CCAMLR

Water transported by the Antarctic Circumpolar Current (ACC) in the Southern Ocean may provide opportunities for Patagonian toothfish (Dissostichus eleginoides) to move downstream between island groups with little energetic cost, and may constrain movement in other directions, influencing population structure and richness. To test whether toothfish stocks were connected or segregated, we used an integrated experimental design to compare age-at-length data sampled at similar times from longline fisheries off the southern South American continent around the Falkland Islands north of the Sub-Antarctic Front of the ACC, and off Kerguelen and South Georgia Islands, situated respectively in the southern Indian and Atlantic Oceans to the south of the Polar Front. We fitted von Bertalanffy (VB) models representing segregation and coupling between management areas to the data, and tested between the models using a likelihood method. Toothfish showed significant differences in VB parameters between the Falkland Islands and both South Georgia and Kerguelen, but no differences between South Georgia and Kerguelen. This evidence suggests that, consistent with the dynamic-physical structure of the ACC, toothfish off the Falkland Islands are segregated from toothfish caught in the other two management areas, but that toothfish stocks at South Georgia and Kerguelen may be connected. Taken with the published genetic data, this evidence strongly suggests that toothfish population structure is related to the physical structure of the ACC and its fronts; that some toothfish populations may be connected between the major Southern Ocean basins; and that consequently, changes in toothfish population dynamics within one management area may have broader impacts through the Southern Ocean.

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.