Preliminary abundance estimates for Antarctic minke whales from three completed sets of IDCR/SOWER circumpolar surveys, 1978/79 to 2003/04

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University of Cape Town

Abundance estimates are provided for Antarctic minke whales from the ship-based IDCR-SOWER surveys using standard distance sampling methodology. The methods of pooling strata and of estimating mean school size have altered since the most recent re-assessment of all these surveys. 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 63.1%, 79.5% and 99.9% of the ice-free area south of 60°S. Abundance estimates from individual surveys were added for CPI and CPII and combined using the ‘surveyonce’ method for CPIII to obtain circumpolar abundance estimates. When closing and independent observer mode estimates were inverse-variance weighted, circumpolar abundance estimates were 594,000 (CV = 0.128), 769,000 (CV = 0.094) and 362,000 (CV = 0.080) for CPI, CPII and CPIII respectively. These estimates are known to be negatively biased because some Antarctic minke whales may be north of 60°S or inside the pack ice during the surveys, and because some whales on the trackline were likely missed. After simple extrapolation to account for differences in the latitudes surveyed during each circumpolar set (but not for increasing proportions of ‘like minke’ sightings), the ratio of the circumpolar estimates was 0.92:1.00:0.39, echoing previous findings of appreciably lower CPIII estimates. These results should be considered preliminary.