CCAMLR

Krill distribution and abundance were estimated from four acoustic surveys, conducted in the vicinity of Elephant Island, Antarctica, from mid-January to mid-March, 1992. The first and last surveys covered a 105 by 105 n.mi. study area centered on Elephant Island; the second and third surveys covered a 60 by 35 n.mi. area immediately north of Elephant Island. During the first survey, krill were distributed in a wide band extending along the north side of Elephant Island and wrapping around the western end; biomass was estimated to be 2. 2 million metric tons. During the second survey, the highest densities of krill were over the shelf extending to the northwest from Elephant Island and including the Seal Island archipelago; high densities of krill also extended off the shelf from the northeast end of Elephant Island into deeper water. Biomass in the smaller survey area was estimated to be 0.7 million metric tons. Three weeks later, high krill densities were still apparent in the vicinity of Seal Island, but the area of high density previously mapped off the northeast end of Elephant Island had retracted considerably; biomass was estimated to be 0.4 million metric tons. During the final survey, conducted six weeks after the first survey, krill were mapped in reduced densities primarily to the west of Elephant Island; biomass over the larger survey area had declined to 1.1 million metric tons.

In sexually dimorphic species, morphometric parameters have separate distributions for males and females, and these often overlap. Whilst discriminant analysis can be used to determine the sex of individuals, it is only able to correctly sex a certain proportion of birds. Two overlapping normal distributions are used to show that there is a difference between the real mean parameter for a sex, and the apparent mean derived by sexing the birds using discriminant analysis.
When discriminant functions are able to correctly determine the sex of birds with greater than 80% success the difference between the true and apparent mean is likely to be undetectable when fewer than 600 birds are sampled. Combining all data for both sexes is considered as a procedure for avoiding the necessity of sex determination, but uncertainty in sex ratios can lead to considerable Type I and Type II errors. Lack of knowledge about the sex ratio between years makes combining the data a very doubtful procedure and use of a discriminant function to determine sex is recommended as being most practically robust.