CCAMLR

Shipboard tracking study for krill-eating predators (penguins and female fur seals) at Seal Island were conducted to identify and evaluate their foraging areas during early January 1990/91. Penguin foraging areas were formed in inshore region, where krill frequently occurred but higher density areas of krill (≥250g/m2) were rather limited. In contrast, fur sea! foraging areas were formed in offshore region, where krill occurred only occasionally but they tended to form extensive aggregations (ca. 2-3 km in horizontal distance) of higher densities (≥250g/m2). In the inshore foraging areas krill showed a diurnal vertical migration, tending to be at deeper depth range (50-100 m deep) in the day time while at shallower layer (20-50 m) at night. In the offshore foraging areas krill showed no diurnal vertical migration, staying close to the surface throughout the day. As for body size and maturity of krill, in the inshore foraging areas middle-sized krill (modal length 43 mm), which consisted mainly of nongravid krill, were dominant with occasional occurrences of juveniles (modal length 21 mm). In contrast, in the offshore foraging areas large-sized krill (modal length 47 mm), of which the majority were gravid females, dominated. Thus horizontal and vertical distributions and population structure of krill were totally different between the two foraging areas. The reasons why the fur seal chose offshore foraging areas over inshore foraging areas were discussed.

Acoustic and net sampling surveys for krill were conducted in the krill fishing area north of the South Shetland Islands from 18 January to 3 February 1991. Distinct offshore-inshore heterogeneities in abundance and maturity of krill were observed. The survey region was divided into four zones; oceanic, slope frontal, neritic and nearshore zones. The mean density of krill was low in the oceanic zone (8.5 g/m2), while, intermediate values in the frontal (37.3 g/m2) and neritic zone (28.1 g/m2), and extremely high in the nearshore zone (134.7 g/m2). The last zone corresponds to the shelf break or on the shelf where topographic eddies were generated, suggesting the hydrodynamic convergence might accumulate krill into this zone. The total biomass over the survey region was estimated to be 1.59 ± 0.45 million t (95% confidence limit), of which 1.22 ± 0.42 million t was concentrated in the fishing grounds ("frontal" + "neritic" + "nearshore"). Information from other studies indicated that krill biomass in this region had been lower than expected until early February 1991. As for maturity stages of krill, spawning krill (modal body length 49 mm) were dominant in the oceanic and frontal zones, whereas less mature krill (modal length 45 mm) dominated in the neritic and nearshore zones. Juveniles, which were scarce in the present stock, were restricted mainly to the nearshore zone. Gravid females were exceedingly abundant in the slope frontal zone with the mean density of 23.9 g/m2 (411,000t), as contrasted with the lower value of 3.7 g/m2 (163,000t) in the oceanic zone and almost absence from the neritic and nearshore zones. This indicates that slope frontal features may be important for the formation of favorable spawning grounds for krill.

The sixth Antarctic survey cruise of R/V Kaiyo Maru was carried out in waters around the Shouth Shetland Islands. where are known as the fishing grounds of krill, during the 1990-91 austral summer. In order to investigate seasonal changes in oceanic structure. two observations were put into operation In the same area after an interval of 40 days. The more characteristic change was clearly recognized in the waters of the insular shelf from the first leg to the second leg. The Antarctic Surface Water over the insular shelf at the first leg entirely indicated the minus values of temperature. 40 days later, however, the temperature in the same waters rised above the plus values. The reason for the phenomenon of the temperature rise is considered that the Warm Deep Water intruded over the insular shelf. The following two processes were considered for the present intrusion. The first was the steady topographic upwelling of the Warm Deep Water, and the second was the wind-driven coastal upwelling. The distribution patterns of temperature, salinity, density. dissolved oxygen and nutrient salts supported this upwelling phenomenon.

Hydroacoustic surveys of the abundance of krill (Euphausia superba) in the Prydz Bay region were undertaken in January/February 1991 and February/March 1992.
The surveys indicated some association of krill with the shelf break in the western part of the survey area but also found that the shelf break was in general not a region of relatively high krill abundance. The mean surface density of krill in 1991 was 15.4 g/m2 and in 1992 was 7.4 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 Australian SIBEX-II data. The Australian SIBEX-II estimate falls in the middle of the range of densities estimated from seven reported surveys between 1981 and 1985 (Higginbottom et al. 1988). The biomass of krill in 3.5 milion km2 of the Prydz Bay region was estimated to be between 4.8 to 5.4 million tonnes in 1991 and between 2.2 and 2.6 million tonnes in 1992. The extent of bias in these results due to the presence of substantial 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.

The current subdivision of the CCAMLR Area is examined and some of the problems inherent in the system are outlined. A rationale for the further subdivision of Area 58 is developed and a scheme is suggested based on : available data on krill distribution patterns, oceanographic features and on fishery data.

Zones of 20 km width are defined around selected colonies of penguins distributed around the coasts of the South Shetland and South Orkney Islands. Krill catches in these zones are shown to have a consistent pattern in Subarea 48.1 but an unpredictable distribution in Subarea 48.2, probably as a result of more variable hydrographic conditions. About 50% of the catch in Subarea 48.1 from December to March was taken within 40 km of the coast, and 90% within 80 km in all years 1988-1990. In 1987 and 1988 75% of the catch in Subarea 48.2 between December and March was taken within 80 km of colonies in the South Orkneys. Estimates of consumption rates, foraging ranges and population sizes from the literature are used to show that for some years, at distances of between 20 and 60 km from predator colonies catches in January and February may be up to 48% of the land-based predator consumption. Whilst the overall ratio of catch to consumption is relatively low (27%), any competition between the fishery and predators as a result of large increases in catch is likely to emerge in these areas earlier than would be expected considering the fishery as a whole.