CCAMLR

A study on the breeding biology and diet of Pintado Petrels Daption capense was conducted at Nyrøysa, Bouvetøya, South Atlantic, from December 1996 to February 1997. Overall breeding success for the total 300 pairs of breeding Pintado Petrels at Nyrøysa was 59 %. Three study colonies were marked out and monitored intensively; in one colony overall breeding success was only 4 % owing to predation by Sub-Antarctic Skuas Catharacta antarctica on unattended chicks, but in another colony success was 85 %. Rates of failure were higher during the chick stage than in the egg stage for both of these two study colonies. The mean hatching date in these colonies was 11 January, and the first and last hatching 6 and 18 January respectively. The mean period for which chicks were permanently attended by parents (guard period) was 14 days. The length of the guard period was negatively related to the date of hatching (expressed as the number of days passed since the first egg had hatched i.e. 6 January). Growth of chicks in the third study colony was similar to that recorded for other populations of Pintado Petrel, as was the diet of chick-rearing adults - dominated by euphasiids, especially krill Euphausia superba. Whole female krill occurred more frequently than male krill in the diets of chick-rearing Pintado Petrels at Nyrøysa, and were larger. It is estimated that during the nestling stage the entire Bouvet0ya population of Pintado Petrel chicks consumes 6 500kg krill.

Acoustic surveys in the vicinity of the sub-Antarctic island of South Georgia revealed the existence of a number of horizontally extensive yet vertically discrete scattering layers in the upper 250 m of the water column. These layers were fished with a LonghurstHardy plankton recorder (LHPR) and a multiple-opening 8 m2 rectangular mid-water trawl (RMT8). Analysis of catches suggested that each scattering layer was composed predominantly of a single species of either the euphausiids Euphausia frigida or Thysanöessa macrura, the hyperiid amphipod Themisto gaudichaudii, or the eucalaniid copepod Rhincalanus gigas. Instrumentation on the nets allowed their trajectories to be reconstructed precisely, and thus catch data to be related directly to the corresponding acoustic signals. Discriminant function analysis of differences between mean volume backscattering strength at 38, 120 and 200 kHz separated echoes originating from each of the dominant scattering layers, and other signals identified as originating from Antarctic krill Euphausia superba, with an overall correct classification rate of 77%. We therefore demonstrate that with the use of echo intensity data alone, gathered using hardware commonly employed for fishery acoustics, it is possible to discriminate in situ between several zooplanktonic taxa, taxa which in some instances exhibit similar gross morphological characteristics and have overlapping length-frequency distributions. Acoustic signals from the mysid Antarctomysis maxima could additionally be discriminated once information on target distribution was considered, highlighting the value of incorporating multiple descriptors of echo characteristics into signal identification procedures. The ability to discriminate acoustically between zooplankton taxa could usefully be applied to provide improved acoustic estimates of species abundance, and to enhance field studies of zooplankton ecology, distribution and species interactions.

The paper, based on the results of italian expeditions to the Ross Sea (December 1989 - Januar 1990 and November - December 1994), presents how is spatial and temporal distribution of krill biomass and what is its value. The hydroacoustic observations were done in the time when investigated area was pratically covered by pack ice (November - mid December) or immediately after ice had receded (end December - January). The process of ice melting began in mid November, near Ross Ice Shelf (so called polynia area) and moved towards North. All the time daytime was prevaled. Krill in aggregations both below ice and in ice free water was observed but below ice were rather single ones. Because in the same time, many times, on the pieces of ices broke up by icebreaker, individuals of krill were observed, it can be supposed that below ice krill occurred mainly in scattered forms very close to surface. Aggregations of krill mainly in the part of Ross Sea between 70° and 75° S but in November and beginning of December closer to 75° S while in the end of December and January closer to 70° S were observed. It can suggested that temporally krill migrated horizontally towards North, parallely to receding of ice cover. It is also possible that part of observed aggregations were not migrated ones but created from krill scattered previously below ice.
Biomass density of krill was calculated per each nautical mile. For comparison the biomass from different years, the whole investigated area was divided in rectangles of 1°x30' .
The results of calculations indicate that biomass of krill in the western part of the Ross Sea (east of longitude 180°), estimated during late spring and at the beginning of the austral summer in 1994 was about 3 min. tons. The investigated area was approximately 30000 nm2 and the mean biomass density about 100 Unm2 . The highest value of biomass density (51761 t/nm2 ) was found the 23 November 1994 at position 174° 59' E, 74° 36' S where a so called superswarm was observed. In November 1994 the investigated area in 80-100% was covered by ice. The above mentioned superswarm was recorded in a small area of ice free water. It can be supposed that below ice krill occurred mainly in scattered forms (as mentioned above) and when the environmental conditions are favorable the coherent forms of aggregation can be formed very quickly.
During the 1989/'90 expedition hydroacoustic observations were done at the end of December and in January in ice-free water. During the first part of echo survey (in the end of December and at beginning of January) in an area of 13000 nm2 a total biomass of 426 thousand tons was estimated (mean biomass density around 32 t/nm2 ). During the second part of echosurvey (January) in an area of about 22000 nm2 the estimated biomass was 847 thousand tons ( mean biomass density around 39 t/nm2 ) . The highest value of biomass density (1041 t/nm2 ) was found in January at position 175° 40' E, 70° 45' S. The results of the italian expeditions indicate that biomass density of E. superba in the Ross Sea (Pacific Sector), contrary to previous opinions, is similar to the ones calculated in Atlantic and Indian Sectors.

