Spawning by Antarctic fish is generally considered to be seasonal and restricted to a brief period during the autumn and winter. Arising from this it has been assumed that the gonadal maturation cycle is also closely tracked to the time of year. The gonad maturation cycle of the mackerel icefish, Champsocephalus gunnari, was investigated using data collected during research surveys and from sampling the commercial fishery. Spawning appears to occur at the same time each year, but the timing of gonadal development is subject to a considerable inter-annual variation. The implications of this variation are discussed with respect to feeding conditions.

Mackerel icefish are widespread on the South Georgia shelf, Antarctica and have been subject to commercial fishing since the early 1970s. They are known to feed predominantly on krill. An index of condition which uses the ratio of the measured total mass to the estimated mass is show to provide a good indicator of local krill density. The index is likely to be little affected by the reproductive cycle unless there is high krill availability during the months around the spawning time and even then the effect is much less than the highest observed values The condition index responds rapidly to changes in hill density and therefore can provide indications of short term variations in krill availability. Condition index provides a useful proxy for krill density that is likely to be of considerable value in interpreting the results from ecosystem assessments such as that in progress under the auspices of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR).

Net samples data from the 15th Italian Antarctic Oceanographic Cruise (Jan-Feb 2000) were analysed to obtain a general picture of the summer distribution pattern, abundance and demography of krill in the western Ross Sea (Antarctica). A midwater sampler-trawl (Hamburg Plankton Net) was used to collect zooplankton and fish larvae at 63 stations. The net was deployed normally as a standard double oblique tow. Mean relative biomass of Antarctic krill Euphausia superba, in the area north of Continental Shelf, was 9.4 (±18%) g/l000m3 of filtered water with a mean density of 11.0 (±16%) indviduals per 1000m3. The high abundance of Euphausia superba (1510 g/1000m3) found in a single haul on the Mawson Bank Area (73°S 173°E) was remarkable. Ice krill Euphausia crystallorophias replaced the Antarctic krill in dominance in the High Antarctic Zone (south of 74°), with a mean relative biomass of 3.0 (±26%) g/l000m3 and mean density of 16.8 (±10%) individuals per 1000m3. The present data have demonstrated that in the Ross Sea during the summer period, the two species of euphausiid inhabit different areas. Lengths of Ice krill and length, sex and reproductive stage of Antarctic krill occurring in discrete aggregations were analysed. This indicated that the composition of the aggregations in terms of the mean length, sex ratio and maturity stage were very variable. The catch data of Euphausia superba was characterized by the complete absence of larval stages, scarce occurrence of juveniles and composed primarily of large adult stages. From the analysis of the krill population structure it emerged that in mid summer, Euphausia superba of age 3+ and 4+ were clearly predominant. While the overall length frequency distribution of Euphausia crystallorophias is characterized by a first mode of juvenile individuals (Age class 1 +) and a second mode consisting of sub-adults and adults (Age groups 2+ and 3+).

The CCAMLR Ecosystem Monitoring Program uses indices derived from data on indicator species and the environment collected by standard methods within the three integrated study regions of the Convention Area. Standardised index values are re-calculated each year as new data become available. Trends and anomalies are presented for data at hand.