CCAMLR

This paper details a potential methodology for performing initial stock assessments for exploratory fisheries and by-catch species, where catch data and mark-recapture data are present but more detailed data, specifically age or length structured data on catches or surveys, is either missing or poorly sampled. As an example case we use the catch data, legal and IUU, and the mark-recapture data in sub-area 58.4.3a to demonstrate the potential uses of the model and also suggest potential catch limits for this stock, based on these results.

Exploratory fisheries for Dissostichus spp. have been operating in these regions for a number of consecutive years with the tagging of toothfish a prerequisite for the legal fishery. This paper presents a detailed study of the catch and effort trends in Divisions 58.4.1 and 58.4.2 and a examines 4 assessment methods based on comparative CPUE trends, local depletions, a constant recruitment model and mark recapture data.

rrent (ACC), especially around the Kerguelen Islands. However, there is little quantitative knowledge of the current field around the islands due to lack of long-term current measurements. We performed a systematic analysis of a total of 28917 points of fishing gear drifts from setting and recovery positions of demersal longlines deployed between 2002 and 2007 for Patagonian toothfish (Dissostichus eleginoides) fisheries in Kerguelen waters. This enabled the construction of a realistic field of depth-averaged time-mean slope currents along the 1000 m isobath all around the Kerguelen Islands. Sevral branches of the ACC are clearly identified, with the strongest depth-mean velocities of 25 cm s-1 east of the islands being associated with the Polar Folar rounding the islands from the south and flowing northward along the inner continental slope immediately east of the islands. These results demonstrate the potential for hitherto unexploited historic longline drift data from demersal fishing grounds to provide valuable quantitative information on the regional circulation.

Large-scale transport of seawater in ocean currents may generate spatially complex population structure through the advection of life stages of marine fish species. To test this, we compared the chemistry of otolith nuclei from Patagonian toothfish (Dissostichus eleginoides), presently managed as spatially discrete populations corresponding to fishery management areas along the Antarctic Circumpolar Current (ACC), which transports water eastward around the Southern Ocean. The chemistry of otolith nuclei, laid down during early life, differed significantly between fishing areas off South America and the Antarctic and between some Antarctic areas. We also found evidence of four groups of fish with different early life chemistry: one associated with South America and three Antarctic groups showing mixing consistent with advective transport along the ACC. These results suggest that toothfish populations are structured by their physical environment; population abundance and persistence may rely on a restricted number of breeding members with access to spawning grounds, whereas fisheries may rely substantially on nonbreeding vagrants transported from fishing areas upstream.

Pronounced environmental trends across fronts suggest that the otolith chemistry of oceanic fish can resolve zones on either side, promoting application to population questions at similar spatial scales. Trace and minor elements laid down immediately prior to capture – along the edges of otoliths from Patagonian toothfish Dissostichus eleginoides – discriminated frontal zones in the Antarctic Circumpolar Current in the Southwestern Atlantic Ocean. Mean values differentiated sampling areas by up to 2.6 standard deviations, suggesting: 1) otolith Mg/Ca enrichment related to fish activity around the Burdwood Bank; 2) Mn/Ca enrichment associated with South America; 3) Sr/Ca linked to the presence of Circumpolar Deep Water; and 4) Ba/Ca to nutrient production and mixing. In the Polar Frontal Zone, meanders or eddies may account for affinities with neighbouring sampling areas, bringing water from the Subantarctic and Antarctic Zones onto the North Scotia Ridge. Moreover, fish age showed a significant relationship with depth and improved cross-validation by 14%, giving 85% classification rates to South American and Antarctic regions, and 57 to 83% to areas along the Patagonian Shelf. These results indicate that otolith chemistry reflects hydrography, detecting oceanic gradients across the slope of continental shelves and between zones separated by strong trends like fronts.