Accurate estimation of krill biomass by acoustic techniques is dependant upon a number of factors including, fundamentally, accurate echo-sounder calibration. The Simrad EK500 scientific echo-sounder used aboard RRS James Clark Ross is regularly calibrated at South Georgia before and after krill surveys, and exhibits acceptable temporal stability in system gain settings. Between Antarctic seasons, this echo-sounder has additionally been calibrated in temperate European waters and, under these conditions, calibrated gain settings differ markedly from those applied in the Antarctic, even after appropriate adjustments for differences in sound speed between locations have been made. Here we present results from multiple Antarctic and European calibration events which suggest that echo-sounder transducer performance is dependant upon ambient water temperature. Highly significant differences in Sv and TS transducer gains were detected at both 38 and 120 kHz between calibrations conducted at the two locations. At 120 kHz, required Sv transducer gains were on average 1.4 dB less at South Georgia (sea temperature 2.0 °C) than in European waters (7.3 °C), and a similar trend was detected at 38 kHz. If European calibration parameters were to be employed on survey around South Georgia, and no account were taken of the differences in gain settings, then integrated 120 kHz echo signals would be under-reported by 2.8 dB, leading in turn to an under-estimation of krill biomass by 52.5 %. Every effort should therefore be made to ensure that echo-sounders are calibrated at a location as close as possible to the area in which surveys are to be conducted. In addition, the implications of temperature variation across a survey area for biomass estimation should be considered carefully.

The research performed was aimed at assessment of fishing fleet impact upon krill population during the months (December-March), being critical to dependent species. The results of fishing intensity estimation are presented on the basis of haul to haul commercial statistics for Russian fleet (data of 2900 trawlings are used) in the South Orkneys area during December 1989 - March 1990. In the above period total krill catch amounted to 88.9 thous. t which constitutes the upper limit of seasonal fishing intensity (2.7 - 88.9 thous.t) obtained for above mentioned four months in 1984-1993 (CCAMLR, 1994). Therefore, the months critical to dependent species (December - March) during 1989/90) were considered as the months of the highest fleet pressure for entire period from 1984 to 1993. Krill fishery was concentrated in the area off the northern Coronation Island. Vessels of PPR type obtained the highest catch. During the period considered average monthly catch per trawling (Q) of those vessels was at the level of 30 t. The highest variability was observed in trawlings duration (τ) and catch per trawling hour (q). Coefficients of variation of average monthly estimates of those parameters during the period considered amounted to: CV(Q)=5.7%, CV(τ)=46%, CV(q)=36.2%. During the fishery period (December 1989 - March 1990) fishing intensity in each month critical to dependent species exceeded no 1 0% of initial krill stock at beginning of each month over entire area of vessels operation. In the fishing grounds where most number of vessels operated, the above estimate exceeded no 15%. Taking in account the estimation method used, those values are considered to be the upper theoretically probable limit. Taking in account the major fishing ground location within a zone of krill regular recruitment, it may be assumed that competition between fishing fleet and dependent predators for krill resources in that area is insignificant, therefore, probable negative impact via trophic chain is low.

