CCAMLR

A tagging programme for Patagonian toothfish (Dissostichus eleginoides) was initiated at South Georgia and Shag Rocks early in 2000. A total of 2486 have been tagged and released during trawl surveys, experimental pot fishing and longline fishing. Some of the fish tagged on longliners in 2002 were injected with oxytetracycline (OTC) to mark the otoliths for age validation purposes. The otoliths of some of these chemically marked recaptures of the 2003 season were examined and the preliminary results are presented here. Two fish marked with strontium-chloride (SrCl2) during the 2002 trawl survey were also recaptured. From recaptures of chemically marked fish our study seems to indicate that the dosages used for the injection of both OTC and SrCl2 did not impact negatively on their survival rate.

During January 2000 a baited video camera system was deployed fifteen times at depths of 719-1518 m around the sub-Antarctic island of South Georgia. Four species of lithodid (Anomura: Lithodidae) crab (Paralomis formosa, P. spinosissima, Lithodes sp. and Neolithodes diomedeae) were attracted to the baits of which Paralomis formosa was the most abundant. Using arrival rate at baits, predictions of odour plume size and observations of walking speed the abundance of the stone crab Paralomis formosa was estimated. Numbers of crabs increased rapidly following bait emplacement, with total numbers observed in the 4.9 m2 field of view exceeding 50 within 200 minutes on three occasions. Current speed was used to predict the area of the odour plume, and by integrating the area to account for scavenger speed the effective area of the odour plume was obtained. The density of crabs, estimated from the increase in crab numbers per unit area of odour plume, averaged 8313 individuals km-2 (range 1100-25600). Density was not significantly correlated with depth, temperature or current speed and variability was attributed to substrate form.

During January 2000 a baited video camera system was deployed fifteen times at depths of 719-1518 m around the sub-Antarctic island of South Georgia. Four species of lithodid (Anomura: Lithodidae) crab (Paralomis formosa, P. spinosissima, Lithodes sp. and Neolithodes diomedeae) were attracted to the baits of which Paralomis formosa was the most abundant. Using arrival rate at baits, predictions of odour plume size and observations of walking speed the abundance of the stone crab Paralomis formosa was estimated. Numbers of crabs increased rapidly following bait emplacement, with total numbers observed in the 4.9 m2 field of view exceeding 50 within 200 minutes on three occasions. Current speed was used to predict the area of the odour plume, and by integrating the area to account for scavenger speed the effective area of the odour plume was obtained. The density of crabs, estimated from the increase in crab numbers per unit area of odour plume, averaged 8313 individuals km-2 (range 1100-25600). Density was not significantly correlated with depth, temperature or current speed and variability was attributed to substrate form.

The scavenging fauna of the South Georgia slope was investigated from 41 baited camera deployment at depths of 471-2235 m during research cruises in September 1997 and January 2000 and 2003. The scavenging fauna was dominated by lithodid crabs, with Patagonian toothfish seen at most deployments. Other scavenging fauna included grenadiers (Macrourus sp.), morids and skates. The data indicate toothfish are absent from depths greater than 1800 m and temperatures less than 1.3 C. The arrival rate of crabs at the bait can be used to estimate density, but the behaviour of toothfish precludes this approach to estimating their density.

On the basis of the results of the trawl-acoustic survey in February-March 2002, the microstructure analysis of otoliths of juvenile icefish of 5.5-13.3 cm in length was carried out. Fish studied represent a mixture of two year-classes of 1999-2000 and 2000-2001. The mean width of the daily growth zone in the first 100 days of fish life was 6 ?k at fish length of 4 cm. By the 200th day the daily growth zone was 4.2 ?k at fish length 7.2 cm, by the 300th day – 3.5?k at fish length 11 cm, by the 400th day – 3.1 ?k at fish length of up to 11.8 cm, by the 500th day – 2.8 ?k at fish length of up to 12.5 cm. The resulting age-length ratio can be used in separation of adjacent year-classes during juvenile fish surveys. The length of the young-of- the-year varies within a broad range as a result of prolonged spawning period and by the end of the first summer of fish life approaches 5.5 – 10.4 cm. This corresponds to the age of 120 – 250 days. The length of 1-year-old fish (1+) by the end of the second summer of fish life attains 12.5-13.3 cm, with the age of 477 – 533 days, and the first annulus radius of 0.7 – 1.3 mm. It was revealed that the period of eggs incubation and larvae hatching of the year-class 2000 – 2001 was about 4 months.

Mackerel Icefish Champsocephalus gunnari Lönnberg is an important component of the ecosystem at South Georgia where its diet is dominated by Antarctic Krill Euphausia superba, it is an important prey species for a number of upper trophic level predators, and is the target of commercial fisheries. Data on the frequency and size structure of C. gunnari in the diet of Antarctic fur seals Arctocephalus gazella and gentoo penguins Pygoscelis papua at Bird Island, South Georgia from 1991-2002 were used to examine the trophic interactions and evaluate the potential impact of predators on the C. gunnari population. C. gunnari occurred in 20 – 40 % of scats from Antarctic fur seals and comprised 48% of the diet by mass of gentoo penguins. The length-frequency distributions of C. gunnari indicated a dominant mode at 180-200 mm total length (1+ age class), with strong cohorts entering the population in 1993, 1996, 1999 and 2001. A recruitment index based on the contribution by mass of the 1+ age-class in the diet of gentoo penguins indicated a higher degree of variability in recruitment than in considered previously. Estimates of consumption of C. gunnari by Antarctic fur seals and gentoo penguins (c. 138 000 tonnes per annum) exceed standing stock estimates (17 –67 000 tonnes). A deterministic population model of cg from 1991 – 2002, using a variable mortality rate scaled by the inverse of krill abundance, replicated the fluctuations in C. gunnari shown by trawl surveys. Changes in the South Georgia ecosystem over the past 2 decades may have increased the level of predator consumption of C. gunnari providing a potential ecosystem based explanation for the lack of a recovery of pre-exploitation population size