CCAMLR

Trends in mean length and maturity stage of toothfish were examined using observer data from the 1998, 1997 and 1996 toothfish seasons. Toothfish are distributed down the shelf slope in relation to their size, an approximately linear relationship existing between depth and fish length. A statistically significant pattern of changing mean length at depth with month was found in both 1997 and 1998 seasons, suggesting movement of animals both up and down slope at particular times of year. Months with a high mean length at depth are May and August, and with a low mean length at depth are April and July. Examination of maturity data suggests that in addition to a major spawning event in late July/August, there may be a small spawning event in April/May. The observed shifts in length at depth may be associated with movement of animals to preferred spawning depths. A general model would be that toothfish form spawning groupings at about 1000-1300m depth. Just prior to spawning, animals move (generally downslope) to this depth, dispersing immediately afterwards, mostly upslope. Behaviour appears to be different at Shag Rocks and South Georgia, but there is some evidence for exchange of animals immediately post spawning between the two areas. There may be two (or more) spawning periods during the winter, with the larger spawning period being late July and August.

A monitoring program of demersal fish in inshore sites of the South Shetland Islands has continued in Potter Cove from 1991 to 1998, covering a continuous sampling period of 15 years and in Harmony Cove, Nelson Island, in the austral summer 1995/96. The decline in trammel net catches of fjord fishes of the species Notothenia rossii and Gobionotothen gibberifrons in relation to the non commercially fished Notothenia coriiceps, which was already reported for the period 1983-1990 in a previous study, is still evident. These results are supported by our knowledge on the diet of the piscivorous Antarctic shag Phalacrocorax bransfieldensis in the South Shetland/Antarctic Peninsula area in this decade. The most likely explanation for the decrease in recruitment to the inshore sub-populations of N. rossii and G. gibberifrons in the last 15 years is the effect of the offshore commercial fishery in the area in the late 1970s. This interpretation is consistent with the information on the historical offshore commercial fishing and with the results of scientific surveys in the area.

A random stratified trawl survey of Mackerel icefish in 2 populations (Plateau and Shell Bank) in the vicinity of Heard Island enabled a revised estimate of yield for the coming 2 seasons in 1999 and 2000. While the abundances of icefish are lower in 1998 than in 1997, the Age 3 and Age 4 cohorts are still sufficiently strong to provide for a yield greater than estimates of the long term precautionary yield. Estimates of yield for the Heard Island Plateau population are 732 tonnes for 1999 and 518 tonnes for 2000. These figures may need to be re-evaluated in light of known catches in the 1998 season since the survey. Estimates of yield were not made for Shell Bank given the very low abundance in this population. Unlike the previous 3 years, the Age 2 cohort in 1998 is very weak and is expected to contribute little to the biomass in subsequent years. If recruitment to Age 2 in 1999 is also weak then the fishery in 2000 will be predominantly on Age 5 fish. After this time, catch limits may need to be set at an estimate of a long term precautionary level and be maintained thereafter unless a further survey again showed that abundant cohorts had been recruited.