CCAMLR

Four Operating Models (OMs) reflecting an “Optimistic”, “Intermediate”, “Less Pessimistic” and a “Pessimistic” current status for the toothfish resource in the Prince Edward Islands region are developed which take account of the different selectivities of past longline and pot fisheries. These models are used for trials of a candidate Management Procedure (MP) which could provide future TAC recommendations for this resource. The MP uses two data sources: the recent trend in longline CPUE and the mean length of the catches made. A specific MP, with its associated control parameter values, is proposed for implementation based upon the results of the trials. Given the importance of an adequate catch rate for the economic viability of the fishery, the choice of control parameter values focused primarily on a reasonable probability of securing a catch rate increase, whatever the current resource status. MP performance is reasonably robust across a range of sensitivity tests, though does deteriorate in conservation terms if steepness h is low. These tests also indicate that monitoring of future catch-at-length information would be necessary to guard against a change in selectivity towards greater catches of older fish.

A new South Georgia bathymetric dataset (SGDB) was compiled from a variety of primary sources including multi-beam swath bathymetry. Seafloor area (km2 <500m depth) within CCAMLR subarea 48.3 was calculated using this new dataset. Total seafloor area within the region closely matched existing estimates derived from nautical charts (and single point sounding data). However, the reliability of existing seafloor area estimates were found to vary spatially and between different depth strata. The new dataset is considered the most accurate and reliable currently available and should be used for future assessments and for assisting with the stratification of surveys.

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.