CCAMLR

To obtain a representative sample of the catch taken by toothfish longliners, CCAMLR observers are recommended to sample a quota of the first 30 fish that are caught during a sampling session. Since toothfish are often aggregated on a line, this means that fish within aggregations are sampled with less catch effort than those outside; or more generally, sampling effort is applied in inverse proportion to toothfish density on the line. The present recommendation therefore indicates sampling with unequal probabilities which, if unaccounted for, can lead to serious biases in estimates of population vital rates. To resolve this, either the probabilities need to be incorporated in the analysis of sampling data, or sampling effort should be proportional to catch effort. As vessels generally record the number of baskets taken while hauling a line, observers can use this to establish a point at which to start a session. Because there is little evidence of a consistent trend in fish sampled over a line, it does not particularly matter if an observer starts sampling before or after the correct basket arrives; but, preferably, the observer should start as close as feasible, and not be consistently late or early.

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.