CCAMLR

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.

Variables of prey availability between inshore and slope/offshore foraging areas were compared to evaluate important factors for the formation of foraging areas of Chinstrap penguins (Pygoscelis antarctica) and Antarctic fur seals (Arctocephalus gazella) at Seal Island. Advantages of foraging inshore are: 1) proximity to breeding colonies and 2) high krill (Euphausia superba) abundance, whereas those of foraging slope/offshore are: 3) less patchy krill distribution, 4) shallow krill distribution, 5) larger krill size and 6) occurrence of energy-rich bioluminescing myctophid fish at night. Chinstrap penguins foraged in two modes during chick-brooding period: daytime foraging chinstrap penguins foraged in the inshore region, while overnight foraging penguins foraged in the slope region. It was suggested that important factors for the formation of daytime foraging areas may be mainly 1), while those of overnight foragers may be primarily 3) and secondarily 6) considering chinstrap penguins may have difficulty in perceiving prey at night. Antarctic fur seals foraged mostly in the slope/offshore region during pup-rearing period. It was suggested that important factors may be mainly 4) and 6). In conclusion, feeding success may not be necessarily associated with dense aggregation of krill, but with distribution pattern of krill which enhance prey capture, and availability of myctophid fish.

During a research cruise of the British Antarctic Survey ship RRS James Clark Ross in February 1996, a substantial number of the ommastrephid squid, Martialia hyadesi, were caught in a pelagic trawl, near the Antarctic Polar Frontal Zone in CCAMLR Subarea 48.3. This paper describes acoustic data gathered during this haul and a survey of the immediate vicinity, in order to characterise an acoustic signature that may be used to identify this species of squid.

The effectiveness of a split-beam echosounder system to delineate single scatterers and accurately measure in-situ target strengths (TS) is dramatically enhanced (98.2 -99.4%) by combining synchronized signals from two or more adjacent split-beam transducers of different frequencies. The accuracy of the method was determined through controlled test tank experiments using multiple standard spheres and 38 and 120 kHz split-beam transducers. Furthermore, by utilizing the angular position information from one of the split-beam transducers, additional corresponding TS measurements can be collected with juxtaposed single-beam transducers. Both methods were utilized to extract in-situ TS measurements of Antarctic scatterers simultaneously at 38, 120, and 200 kHz. Differences in the resulting TS measurements provided information about the identity of constituents in a mixed species assembly.