The results of analysis of haul by haul catch statistics of Soviet commercial fleet during 1989/90 season (totally 3614 hauls for the period from 1.11.1989 to 12.06.1990 are presented. Actually during the whole season fishing vessels worked in one fishing ground off the north-western edge of Coronation Island. Incidentally, CPUE shows regular intraseasonal variations from minimum (3.2 t/hour) in November to maximum (9.6 t/hour and 11.0 t/hour respectively) in February and March, and again decreases to 6.9-7.1 t/hour by May-June. Stability of the fishing ground is stipulated by peculiarities of the area hydrodynamics, such as availability of sustained doubling current around Islands and high water disturbance caused by topogenic effect. No temporally and spatially sustained krill aggregations (atractive to fishing vessels) are likely to form outside the main area (excluding November 1989). In November the commercial fleet fished oceanic krill aggregations formed in the major flow transporting krill northeastwards. The drift of fleet fishing the above aggregation was observed. Maximum CPUE (3.8 t/hour) was observed in the beginning of the period with gradual subsequent decrease to the level inacceptable to the fleet (2.5 t/hour) in the next to the last five days of November. After disintegration of that aggregation, existed for about 25 days, commercial vessels returned to Coronation Island. Krill drift velocity estimated on the basis of fishing ground shift velocity, amounted to 7.4 km/day or 8.7 cm/sec. The above value is compatible, however below to the drift velocity off Elephant Island (11-13 cm/sec), obtained earlier (Sushin, Myskov, 1992).

The results of hydroacoustic survey of krill biomass assessment in Subarea 48.2 adjacent to Coronation Island, carried out in February-March 1996 in Russian RN "ATLANTNIRO" are presented. Taking in account close relation of krill and oceanographic structure of waters and currents, hydrorological conditions during surveys are also discussed in the paper. Average weighed density of krill in the study area amounted to 17 g/sq.m, obtained on the basis of diurnal surveys, and average weighted krill density in the day-time amounted to 30.4 g/sq.m are comparable with density values, observed in the process of interannual krill density variability in other areas of Scotia Sea. However, density values obtained seem to be considered as underestimated and typical only for the late summer season, since in the early summer season krill density may be higher. Krill biomass estimate (1.12 ± 0.093 mln.t) in acoustic survey area, obtained in the basis of average weighted density in the day-time and night hauls, is characterized by relatively high accurancy (CV = 4.1 %). However, this estimate is likely to be biased towards underestimation which is explained by systematic error caused by krill abundance underestimation during night hauls. We consider, that krill biomass, estimated on the basis of average weighed krill density in the day-time (30.4 g/sq.m) provides a more accurate value of krill abundance in the study area. This biomass amounted to 2.00 ± 0.573 mln.t with a coefficient of variance 12.5%.

Stratified random acoustic surveys, each comprising 10 parallel transects, were conducted within two 80 x 100 km areas over the shelf-break to the north-east and north-west of South Georgia during December/January 1996/97. Netting suggested that the mean lengths of krill within the eastern and western survey boxes were 36.1 mm and 45.4 mm respectively. These lengths were used to derive box-specific krill target strength (TS) values of -38.89 and -38.59 dB kg-1. Acoustic signals believed to be attributable to krill were identified on the basis of the difference between mean volume backscattering strength at 120 and 38kHz (mvbsl20 kHz - mvbs38kHz between 2 and 12 dB indicative of krill). Mean volume backscattering strengths at 120 kHz were scaled by the appropriate TS value to determine mean krill biomass for each transect. Transect values were used to derive weighted mean (and variance) estimates of krill biomass within each survey area. The estimates were 58.28 g m-2 (56.31) and 25.17 g m-2 (18.44) for the eastern and western boxes respectively. These mean density values were similar to those obtained during the n10st recent previous surveys of krill biomass (January 1996) in these areas, but substantially higher than estimates derived from surveys there in January 1994.

Between 1980 and 1997 British Antarctic Survey carried out 11 cruises in the region of South Georgia. Here we summarize the length-frequency distributions of krill caught on each of these cruises and look particularly at differences between krill taken from the eastern and western ends of the island. Cluster analysis revealed that the length-frequency distributions could be grouped into 5 main types with relatively simple biological characteristics: cluster 1 were medium-sized year 2+ krill (mean size 39.7 mm); cluster 2 were a mixture of year classes (mean size 37.7 mm); cluster 3 were large krill probably 3+ and older (mean size 49.5 mm); cluster 4 and 5 were small krill, mostly year 1 + either with or without some older size classes (mean sizes 27.2 and 24.5 mm respectively). Principle components analysis (PCA) provided good separation of these clusters using the first two axes (80 % of the total variance). The PCA for all cruises combined showed that there were no obvious differences in the type of krill found in different water depths, although there were some indications that differences did occur between different water masses. Detailed inspection of the individual cruises revealed firstly that the smallest krill were found in Weddell Sea water and that the length-frequency distributions at the western end of the island contained more large krill that those from the eastern end of the island. We consider such differences may arise because krill may originate from either the Weddell Sea or Bellingshausen Sea, may experience different conditions during transport to South Georgia and at either end of the island.