar. While the status of Antarctic blue whales is well known, the past catches, current numbers and status of pygmy blue whales are poorly determined. A major problem in the Southern Hemisphere is the inability to distinguish between Antarctic and pygmy blue whales during at-sea encounters. This problem affects current estimates based on surveys south of 60°S, which are assumed to be Antarctic blue whales but could include an unknown proportion of pygmy blue whales. Two studies were undertaken to address this issue. In the first study I compiled a database of the relationship between length and corpora luteum counts (marks in the ovaries which accumulate with age). A logistic plus negative binomial mixture model estimated that only 0.4% of blue whales south of 60°S were pygmy blue whales. An additional finding based on translated USSR logbook data was the first good estimate of the length at sexual maturity for pygmy blue whales. In the second study, I examined historical catch length frequencies of sexually mature females to divide these catches in various regions into the two subspecies. An additional parameter allowed for the estimation of the proportion of catch lengths that had been rounded to the nearest 5 ft interval. Length distributions were usually unimodal and indicated that each region contained either pygmy or Antarctic blue whales, and only rarely a mixture of the two. The estimated proportion of pygmy blue whales south of 52°S was 0.8%. Pregnant females caught at Chile followed a unimodal distribution but were inexplicably short for Antarctic blue whales, and too long for pygmy blue whales. The Chilean data was best explained by the hypothesis that this population constitutes a new and as-yet undescribed subspecies. Evidence from genetics, acoustics and a compilation of positional data also supports this possibility.