CCAMLR

Reconstruction of size and weight composition of Antarctic toothfish (Dissostichus mawsoni) from the data on processed commercial catches of longliners using conversion factor

Last two years two different approaches were used for stock assessment of Antarctic toothfish in the Ross Sea. One of them, the CASAL model, (Dunn & Hanchet, 2007; Bull et al., 2007) is mostly based on likelihoods and potentially could insure proper mutual weighting of signals from all available sources of information incorporated into the model. The second one, the TISVPA model (Vasilyev, 2005, 2006; Vasilyev et al., 2006, 2007), takes care about robustness of analysis and includes a number of features aiming at consistent assessment using real (that is usually noisy and containing outliers) data. Besides that, some sorts of data, e.g. tagging data, are used in these models in quite different ways. Comparison of the results shows also that the input to the solution from different sources of information was also quite different: while in TISVPA all sources of data gave rather coherent signals about the stock (Vaslilyev et al., 2007), in CASAL the solution was mostly supported by signals from tagging data (Dunn & Hanchet, 2007), especially strong being the influence of fish tagged in 2006 and caught in 2007, driving the stock estimate down.

1. A dataset of all possible combinations of release nation, recapture nation, release year and recapture year for tags released and recaptured in the same SSRUs on the slope of 88.1 was compiled for the years 2003-2006. Recapture rate was expressed as tags captured/tags released/fish scanned (caught), all in numbers. 2. The overall size of the dataset was 734 combinations of release year, recapture year, SSRU, release nation and recapture nation, with 193 recaptures. Despite this size, fishing has not been consistent enough between nations to allow the analysis to be definitive. In many cases, release or recapture nation effects were not significant. In the cases where significant differences existed, recapture rates were usually highest with New Zealand tagged and recaptured fish, although there was some evidence for suggesting that recapture rates are highest when the fleet and tagging fish is the same. 3. This method could be used to identify groups of nations that have similar reporting rates, for inclusion in the Ross Sea stock assessment.

This paper outlines a method of calculating suitable tagging rates and total allowable catches (TACs) that would be expected to yield a pre-specified precision in a resultant abundance estimate. With respect to the tagging-based abundance estimator, we use the Lincoln-Petersen method and derive a formula that gives the expected coefficient of variation of the abundance estimate in terms of the number of releases and recaptures, which can in turn be expressed in terms of the tagging rate per tonne caught, the catch taken and the postulated underlying exploitable biomass. This relationship is shown to be extremely useful in terms of defining suitable catch levels and tagging rates required to obtain a given precision in the Petersen abundance estimate. To show the reliability of the precision relationship we predict the expected abundance precision using the mark and recapture data for Patagonian toothfish (Dissostichus eleginoides) in sub-area 48.3, and compare this to the precision in the abundance predicted by the integrated stock assessment. To show the usefulness of the methodology to new and exploratory fisheries we estimate the required TAC, for a given range of observed tagging rates, that would give us an abundance estimate with a coefficient of variation of 30% for the Patagonian toothfish stock in sub-area 48.4.

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.