CCAMLR

Preliminary computations are carried out for the krill yield model of Butterworth et al. (1993), adjusted as requested by the fifth meeting of WG-KRILL. This involves modifying the input distributions for the lengths at recruitment and maturity, natural mortality and the extent of recruitment variability. Sensitivity tests are carried out to assess the consequences of avoidance of gravid females by the fishery, and higher natural mortality for younger ages of krill. Results do not differ greatly from those of comparative calculations in Butterworth et al. (1993), except at higher harvesting intensities for which greater resource depletion is indicated. Comparatively greater depletion is also evident for males when gravid females are avoided, and if natural mortality is higher for younger ages.

Between year variability of krill year class success and recruitment during the 1975-1994 period are described based on data from German expeditions and US AMLR cruises in the Elephant Island area. Recruitment indices based on the relative abundance of the 1+ year class during each year indicate good recruitment of the 1980/81, 1985/86, 1987/88 and 1990/91 year classes; exceedingly poor recruitment occurred for the 1976/77, 1982/83, 1983/84, 1988/89 and 1991/92 and 1992/93 year classes.

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%

Laboratory studies have shown that Antarctic krill Euphausia superba shrink if maintained in conditions of low food availability. Recent studies have also demonstrated that E. superba individuals may be shrinking in the field during winter. If krill shrink during the winter, conclusions reached by length-frequency analysis may be unreliable. In this study, the correlation between the body-length and the crystalline cone number of the compound eye was examined. Samples collected in the late summer show an apparent linear relationship between crystalline cone number and body-length. From a laboratory population, it appears that when krill shrink, the crystalline cone number remains relatively unchanged. If crystalline cone number is little affected by shrinking, then the crystalline cone number may be a more reliable indicator of age than body-length alone. The ratio of crystalline cone number to body-length offers a method for detecting the effect of shrinking in natural populations of krill. On the basis of the crystalline cone number count, it appears that E. superba shrink during winter.

The reproductive state and size composition of Euphausia superba collected in the Indian sector of the Southern Ocean from 1985 to 1990 were analyzed to estimate its growth, life span and mortality rates. The duration of the life cycle of E.superba exceeded 5 years in the Cosmonaut Sea and 6 years in the Cooperation Sea. Assuming growth for only 180 days per year, growth rates ranged from 0.120-0.133 mm.d-1, during the first year of life, to 0.019-0.022 mm.d-1 during the fifth year. Von Bertalanffy growth curves calculated for different areas are similar to those obtained by Australian researchers in the Prydz Bay region for 1981-1985. In mid summer, E.superba of age 2+ to 4+ were predominant in all hauls made south of the Antarctic Divergence, while north of the Divergence the krill stock was clearly dominated by individuals of age 4+. The coefficients of natural mortality (M) of E.superba in the Indian sector of the Southern Ocean, calculated by the methods of Alverson & Carney, Richter & Efanov and Beverton & Holt, varied from 0.72 to 0.87, from 0.52 to 0.57 and from 0.76 to 2.92, respectively. The value of age-dependent natural mortality of E.superba derived using Zikov & Slepokurov’s method ranged from 0.52, during the maturation period, to 1.1-2.41, during the first and last years of life. Based on long-term observations, the relationship between E.superba age composition and its spawning success is examined for the coastal areas of the Cooperation and Cosmonaut Seas.

Maps of krill (Euphausia superba) density derived from acoustic survey data and the distribution of fishing effort in the Chilean krill fishery indicate an area of high krill density wrapping around the northwestern end of Elephant Island during the austral summer of 1992. In this area, the distribution of catch-per-fishing-time and krill density (measured acoustically) show similar forms. Search time could not be used to estimate other aspects of krill distribution pattern because fishing operations are limited by processing efficiency rather than availability of krill. Analysis of the acoustic survey data suggested characteristic distribution pattern scales of 1.7 and 4 n.mile. Estimates of a composite index of krill abundance (SC-CCAMLR-VIII, 1989) were derived for two surveys from these data sets. In addition, the data suggest that the abundance of krill in the Elephant Island area can change rapidly, and when krill do come into to the area they are most often found in water 100m-500m depth along the shelf break north of Elephant Island, particularly in the area where it wraps around the western end of the island.