CCAMLR

The results of icefish ecology were summarized for the period of above 20 years of South Georgia Subarea study. Ecological aspects were considered taking into account temporal-spatial variability. The functional structure of Ch.gunnari distribution area was described on the basis of juvenile fish distribution at different stages of physiological state. Juvenile fish mainly distribute in the south, immature fish - in the cast and southwest, while adult at the north of the shelf. It was revealed that the life mode changed from pelagic one occurring at early life stages, to near-bottom - pelagic one after 20 cm length reaching and the bottom one at length above 40 cm. Ch.gunnari undertakes vertical migrations: young fish above 4 cm in length distribute within 75 cm from the bottom, and migrate into the less depths in the day-time and to the bottom at night. Immature fish distribute in the water column for the most time and is fished with midwater trawl. Its diurnal migrations pattern is similar to that of the larger fish. Diurnal migrations of adult fish are opposite to those of juveniles: it migrated to the bottom in the day-time and into the water column at night. Mature fish form dense aggregations near the bottom where it is fished with bottom gears. The annual cycle of fish is subdivided into 3 periods: feeding (October-March), spawning (April-June) and wintering (July-September). Feeding migrations of fish above 25 cm in length occur in the northern shelf and associate to migrations of krill being the main, food item of icefish. Two major directions of prespawning migrations were revealed: from the north-east to coastal areas in the northern and north-eastern part and to the open shelf areas in the southeast, while in the north-west part it migrated westwards and south-westwards. Post-spawning migrations had the opposite direction. Overwintering of icefish occurred at the depths of 200-250 m far from the coast mainly northwards of the Island.

The results of experiments on the South Georgia Shelf carried out in the period of 1986 to 1990 have been presented. Six (6) daily stations were occupied where vertical distribution of fry and young individuals (including adult specimens) of Champsocephalus gunnari, Champsocephalus aceratus, Pseudochaenichthys georgianus (Channichthiidae family) and Nototheniops larseni (Nototheniidae family) was studied. The most representative data were obtained for Ch.gunnari and N.larseni. The vertical migration trend was the most distinctive in Ch.gunnari fry. The individuals used to ascend towards the water column before the sunrise, being finished their return migration before the sunset. Juvenile and adult individuals were found in the pre-bottom layer by day, being presented their small quantities in the water column, including the superficial layer, at night. N larseni fry vertical migration pattern was poorly expressed during 24 hours. It was distinctly observed in juvenile and adult individuals: they were kept close to the bottom by daylight and were not caught within the pre-bottom layer. At night (after the sunset) they ascended into the water column where they were found in the superficial layer. No substantial variations in middle size fry of both species were found at different depth levels during 24-hour periods. Ch. aceratus fry were found within the whole water column during 24-hour periods, being mainly observed within two lower depth levels. Juvenile and adult individuals were kept by day close to the bottom and within the whole water column at night. However major catches were taken within the limits of the pre-bottom depth level, being higher as compared to those taken by day. Ps.georgianus fry were caught during 24-hours nearly in equal quantities in the pre-bottom layer by day and at higher depth levels at night. Juvenile and adult individuals were almost not found in the day-time, being observed within the whole water column at night and mainly caught within the pre-bottom layer.

1. Bottom trawl surveys on the South Georgia shelf indicate episodic declines in the abundance of the mackerel icefish Champsocephalus gunnari that, since 1990, are not directly attributable to commercial fishing.
2. The greatest effect has been observed in years when krill are known to have been scarce on the South Georgia shelf.
3. It is thought that C. gunnari survivorship is closely related to, but indirectly influenced by, krill availability.
4. Examination of food chain relationships indicates that predation by fur seals is likely to play a major role in the observed changes in C. gunnari abundance.
5. Periodic shifts in food chain relationships, involving krill, C. gunnari and fur seals, are postulated that could explain the episodic declines in the abundance of C. gunnari in the absence of fishing.
6. The study demonstrates how a key predator can have an over-riding impact on a commercial fish species, highlighting the importance of the ecosystem approach to fisheries management.

It has been suggested a method of GY-model adjustment to the additional data of observations represented as abundance indices. Catch per effort values or results of inventory surveys can serve as such indices. Adjustment basic concept is a selection of those GY-models from realization sets a certain function of which runs through the set of points defined by confidence intervals of abundance index estimates. It has been shown as an example of D.eleginoides from Subarea 48.3 that GY-model adjustment to the catch per effort values should give estimates of potential yield (2500 tons), essentially differing from those obtained by a routine model usage (3500 tons).

In February-March 1999, a Russian fishing vessel "Zakhar Sorokin" carried out fishery for Champsocephalus gunnari in CCAMLR subarea 48.3. During the processing of catches taken in the area of the northwestern slope, two species of parasites were revealed: copepod Eubrachiella antarctica and leech Trulliobdella capitis Brinkman, 1948. Statistical data collected during examination of more than 3.000 specimens of icefish has shown that the average infestation of fish with copepods constituted 24.4% and with leeches - 18.5 %.

From 16 Fenruary to 10 March, 1999, a Russian large capacity trawler "Zakhar Sorokin" carried out fishery for icefish Gunnari in CCAMLR subarea 48.3. During the cruise, 88 hauls were done and 264,921 t of icefish were caught. 86% of catch were taken from 28 February to 3 March on the nothwestern slope of the South Georgia Island, where icefish fed actively on Antarctic krill and, therefore, formed dense commercial concentrations. In this period, catches constituted 3,4 t per trawling hour and 57 t per day of fishing. Icefish 27-33 cm long were the basis of catches. Total catch of other fish species constituted 9,171 t.
Due to opinion of the captain of a Russian vessel and Russian scientists, having large experience in fisheries and research in this area, the results of the cruise have proved a complexity of fishery for this species but are not an evidence of bad status of Champsocephalus gunnari stock in the subarea 48.3

To examine consistency between laboratories in age estimation of Patagonian toothfish, otoliths were taken from 100 fish caught in the South Atlantic, and transverse sections prepared by baking, then grinding from the anterior and posterior sides of the otolith. Ages were estimated from the otoliths by one reader from the Center for Quantitative Fisheries Ecology at Old Dominion University and one reader from the National Institute of Water and Atmospheric Research, without auxiliary information: the readers generally read the sections similarly, with 84.7% of the estimates falling within two years or less of each other. Variation in estimates between readers did not increase markedly with age; however. the CQFE reader tended to give higher age estimates than the NIWA reader consistently over the full age range, indicating differences in the criteria used to interpret the first few years of age, possibly in identification of the first annulus.