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SC/69B/SH/08
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Resource ID
22151
Access
Open
Document Number
SC/69B/SH/08
Full Title
A timing hypothesis explains the mystery of the missing blue whale calves
Author
Trevor A. Branch
Authors Summary
Paper SHxx presents data showing that only 2.3% (95% CI 2.0-2.6%) of sighted individuals are mother-calf pairs in blue whale populations, which is far lower than expected from a species that gives birth every 2-3 years. Accounting for males and immature individuals, it would be expected that 7-12% of sightings should be mother-calf pairs. A variety of hypotheses are examined for this discrepancy, with a timing hypothesis being the most consistent with the observed data. This hypothesis proposes that most blue whales give birth shortly after departing their summer feeding aggregations, and then wean the calves seven months later just before returning to the feeding areas. A small proportion of calves are born in the non-calving months. Since most field studies are focused on summer feeding areas where blue whales are most aggregated, during the period between peak weaning and peak calving, few mother-calf pairs are sighted. This hypothesis matched the observed low proportions in summer studies (except for the higher proportions in New Zealand), and the higher proportions in winter studies of blue whales. Proposed future work could be used to test this hypothesis including monthly analysis of mother-calf proportions, and data on calf sizes by month.
Publisher
IWC
Publication Year
2024
Abstract
Few mother-calf pairs are sighted in blue whale populations worldwide, averaging only 2.3% (95% CI 2.0?2.6%) of sighted individuals, despite pregnancy data suggesting that 33?50% of mature females produce a calf every year. Multiple hypotheses were developed and examined to explain this apparent low proportion of sighted calves. The observed rates are too low to be explained by hypotheses of low fetal survival, low calf survival, low birth rates, or calf separation from mothers. However, accounting for males and immature individuals in the population reduced the expected proportion of mother-calf pairs to 7?12%. It is also possible that mother-calf pairs avoid aggregations of blue whales, which is where field studies are concentrated. A timing hypothesis was able to explain most of the remaining observations by assuming that blue whales produce calves shortly after departing their summer feeding grounds and wean their calves seven months later as they return to their summer feeding grounds, in combination with some variability around calving dates, and a small proportion of calves being born at any time of the year. A conceptual model combining these elements predicted that observed mother-calf proportions should peak in winter months at 8?9% and be at a minimum of 1?4% in summer months. These predictions matched the low proportions observed in seven out of eight summer feeding regions (1.5?3.5%), but not off New Zealand (9.8%), while in the two winter studies, the observed proportion was close to predicted in the Timor Trough (9.3%), but lower in the Galapagos (3.8%). The lowest proportion was reported from Sri Lanka (0.7%), where blue whales reside year-round. These results suggest that the mystery of the missing blue whale calves can largely be explained by peak calving immediately after leaving summer feeding grounds, and peak weaning on or before their return.