CCAMLR

Hydroacoustic surveys of the distribution and abundance of krill, Euphausia superba Dana, were undertaken in the Prydz Bay region, Antarctica. Three surveys were carried out south of 60°S within an area of 1.28 x l06 km2 between 55°E and 95°E, during the austral summer months of 1980-81, 1983-84 and 1984-85. The surveys included a total of 25 transects and covered a distance of 22 500 km over the three seasons. The surveys formed part of the Australian contribution to the international BIOMASS program.
The results of the surveys and a discussion of the theory and methods employed including sources of error in the echointegration technique are presented. Quantitative data are presented on the abundance (biomass) of krill in the Prydz Bay region as a whole and the weight density of krill along each cruise track is presented in graphical form. These data demonstrate a patchy density distribution with surface densities reaching a maximum of over 100 g/m2 along the coastal margin.
Estimates of biomass for the entire study region of 1.28 x l06 km2 were 1.6, 3.5 and 3.7 million tonnes for the 1980-81 (FIBEX), 1983-84 (ADBEX II) and 1984-85 (SIBEX II) cruises respectively. It could not be concluded, however, that there had actually been changes in the abundance of krill in the region over the period between the surveys because of the wide confidence limits on the abundance estimates.
The precision with which changes in abundance can be detected can be improved by underway identification of target species, better calibration and improved survey design. More accurate estimates of biomass also require more accurate target strength measurements.

Current knowledge of the ecology, biology and life history of the Antarctic krill Euphausia superba Dana is reviewed. Emphasis is placed on recent developments, particularly on the results of various national and international research programmes loosely co-ordinated under the auspices of BIOMASS (Biological Investigations of Marine Antarctic Systems and Stocks). Attention is focused on topics which are either directly or indirectly applicable to the effective management of krill exploitation within the provisions set out by Article II of the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR).
The hydrography of the Southern Ocean is briefly reviewed and the horizontal distribution of krill on various scales ranging from the global to individual aggregations (e.g., swarms) is discussed. Factors likely to influence the distribution on all scales are discussed and the question of stock separation considered.
Recent attempts to assess the krill standing stock directly are examined, with particular emphasis being given to the First International BIOMASS Experiment (FIBEX) acoustic survey of 1981. General problems in assessing krill acoustically and by other direct methods are discussed, and some solutions proposed to the various associated problems.
A broad overview is given of recent progress in estimating krill production through studies of age and growth, energy budgets for individual animals, and predator consumption rates. Various models of interactions between krill and its major predators, including Man, are considered. It is concluded that neither the production estimates nor the models are adequate at present for setting optimum harvesting levels for krill.
Special attention is given to the phenomenon of aggregation in krill. Physical and biological characteristics of different types of aggregation, and the effect on them of environmental factors are discussed, as are various models of aggregation formation, maintenance and dispersal.
Finally, attempts are mads to consolidate current knowledge of aspects crucial to the ecosystem management approach espoused by CCAMLR. It is concluded that deficient information on Antarctic marine ecosystem components, and on interactions between them, precludes the implementation of a realistic multi-species management approach at present. The best prospect of obtaining the information needed for the future development of the required models appears to be through intensive studies in a number of relatively small, preferably biologically distinct, areas.

A model is set up for the operation (which includes both searching and fishing) of a Japanese krill trawler over a half-month period. It is based on an underlying krill distribution model whose parameters are determined primarily from the scientific FIBEX surveys. Output from the model of the operation is compared with (and partially tuned to) statistics for a sample of data from the commercial fishery. A major inconsistency is found: haul times are a factor of 4-5 times greater in reality than in the model. Two ad hoc model modifications are introduced to eliminate this inconsistency: artificially elongating krill swarms, and allowing hauls to continue through more than one swarm. Twenty four candidate abundance indices (generally of a CPUE form) for krill biomass in the 600 n.mile square oceanic sector modelled are considered, and their performance in response to a variety of ways in which the overall krill biomass might decline is investigated. Generally the indices respond by dropping relatively less than the proportional biomass decrease. Catch statistics collected at present (centred primarily an catch per fishing time) are of low utility in detecting biomass decline. Combination catch rate indices incorporating within-concentration search time give improved performances, but are able to monitor changes in within-concentration krill distribution parameters only. Indices that distinguish primary searching time from secondary searching time (searching while waiting to finish processing) within concentrations perform better, but collection of the requisite data may not be practical. Other approaches (e.g. research vessel surveys) need to be considered to monitor changes in the number, distribution and size of krill concentrations, both because there are doubts about the reliability of indices based on concentration searching time (which do respond to such changes), and because such indices are relatively imprecise. Priority needs to be given to improving the krill distribution model underlying the analysis; this probably requires that scientific surveys be planned to operate in small areas concurrently with fishing vessels.

This paper includes information on the trend of the elephant seal population at Stranger Point (25 de Mayo-King george I.) between 1980-87(excluding 1981). A decline in the population was observed in 1980-82 and 1987. The female component was mainly affected in 1 980-82 (r=-0.568) while the male component was affected in 1987 (r=-0.463). A possible influence of El Niño Southern Oscillation (ENSO) events upon the numerical changes in the studied population is postulated.

Many data on the diets of important vertebrate predator species in Prydz Bay and around the sub-Antarctic Heard and Macquarie Islands have been collected during the last seven years. Published and unpublished results are collated and summarised, and several important points emerge.
In Prydz Bay, on which is centered the CCAMLR Ecosystem Monitoring Programme's priority study area, Euphausia superba forms a rather low proportion of the diet of most vertebrate predators. Euphausia crystallorophias and the fish Pleuragramma antarcticum are important in most diets, and many predators can switch between prey species. The use of these predators to monitor the state of prey populations is thus very limited, although continuing studies will be useful to assess the natural varibility of the system.
Around the sub-Antarctic islands, the four penguin species feed more heavily on fish, especially myctophids, than at other comparable localities.