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.
Abstract:
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.
