Browsing by Subject "mixture models"
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- ItemRestrictedSeparating pygmy and Antarctic blue whales using long-forgotten ovarian data(Wiley, 2009) Branch, Trevor A; Mikhalev, Y A; Kato, HPygmy blue whales (Balaenoptera musculus brevicauda) are ≤24.1 m and are generally found north of 52°S in summer, whereas the more southerly Antarctic blue whales (B. m. intermedia) may exceed 30 m. Previous assessments have assumed that catches and recent surveys south of 60°S recorded Antarctic blue whales, but these may have included pygmy blue whales. Here, we use ovarian corpora, which accumulate with ovulations and hence with length, to separate these subspecies. The resulting Bayesian mixture model, applied to 1,380 Northern Region (north of 52°S and 35°–180°E) and 3,844 Southern Ocean (south of 52°S) blue whales, estimated that only 0.1% (95% credibility intervals 0.0%–0.4%) of the Antarctic region blue whales were pygmy blue whales and, unexpectedly, found significantly lower lifetime ovulation counts for pygmy blue whales than for Antarctic blue whales (7.6 vs. 13.6). Over four decades, despite substantial depletion of Antarctic blue whales, there was no trend in the estimated proportion of pygmy blue whales in the Antarctic. Several lines of investigation found no evidence for sizeable numbers of pygmy blue whales in ovarian corpora data collected in the 1930s, as was previously hypothesized.
- ItemRestrictedSeparating southern blue whale subspecies based on length frequencies of sexually mature females(Wiley, 2007) Branch, Trevor A; Abubaker, E M N; Mkango, S; Butterworth, Doug SWhen sexually mature, Antarctic (true) blue whales are substantially longer than pygmy blue whales. To estimate the proportions of these two subspecies in various regions, Bayesian mixture models were fitted to catch length frequencies of sexually mature females. The extent of rounding to 5-ft intervals was also estimated. Antarctic blue whales dominated (99.2%) pelagic catches south of 52°S, whereas pygmy blue whales dominated (99.9%) north of 52°S and in 35°–180°E. South of 60°S, only 0.7% (95% credibility interval 0.5%–1.0%) were pygmy blue whales, lower than the 7% upper bound currently assumed. Shore-based catches from SW Africa and those before 1937 from South Georgia and the South Shetlands were estimated to contain 90%–92% Antarctic blue whales. Actual proportions were probably higher, but these data show evidence of rounding (up to 19% of records), poor length-estimation methods, and other problems. The mean length of sexually mature female Chilean blue whales (77.1 ft, 23.5 m) was intermediate between pygmy (68.9 ft, 21.0 m) and Antarctic blue whales (83.4–86.3 ft, 25.4–26.6 m). A good fit to these data was obtained only by assuming that the Chilean whales are a separate subspecies or distinctive population. This finding is also consistent with their discrete distribution, and genetic and call type differences, compared to Antarctic and pygmy blue whales.