CCAMLR

The distribution of krill catches in relation to predator colonies in Subareas 48.1 and 48.2 is shown. 74-90% of catches in Subarea 48.1 are taken within 100 km of predator colonies between December and March, and these are between 10 and 18% of the estimated total penguin consumption in this period. The pattern of fishing is very consistent in all years 1988-1991 in Subarea 48.1, but is more variable in Subarea 48.2 where 1989 and 1990 show highly mobile fishing patterns. 53-78% of total catches in Subarea 48.2 are taken within the ’critical period’ defined above, and these are between 2 and 45% of the estimated total penguin consumption. The largest catch taken in this critical period was 94 860 tonnes in Subarea 48.1, in 1989, and 88 139 tonnes in Subarea 48.2 (1991).

This paper outlines the theory and procedures for calibrating an echo integration acoustic system with a standard sphere. It presents the results of an extensive calibration of a Simrad EK500 scientific echo sounder with a 120 kHz sprit-beam transducer in a refrigerated 10m deep tank. Calibration parameters are studied in relation to sphere material (WC and Cu), water temperature (0.5-5.5°C), transmitted pulse length (0.1, 0.3, and 1.0 ms), target depth (0.8 - 7.5 m), and time (149 days). A discussion follows concerning the ramifications of calibration errors and variability on the accuracy of acoustic biomass estimation.

CCAMLR Conservation Measure 32/X sets a 1.5 million metric ton precautionary catch limit on krill (Euphausia superba) in Statistical Area 48. The measure also asks the Scientific Committee to provide the Commission with advice on how precautionary limits could be applied to subareas or local areas. Nine alternative methods of determining subarea or local area krill catch limits are evaluated relative to six criteria: 1) the degree to which information on biological relationships is considered, 2) the cost of data collection, 3) the reliability of required information, 4) the ease of enforcement, 5) the effects on current fishing patterns, and 6) the potential for delay in implementing the alternative. The probability of adverse impact on dependent species is minimized when a high amount of biological information is considered and the potential for delay is low. Therefore, we consider the following tradeoff to be important: choosing a biologically explicit alternative and delaying implementation, or choosing a biologically unrealistic alternative and implementing a management scheme immediately. We recognize that other tradeoffs may be equally important. Alternatives that allocate the 1.5 million ton limit by evenly dividing the catch among subareas or by using historical catches to set limits can be categorized as having a low potential for delaying implementation, but they ignore information on biological relationships. Alternatives based on protective zones, critical periods, predator censuses, and predator-prey models include large amounts of biological information, but may not be practical in the near future. Alternatives based on continental shelf area, simple pulse fishing, and krill surveys are not biologically explicit and result in delayed implementation. None of the alternatives are categorized as being both biologically explicit and immediately available for implementation. However, two of the alternatives (i.e. protective zones and critical periods) are unsatisfactory only because they would alter current fishing patterns. These two alternatives could be implemented immediately if the member nations are willing to tolerate changes in current fishing patterns.

Length-weight relationships for krill Euphausia superba are listed for ash-free dry weight, dry weight, and wet weight as well as relationships for other Antarctic euphausiid species. Information on sex and dominant maturity stages underlying the data are supported. The influence of seasonal changes in length-weight relationship parameters is discussed. Recommendations are given for the use of the listed lenght-weight relationships.

Krill stock composition and distributional patterns in the vicinity of Elephant Island during austral summer 1988-1992 are described. Changes in both size and maturity composition over the five year period indicates strong recruitment from the 1987/88 and 1990/91 year classes and poor recruitment from the 1989/90 and 1990/91 year classes. Year class success may be related to the abundance of large mature stages and/or the development state of mature females during early summer. The overall distributional patterns during each year indicate that the older age classes were associated with oceanic/Drake Passage waters while younger classes were associated with water masses to the south. Between-year differences in distribution patterns reflected changes in year class success and maturity stage composition.

Empirical estimates for the target strength of krill are extracted from the literature. These are confined to measurements on aggregations of live euphausiids, which should avoid a frequent cause of bias in single-animal measurements, namely thresholding. Theoretical estimates for the target strength are derived from the deformed-cylinder scattering model assuming specific sets of physical and orientational parameters, for which there is an empirical basis. The theoretical estimates show a non-monotonic dependence of target strength on both animal size and transmit frequency, notwithstanding admitted shortcomings. Some recent single-animal measurements of target strength for live euphausiids and euphausiid-re1ated specimens, made under high signal-to-noise-ratio conditions, are consistent with the general pattern. Several specific recommendations are made for future, improved determinations of krill target strength. Based on the comparisons, general prediction curves for the target strength are presented that are applicable to a wide range of lengths, acoustic frequencies, and orientation parameters.

Main outlines of E. superba material collection (sampling from commercial catches) and positions of samples examination are reviewed and described. These positions include mass-measuring of body length, examination of maturity stages composition (biological analysis), feeding activity observation and body weight determination. Periodicity of sampling and volume of samples for every position of observation are included. Several proposals regarding to Formats filling and Report of biologist-observer construction are included also. These proposals are suggest to consider during WG-KRILL-92.

Investigations of diurnal changeability of size composition of E. superba was carried out during the working period of commercial trawler ‘Grigory Kovtun’ in the fishing area near the South Orkney Islands during March-June 1990. observations at 6 time stations were fulfilled. Every station consisted of series of catches by mean the usual commercial trawl (9-12 tows per every station). Increasing of average size of animals during light time was observed at several stations. These changes are related and were determined by increasing of males proportion as well as by difference between average length of males and females. It were namely males, which first deign diurnal vertical migrations inside of swarms. When diurnal migration are absent (particularly at most later season) the diurnal changes in E. superba size composition became feebler expresses or stop. Significance of these observations in relation to data on E. superba’s size composition accepted during usual surveys (collecting E. superba from every point of survey only once) is considered.
Gradual decreasing of average size of E. superba from end March to June was observed in operation region. As may suppose, these changes are related with drift of swarm, but not with progress of life cycle at winter postspawning non-feeding season. They hardly can be related also with selective stress of fishery activity upon larger specimens of E. superba.