As for other baleen whales, the IWC’s management of Antarctic minke whales has been based on six Areas, I through VI, which are longitudinal pie slices 50°–70° wide. The population structure is poorly known, but recent analyses suggest a genetic distinction between whales in the Indian Ocean sector of the Antarctic (west of 165°E) and the Pacific Ocean sector (east of this line) with presumably some overlap (Pastene 2006b). With the exception of the two marked whales mentioned above, the relationship between the Antarctic distribution and putative breeding areas is largely unknown.

Ship-based summer surveys of the area south of 60°S have been conducted each summer since 1978/79, under the auspices of the IWC SC, under the International Decade of Cetacean Research (IDCR) —later Southern Ocean Whale and Ecosystem Research (SOWER) — programme, and covering a different Area (or part Area) each year (Matsuoka et al. 2001). These have been used to estimate minke whale population sizes, on the assumption that the bulk of the population is found south of 60°S in the survey season (Branch and Butterworth 2001). The survey vessels do not enter the pack ice, where minke whales are known to occur to some extent (Shimada and Kato 2006).

The IWC SC conducted a major assessment of Antarctic minke whales in 1990, and a population estimate of 760,000 was adopted, based on results of the IDCR surveys conducted in the seasons 1982/83 through 1988/89 (IWC 1991). Results of subsequent surveys indicated lower abundances (see Table 1 in linked PDF document, which constitutes an integral part of this assessment), leading the Committee to conclude in 2000 that the estimate of 760,000 was no longer a valid estimate of current abundance (IWC 2001, p. 31). The Committee has not yet adopted a new current estimate, pending the development, testing and implementation of improved analysis methods. Estimates calculated using the standard methodology for comparative purposes (Branch 2006, IWC 2007) were:

After adjusting for the different coverage of the three sets of surveys (which is not simply a multiplication, because the areas unsurveyed in the earlier years tended to be further north), the ratio of the three abundance estimates for the three periods was 0.97:1.00:0.39 (see Table 1 in attached PDF document).

The Committee has to date (January 2007) been unable to determine whether the apparent decline was real or artifactual. The Committee considered the two most likely confounding factors to be: (i) a reduction in sighting efficiency (e.g. due to smaller school sizes and possibly less experienced observers) and (ii) changes in ice extent, such that fewer whales occurred in surveyable open water.

Two additional points were noted (IWC 2007):
(i) The decline was specific to minke whales; estimates for other species (blue, fin, killer, humpback) increased over the period; an explanation for the decline would need to account for this.
(ii) Abundance estimates for Areas IV and V obtained by the Japanese scientific permit whaling programme (JARPA) for the seasons 1989/90 through 2004/05 showed no statistically significant trend.
Further work on the issues of sighting efficiency and whale/ice distribution is underway. The Committee expects to adopt revised abundance estimates in the near future (2008) and to determine whether and to what extent the apparent decline is real.

Some authors have proposed that the population of Antarctic minke whales had been increasing up to the late 1960s (e.g. Mori and Butterworth 2006), but this is controversial.

Unknown