CCAMLR

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.

Acoustic surveys to estimate krill biomass require that sound backscattered by krill be identified and distinguished from all other types of backscatter. Traditionally this has been achieved by sampling characteristic types of acoustic target with nets. More recently partitioning of backscatter into species groups has been attempted using differences in echo strength at two acoustic frequencies (ΔMVBS). Here we compare net and acoustic data from 2 cruises around South Georgia in 1996 in order to assess the functionality of acoustic target identification techniques. Mean volume backscattering strength (MVBS) data at 120 and 38 kHz were collected with a Simrad EK500 echo sounder, and net samples were collected with an RMT8. Around 80 % of the targets thought to be krill on the basis of their appearance on echo-charts were also identified as krill on the basis of their difference in backscatter at 38 and 120 kHz (ΔMVBS = MVBSI20 - MVBS3S; where ΔMVBS between 2 & 12 dB). Biomass values estimated from krill identified using echo-chart appearance or dB difference were broadly similar (regression: dB classification = 0.94 visual classification, r2 = 0.99). Krill size was predicted from scattering models using the 2 frequency data and compared with that obtained in net hauls. This comparison revealed that a simplified bent cylinder model was a better predictor of krill length (predicted length = 8.79 + 0.685 observed length, r2 = 0.77) than a fluid-filled sphere model.

Two Monte Carlo simulation models are currently available to CCAMLR for estimating krill yields according to decision rules that relate spawning stock status to the median unexploited spawning biomass, SB0. The first model has been developed specifically for krill, the krill yield model, while the second is a generalised yield model. The decision rules relate to the effects of a specified long-term annual yield (γ) on (i) the probability of the spawning stock being reduced below a set proportion of SB0 during a projection run (probability of critical depletion) and (ii) the median status of the spawning stock (as a proportion of SB0) at the end of the projection run (level of escapement). The krill yield model uses a biased estimate of SB0 with a method for correcting this bias in the level of escapement. The generalised yield model uses an unbiased estimate of SB0 and, thus, requires no correction.
This paper examines the implications for estimates of krill yield from using the biased estimate of SB0 in the krill yield model compared to the unbiased estimate in the generalised yield model. The results show that the biased estimate of SB0 results in a biased estimate of the probability of critical depletion while the level of escapement is not appreciably sensitive to the method of estimating SB0. This bias in the krill yield model will result in the catch level, γ, given for a set probability of critical depletion will be too high. The relationship between fishing season and spawning season will influence the sensitivity of the krill yield model to this bias. In this case, a greater overlap between fishing season and spawning season will result in less bias occurring.

The 1997 Workshop on International Coordination was convened by Suam Kim (Korea) at 0900 on 14 July 1997 at the Southwest Fisheries Science Center, La Jolla, California. In attendance were Sung-Ho Kang (Korea), Hyungmoh Yih (Korea), Mikio Naganobu (Japan), So Kawaguchi (Japan), Volker Siegel (Germany), Anthony Amos (USA), David Demer (USA), Christopher Hewes (USA), Roger Hewitt (USA), Osmund Holm-Hansen (USA) and Valerie Loeb (USA). Attendees and addresses are listed in Table 1.1.
During the 1996/97 field season Germany, Korea and USA conducted surveys in the Elephant Island area. It was agreed during a planning session at the 1996 meeting of WG-EMM to conduct observations at a common set of stations along the 55°W meridian north and south of Elephant Island. These stations correspond to Stations 60-67 on the U.S. AMLR grid which has been occupied twice each austral summer since 1991. Table 1.2 lists the cruise dates, the dates that the common stations along 55°W were occupied, the survey areas, the types of observations conducted and the equipment used by each member country…[contact the Secretariat for a full version of the abstract]

This work presents the initial comparative analysis of acoustic and net haul krill density estimates for the RMT 8 oblique (0 to 200m depth) trawls conducted during January-March 1996 as part of a hydroacoustic biomass survey for Antarctic krill in CCAMLR Division 58.4.1.
The acoustic krill density estimates were consistantly larger than the net estimates, otherwise no relationship was found between the two data sets. Several orders of magnitudes were seen in the range of variation between acoustic and net krill density estimates. This variation was reduced by subsampling the data set, excluding sites where krill represented less than 90% of the catch. This reduction in variation was thus largely attributed to the removal of the mixed species component of the samples.

The Patagonian toothfish, Dissostichus eleginoides, has been reported as prey of Elephant seals in the Heard Island region. A question was raised at SC-CAMLR-XV concerning what affect the fishery for toothfish in Division 58.5.2 might have on the abundance of the age classes of toothfish preyed on by these seals. This paper provides an updated analysis of the age of toothfish taken by Elephant seals at Heard Island and uses the generalised yield model (Constable & de la Mare, 1996) to examine the level of escapement of fish preyed on by these seals under the current Total Allowable Catch of 3800 tonnes.
Of 21 otoliths found in Elephant Seal stomachs at Heard Island that have previously been identified as being from Dissostichus eleginoides, 18 were from one stomach and, of these, only 7 could be reliably identified as being from toothfish. There appeared to be three pairs among the seven identified otoliths, based on the similarities of size and erosion state. Estimates of age were obtained from these otoliths. The importance of Dissostichus eleginoides as prey of the Elephant seals at Heard Island is unclear given the small number of otoliths that are of sufficient quality to be identified reliably and related to a specific size class of fish. However, it seems that toothfish in the age range of 2-6 years are likely to be vulnerable, to some extent, to predation by Elephant seals.
The evaluation of escapement of young fish (ages 2-6 years old) under the current catch limit of 3800 tonnes in the trawl fishery in Division 58.5.2 indicates that escapement in biomass is more than 85%, well above the 75% level adopted by CCAMLR for other prey species such as for krill, Euphausia superba, generally and icefish, Champsocephalus gunnari, at Heard Island.

The Elephant Island long-term data set was updated for krill density and proportional recruitment. Krill biomass (from net sampling surveys) and absolute recruitment are introduced as additional indices for this time series. New survey results of the past two seasons indicate a very successful 1994/95 and an average 1995/96 krill year-class (below average for proportional recruitment, average for absolute recruitment). After a period of low krill density in the area for almost a decade, krill density and biomass have increased. Caution is expressed whether this observed increase represents a long-term recovery of the stock.