Abundance estimates for Antarctic minke whales from three completed circumpolar sets of surveys, 1978/79 to 2003/04
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2006
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University of Cape Town
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Abstract
Abundance estimates are provided for Antarctic minke whales from the ship-based IDCR-SOWER surveys using the standard distance sampling methodology applied in the past in the Scientific Committee. Agreed methods of pooling strata and of estimating mean school size have changed since the most recent published assessment of these surveys by Branch and Butterworth (2001a). The IDCR-SOWER surveys are grouped into three completed circumpolar sets of cruises: 1978/79–1983/84 (CPI), 1985/86–1990/91 (CPII) and 1991/92–2003/04 (CPIII), which respectively covered 64.3%, 79.5% and 99.7% of the ice-free area south of 60°S. Circumpolar abundance estimates are obtained by summing individual surveys in CPI and CPII (each covered one IWC Management Area), and by combining CPIII surveys (some overlapped) using the ‘survey-once’ method—by selecting the single survey offering the best or most recent coverage. When calibrated closing and independent observer mode estimates were inverse-variance weighted, circumpolar abundance estimates were 645,000 (CV = 0.143), 786,000 (CV = 0.094) and 338,000 (CV = 0.079) for CPI, CPII and CPIII respectively. These estimates are negatively biased because some Antarctic minke whales are north of 60°S and inside the pack ice during the surveys, and because some whales on the trackline are missed. After simple extrapolation to account for differences in the latitudes surveyed during each circumpolar set and for the increasing proportions of ‘like minke’ sightings, the ratio of estimates from the three CPs is 0.97:1.00:0.39, echoing previous findings of appreciably lower CPIII estimates. CPIII estimates for individual IWC Management Areas are similarly low, ranging within 18–52% of CPII estimates for Areas I–V, although 159% of CPII for Area VI. Explanations for the appreciably lower abundance estimates include a higher proportion of minke whales within the pack ice and a greater proportion of whales missed on the trackline, but any such hypothesis needs to be reconciled with higher abundance estimates in CPIII than in CPII for blue, humpback, fin, sperm and killer whales based on the same surveys.
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Reference:
Branch, T. A. (2006). Abundance estimates for Antarctic minke whales from three completed circumpolar sets of surveys, 1978/79 to 2003/04. Cambridge, UK: International Whaling Commission (paper SC/58/IA18).