CCAMLR

Samples of fish collected from 1983 to 1992 at Potter Cove, King George Island, South Shetland Islands, are used to study the decline in abundance previously reported fro fjord Notothenia rossii, in association with changes in the size and age structure of the juvenile population. Mean lengths increased in correlation with poor recruitment to inshore waters and sharply decreased as stronger year classes entered the cove. In comparison, Notothenia neglecta, a non commercially fished species with similar ecological habits in the fjords, showed little variations around the expected overall mean size, without any significant trend. Thus, the changes in the juvenile N. rossii population are not thought to be caused by local factors, but related with the operations of the commercial fishery during the late seventies in the area, mainly around Elephant Island. Data from 1991 and 1992 might be indicating the beginning of a recovery of the N. rossii population in Potter cove.

The paper presents calculations of water flow velocity through the codend under the assumption that the flow is uniform and equal on its entire surface. The calculations were made for different ratios of cod end length to its diameter, mesh bar length to mesh bar diameter, and for various mesh opening coefficients. The results obtained were analysed with regard to the impact of flow velocity on hydrodynamic forces opening the codend. These forces, different for different cod end constructions, have great impact on selection properties. The greater velocity of water outflow is effecting the wider opening of the codend meshes and a greater probability of undersized - juvenile fish escaping.
An idea of a new setup of meshes in the codend, changing the effect of operation of longitudinal forces (hydromechanical drag forces) was presented. Closing forces become mesh opening forces, which should ensure greater mesh opening coefficient and better selectivity of the codend of the trawl.

The Antarctic fish fauna is relatively impoverished with fewer than 300 species recorded from the whole Southern Ocean. Demersal forms predominate but many of these have extended early life histories as pelagic larval and juvenile phases. Only 30 larval stages of the 103 fish species occurring at South Georgia have been identified but these represent all of the dominant and economically important forms. The larval stages occur in succession throughout the year suggesting niche separation to avoid competition. The ichthyoplankton distribution exhibits a marked division between oceanic and neritic coincident with the continental shelf-break. The neritic larval assemblages are more diverse and abundant with greater proximity to the coast. Studies on ichthyoplankton temporal distribution in the fjord, East Cumberland Bay, and the adjacent shelf areas at South Georgia show both large seasonal variations and marked interannual variations in species composition and abundance. The mechanisms controlling the larval fish assemblages are not known but the observed interannual variations must reflect interactions within the neritic ecosystem and so have important implications for subsequent recruitment at South Georgia. By-catch of young fish during the krill fishery further perturbate recruitment success and may delay the recovery of fish populations from over-exploitation by commerical-scale fin-fisheries over the past 30 years.

Discussed in this paper is an investigation on juvenile antarctic fish caught during krill fishing by the BAT GrigOly Kovtun in the region of the South Georgia Islands during the period May-June 1992.
Two species were predominant in the by-catches: Champsocephalus gunnari and Nototheniops larseni. The frequency of occurrence of juvenile fish was 18.2% when considering all krill tows and 45.5% when only tows made in shelf waters were considered. Juvenile fish abundance, normalised to 1 t of krill, ranged from 700 to 18 900 individuals. In the case of C.gunnari, average values were 966±225 ind/1t krill and 2434±579 ind/1t krill for all trawls and for shelf trawls, respectively. Similarly, for N.larseni the corresponding averages were 557±103 and 1388±248.
The mean standard length of C.gunnari was 73-80 mm in May-June and 97 mm in late July. The mean growth rate of this species over this period is estimated at 0.35 mm per day. In May-June N.larseni juveniles were represented by fingerlings (mean length of 42-47 mm) and yearlings (72-73 mm). In late July the mean length of fingerlings increased to 50 mm. The mean daily length increase in N.larseni is estimated at 0.09 mm.
Juveniles of Euphausia superba dominated by mass the diet of both C.gunnari fingerlings and N.larseni yearlings. The food ball in N.larseni fingerlings consisted mainly of Chaetognatha, Copepoda and furcilia of Thysanoessa spp.
According to the results obtained from krill fisheries of the Ukrainian fleet in the South Georgia area (35 500 t within the period from May to August) the total elimination of C.gunnari and N.larseni is estimated to be 34.3±8.0 and 19.8±3.7 million individuals, respectively.

In an investigation of the age structure of a population of Notothenia coriiceps Richardson at the South Orkney Islands, Antarctica, the microstructure of otoliths was examined. Progress is reported on a method used for processing large numbers of otoliths using a scanning electron microscope (SEM). Compared with previous SEM techniques, which normally prepare otoliths individually for viewing, this allowed larger sample sizes to be examined. Compared with similar light microscope techniques, this method gave enhanced resolution particularly for discerning edge structures, and thus may facilitate present methods of ageing fish populations, especially for fishery work in the Antarctic where otoliths are often small and difficult to interpret using conventional techniques.

To validate the timing of annuli in otoliths of immature Notothenia coriiceps Richardson. a time-series of samples were taken over a complete year. Light and scanning electron microscopy (SEM) techniques were used to examine the structure of sectioned otoliths. Six growth regions were identified in the otolith sections and micro-increments were also evident. The timing of growth and annual nature of annuli revealed by SEM were demonstrated. Annuli revealed by SEM and light microscopy techniques were shown to correspond, supporting the hypothesis that annuli visible by using light microscopy represent one year. Using SEM the potential errors due to light illumination artefacts and the pseudo-hyaline features could be avoided.