International Whaling Commission/International Decade of Cetacean Research (IWC/IDCR) Southern Minke Whale assessment cruise, 1978–79

Abstract

The Minke Whale (Balaenoptera acutorostrata) represents the last major baleen whale resource of the Southern Ocean. Concerted exploitation for Minke Whales began in the 1971–72 season, so that it is especially important now that management of this species be placed on an adequate scientific basis.Despite the accumulation of a considerable amount of basic information on the biological and population parameters of the species remarkably early in its history of exploitation (Ohsumi and others, 1970; Ohsumi and Masaki, 1975; Masaki, 1979), reliable estimates of stock size have proved extremely elusive. Initially, population estimates had to be based entirely on sightings (Ohsumi and Masaki, 1971) as trends in catch per unit of effort were not discernible. These estimates were variable, and sensitive to the extrapolation procedures used to cover areas or seasons for which no data were available. Eventually a decline in catch per unit effort was detected in one Southern Hemisphere Area, Area IV (70°E to 130°E), although this was not statistically significant (IWC, 1977). Most of the recent population estimates have been calculated (directly or indirectly) from an analysis of the decline in Area IV, with the results extrapolated to other Southern Hemisphere areas, based on relative densities from sightings information. Despite a rigorous analysis of catch per unit effort data at the Seattle meeting of the Scientific Committee of the International Whaling Commission (IWC) in May 1978, there was much variability in the Minke Whale population estimates—as much as 3.5 times, depending on the assumptions made (IWC, 1979)—causing the adequacy of assessments based solely on data from catch per unit effort to be questioned. The variability arose with the adoption of a deliberately conservative regime of exploitation (which should produce only a small change in relative abundance), and the reduced amount (and localized nature) of the effort used. Moreover, the only independent data available was based on sightings, and was too variable to place any confidence in detecting significant changes in abundance.