Hydroacoustic surveys of the distribution and abundance of krill: Prydz Bay region – FIBEX, ADBEX II and SIBEX II

Hydroacoustic surveys of the distribution and abundance of krill, Euphausia superba Dana, were undertaken in the Prydz Bay region, Antarctica. Three surveys were carried out south of 60°S within an area of 1.28 x l06 km2 between 55°E and 95°E, during the austral summer months of 1980-81, 1983-84 and 1984-85. The surveys included a total of 25 transects and covered a distance of 22 500 km over the three seasons. The surveys formed part of the Australian contribution to the international BIOMASS program.
The results of the surveys and a discussion of the theory and methods employed including sources of error in the echointegration technique are presented. Quantitative data are presented on the abundance (biomass) of krill in the Prydz Bay region as a whole and the weight density of krill along each cruise track is presented in graphical form. These data demonstrate a patchy density distribution with surface densities reaching a maximum of over 100 g/m2 along the coastal margin.
Estimates of biomass for the entire study region of 1.28 x l06 km2 were 1.6, 3.5 and 3.7 million tonnes for the 1980-81 (FIBEX), 1983-84 (ADBEX II) and 1984-85 (SIBEX II) cruises respectively. It could not be concluded, however, that there had actually been changes in the abundance of krill in the region over the period between the surveys because of the wide confidence limits on the abundance estimates.
The precision with which changes in abundance can be detected can be improved by underway identification of target species, better calibration and improved survey design. More accurate estimates of biomass also require more accurate target strength measurements.