CCAMLR

During the XIII Italian Expedition to the Ross Sea in late spring and early summer (December-January 1997-98), for biological validation of hydroacoustic recordings, 35 net samplings were carried out. For all hauls both parameters: biological (length AT, sex and maturity stage of the gonads of the Euphausia superba) and acoustical (differences in volume back-scattering strength of krill aggregations MVBS at 38, 120 and 200 kHz) were examinated. Comparing the depth of the net and acoustic recordings during the hauls.
The surveyed area extended from 71° to 76°' S in latitude and from 164° E to 177° W in longitude. The aim of the paper is: (1) to compare the species/length composition of catch data and the one obtained by processing acoustic data, acquired during the net sampling; (2) to present density distribution maps of Euphausia superba and Euphausia crystallorophias obtained by processing acoustic data; (3) to analyse the structure of Euphausia superba population on the basis of biological variables, extracted from catch data;.
The analysis of the biological parameters seems to indicate statistically significant differences in them between the hauls, while within each haul the parameters are homogeneous. The maturity stage of krill in general increase with decreasing of latitude (from 76° to 71°S).
The analysis of acoustic and catch data indicate that: (1) the acoustic discrimination between the two predominant species, based on the multi-frequency method, is very close to the one based on net samplings; (2)The acoustic estimation of the krill mean length, based on the fluid sphere method, is pretty close (around 9%less) to the mean catch values; (3) The numerical abundance of the krill, estimated by the fluid sphere method, is 20-100 times higher than the one estimated from the catch.
The geographical biomass distribution of Euphausia superba and Euphausia crystallorophias, estimated acoustically, overlap in this period of year.

Net samples data from the 15th Italian Antarctic Oceanographic Cruise (Jan-Feb 2000) were analysed to obtain a general picture of the summer distribution pattern, abundance and demography of krill in the western Ross Sea (Antarctica). A midwater sampler-trawl (Hamburg Plankton Net) was used to collect zooplankton and fish larvae at 63 stations. The net was deployed normally as a standard double oblique tow. Mean relative biomass of Antarctic krill Euphausia superba, in the area north of Continental Shelf, was 9.4 (±18%) g/l000m3 of filtered water with a mean density of 11.0 (±16%) indviduals per 1000m3. The high abundance of Euphausia superba (1510 g/1000m3) found in a single haul on the Mawson Bank Area (73°S 173°E) was remarkable. Ice krill Euphausia crystallorophias replaced the Antarctic krill in dominance in the High Antarctic Zone (south of 74°), with a mean relative biomass of 3.0 (±26%) g/l000m3 and mean density of 16.8 (±10%) individuals per 1000m3. The present data have demonstrated that in the Ross Sea during the summer period, the two species of euphausiid inhabit different areas. Lengths of Ice krill and length, sex and reproductive stage of Antarctic krill occurring in discrete aggregations were analysed. This indicated that the composition of the aggregations in terms of the mean length, sex ratio and maturity stage were very variable. The catch data of Euphausia superba was characterized by the complete absence of larval stages, scarce occurrence of juveniles and composed primarily of large adult stages. From the analysis of the krill population structure it emerged that in mid summer, Euphausia superba of age 3+ and 4+ were clearly predominant. While the overall length frequency distribution of Euphausia crystallorophias is characterized by a first mode of juvenile individuals (Age class 1 +) and a second mode consisting of sub-adults and adults (Age groups 2+ and 3+).

The paper describes the criteria, based on the multiple frequency analysis, used in the Italian expeditions to the Ross Sea to discriminate Euphausia superba (krill) aggregations from Euphausia crystallorophias ones and from other scatters. They are based on the experimental data acquired in the expeditions 1989/90, 1997/98 and 1999/2000. These criteria are presented, discussed, compared with literature and applied to estimate krill biomass in the area of Ross Sea (22000 nm2), investigated from 19 to 26 December 1997. It results that krill biomass estimated by the criteria based on multiple frequency method is 8.87% lower than krill biomass estimated using conventional methods (net samplings and/or echogram analysis).

The data on sea birds distribution and abundance collected on the route Kaliningrad (Baltic Sea) - Falkland Isl. - South Georgia Isl.- South Sandwich Isl. - South Georgia Isl. - Montevideo - Kaliningrad from 1999 November to 2000 April are analyzed. Significant shift of the southern boundary range of the great shearwater Puffinus gravis up to South Sandwich Islands (about 7 degrees) is revealed. The fact may be explained by abundance fluctuation or shift of the Southern Polar Front to the South. Unusual behavior was observed in blackbrowed albatross Diomedea (Thalassarche) melanophris. It was observed diving for fish and swimming underwater at the depth 1.5 m during 15 seconds. Near South Sandwich archipelago very low abundance area of blackbrowed albatross is revealed. The phenomenon may be explained by avoidance by BA in the area of the cold Weddell Sea waters penetrating North along the archipelago.

