CCAMLR

The elemental structure of growth increments in the otoliths of fish reflects the composition of water passing across the gills: as a result, elemental signatures can potentially be used to reconstruct the environmental history experienced by fish. To test whether the otolith elemental signatures of Patagonian toothfish (Dissostichus eleginoides) can discriminate spatial differences, we compared signatures from the outer edges (which are laid down during the interval leading to capture) of otoliths taken from toothfish sampled from management areas off southern Chile, the Falkland Islands, South Georgia, Kerguelen and Macquarie Islands. Edge Ba/Ca values were higher for toothfish caught south of the Subantarctic Front (SAF) in 1996 and 1997 compared to those captured off the South American continent, whereas Mn/Ca values were lower. Edge signatures also showed differences between samples taken south of the SAF, both across the Polar Front and across ocean basins. A sample taken west of South Georgia in 1998 showed similarities to the samples taken in 1996 and 1997 off South America, but very different Mg/Ca concentrations from all other samples.With further development, otolith elemental signatures show promise for identifying the site of capture of sampled toothfish, and for use as retrospective spatial markers to trace toothfish population structure and movement.

Using the High Resolution laser-ICPMS at Old Dominion University, we sampled the edge of otoliths from Patagonian toothfish (Dissostichus eleginoides), and examined whether the elemental signatures could discriminate between geographic locations along the Scotia Arc and along the eastern South American continental shelf near the Falkland Islands. Furthermore, we examined the elemental signatures of material in the otolith nuclei, formed during the early life history: similar nucleus signatures can be expected between fish which were spawned on the same spawning ground, even if the samples have subsequently been taken from different locations. We found signatures on the otolith edge of fish from South Georgia and Shag Rocks showed distributions characterized by lower levels of Mn/Ca than samples from the FOCZ. Signatures from the nucleus showed separation between the fish from South Georgia and Shag Rocks, and fish caught further west, implying a stock boundary. Although fish caught off the eastern North Scotia Ridge showed nucleus signatures similar to FOCZ fish, the distribution of their edge signatures was similar to South Georgia fish, suggesting that the elemental signature is due to an environmental effect rather than a genetic one.

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.