Estimation of the biomass of krill in Prydz Bay during January/February 1991, February/March 1992 and January/ February 1993 using echo integration

Hydroacoustic surveys of the abundance of krill (Euphausia superba) in the Prydz Bay region were undertaken in January/February 1991, February/March 1992 and January/February 1993. The surveys indicated some association of krill with the shelf break in the western part of the survey area and the centre of the Prydz Bay region but also found that the shelf break was in general not a region of relatively high krill abundance. The mean surface density estimates of krill in 1991, 1992 and 1993 were 16.6, 10.3 and 7.7 g/m2 respectively. These densities are large compared to estimates of 1.95, 3.45 and 1.78 g/m2 for statistical areas 48.1,48.2 and 48.3 (SC-CAMLR 1991, p47)but small compared to 20.2 g/m2 estimated for January 1985 from the SIBEX-II data. The Australian SIBEX-II estimate falls in the middle of the range of densities estimated from seven surveys between 1981 and 1985 (Higginbottom et al. 1988). The distribution of the Sa values for the lower biomass years 1992 and 1993 were similar and distinctly different from the higher biomass year of 1991. The difference is characterised by the high percentage of lower Sa values (60% < 10 m2/nm2) for 1992 and 1993 compared with the relatively even distribution of medium Sa values (10 to 200 m2/nm2) over the survey region in 1991. The extent of bias in these results due to the presence of substancial biomasses of species other than Euphausia superba in the survey areas (Williams et al. 1983; Ikeda et al. 1984, 1986; Hosie et al. 1988) could not be determined