Late in 1999 and early in 2000 there was observed a significant shifting of Falkland Current core towards the shelf and abnormally large number of icebergs on the South Georgia Island shelf. In this connection, retrospective hydrometeorological data on the areas of South-West Atlantic Antarctic were examined from 1970 to 2000 March. The analysis of atmospheric pressure average monthly fields by principal component method, and analysis of interannual variations of atmospheric circulation indices showed that since 1996 atmospheric process unusual development had been observed. In 1996-1998 an anomalous increase of meridional air transport southern component was recorded. Similar situations were not observed for the whole period under investigation. As the result, the Antarctic Circumpolar Current regime could be changed which, in turn, provoked variations in hydrodynamic conditions of krill habitat within insular areas. Small number of krill aggregations near South Georgia Island in January-February 2000 may serve as an indirect confirmation

Basing on the analysis of major water mass boundaries and their thermochaline characteristics, T,S diagrams' typification has been realized for hydrological stations carried out in the area of South Orkney Isls and Shetland Isls in January-February since 1970. Three types of years have been singled out for the area of South Orkney Isls. The first type was characterized by presence of intermediate cold layer. In some years of this type (1 97 1, 1972, 1978, 1982) the layer had positive temperature, while in other years (1980, 1981) the layer temperature was negative. The second year type (1979, 1984, 1987) was characterized by rather narrow range of water temperature from the surface to the bottom, being observed at the same time, higher salinity. The third type (1970, 1976, 1984, 1989) was characterized by higher degree of principal water mass mixing within the area and lack of distinct boundaries between the same. Two year types were revealed in the area of South Orkney Isls. Availability of a warmed up layer of summer modification and a cold intermediate layer was characteristic of first year type. Lack of surface water of summer modification was characteristic of second year type (1 970, 1979, 198 1, 1985, 1989).
As water structure within those areas was considered to be the consequence of hydrodynamics processes and an indirect index of hydrodynamic situation observed, the results of typification could be used when analysing krill aggregations' distribution and density for the period of years under examination.

There were carried out hydroacoustic, biological and oceanographic surveys in subarea 48.4 near South Sandwich Islands. The main water masses were revealed and their boundaries determined: waters of Antarctic Circumpolar Current, Weddell Sea and Secondary Frontal zone of Weddell Gyre. The schematic maps of geostrophic waters circulation are made. The joint analysis of environment parameters and krill distribution is carried out. There are determined three krill groups which differ each other by length frequency, maturity stages and waters inhabited. Distribution of the 1st krill group (juveniles 25-29 mm) well corresponds to meanders of Weddell Current to West of archipelago and distribution of the 2nd group (41 -47 mm) to Frontal Zone of Weddell Gyre. In small eddies on FZWG boundary takes place concentration of krill of the 3rd group (50-56 mm) transported there by ACC waters. Such concentrated schools are transported to South from East of archipelago, and may be mixed with krill of the 1st group. It is very probable that krill concentrated in stable eddies to East of archipelago may be kept there for a rather long time, maturation and spawning may take place. Newborn larvae may drift further providing krill recruitment for the range parts situated down the Current

A precautionary catch limit for krill was established for CCAMLR Division 58.4.1 in 1996 based on the results of an acoustic survey (BROKE) of the southernmost 873,000 km2 of this area (CCAMLR 1996). Detailed subsequent analysis of the results of this survey have indicated that Division 58.4.1 is not meridionally homogeneous in its oceanographic or biological characteristics. Division 58.4.1 can be divided into two approximately equal sized subdivisions: 80-115?E and 115-150?E on the basis of environmental data. Krill is more abundant in the western subdivision and its biomass is more latitudinally extensive in this area. This observed pattern appears to be historically stable. Because of the unequal distribution of krill in this Division, and because of its extremely large size (4.68 million km2) we propose the formal subdivision of Division 58.4.1 into two based on the analysis of the scientific data collected on BROKE. The revised krill biomasss estimates for these areas are: 3.04 million tonnes for the western portion and 1.79 for the eastern one. We suggest the calculation of new precautionary catch limits for the two subdivisions of Division 58.4.1 based on the revised krill biomass estimates.