CCAMLR

Chaenocephalus aceratus and Pseudochaenichthys georgianus have been exploited in Subarea 48.3 since 1977 but have never been fully assessed by the Working Group on Fish Stock Assessment because of a lack of reliable biological and catch data. This paper attempts to reconstruct the fishery for these two species by assuming that 75% of the ‘unidentified fish’ caught by the Soviet Union in the years 1977 to 1988 consisted of catches of C. aceratus and P. georgianus. Biological, age-length and length data from Polish research and commercial sources has been used with the new catch data in a VPA (Variable Population Analysis). The analyses show good agreement with historical biomass estimates from surveys, and indicate that the stock of C. aceratus has decreased from 18 000 tonnes to 6 000 tonnes, and that of P. georgianus from 40 000 tonnes to 10 000 tonnes, over the period of exploitation. Projections indicate that C. aceratus is unlikely to sustain a high level of catch even when it recovers, although P. georgianus may sustain a fishery of about 2 000 tonnes. The current TAC (Total Allowable Catch) (bycatch) limit of 300 tonnes would seem to be appropriate to allow recovery of both species in the near future.

Differences in the target strength of adult Antarctic krill (Euphausia Superba) at 38 and 120 kHz have been inferred from differences in the mean back-scattering strength of swarms simultaneously insonified at these two frequencies in studies off the South Orkneys and Elephant Island in March 1990. Krill in these areas varied in total length between 36 and 60mm. Backscattering strengths at 120 kHz ran consistently about 7 dB higher than at 38 kHz, a difference which was regarded as too large to be explained by possible experimental error, and which was therefore attributed to real differences in average target strength at these two frequencies. The results are in good agreement with recent experimental work on the target strength of encaged E. Superba at 38 and 120 kHz but are in major conflict with the 120 kHz - to - 50 kHz target strength conversion factor used at the Post-FIBEX Acoustic Workshop in 1984.

This paper uses actual krill length frequency data in an attempt to address the problem of determining adequate sample size to obtain representative krill length frequency distributions from commercial catches. In addition, the possible effect of two other factors (within-trawl variability and sample decomposition) which may influence length frequency data quality are also considered. The attention of WG-Krill is drawn to the following:
* All measurements of length from commercial krill catches should as far as possible be made by a single observer/vessel;
* WG-Krill should consider the minimum length differences between-catches which should be detectable. As far as possible, meaningful account must be taken of the desired biological characteristics which the Group wishes to discern (e.g. length-with-age). At present, and in the interests of both statistical rigour and the efficiency of measurement, it is proposed that length measurements from commercial catches should be grouped into 2 mm size classes;
* The biological implications associated with the detection of specific differences in length should be considered along with the need to collect maturity stage information.
* For most purposes a minimum sample size of at least 100 animals/trawl is necessary to obtain statistically meaningful differences between-samples.
* The problem of how frequently to sample commercial krill catches still requires consideration.