WG-FSA-SAM-05

A descriptive analysis of the toothfish tagging programme carried out in the Ross Sea since 2001 is presented for the first time. Tag-release and tag-recapture data are presented for both toothfish species for Subareas 88.1 and 88.2 for New Zealand vessels only. This is because data from non-New Zealand vessels were unavailable at the time of the analysis. A total of 4903 Antarctic toothfish Read More
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Abundance estimates from tag-release and tag-recapture data require that the number and type of errors in data for analysis are minimised. Accurate recording of the sequence numbers on tags at time of release and recapture form an important part of data accuracy. Check digits, as a part of the tag sequence number, can assist in the identification of errors in recorded data.
A computer Read More
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This report outlines the development of a Bayesian sex and age structured population model for the assessment of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea (Subareas 88.1 and 88.2), and initial progress towards evaluation of spatially explicit models. Three model scenarios were investigated. The first scenario considered the Ross Sea fishery as a single homogeneous area (single Read More
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Response-biased sampling in the context of regression modelling occurs when sample units are selected with a probability that is a function of the response. In sampling to obtain fish to both age and measure for length there are two potential response-biased sampling processes when length is the response and age is the predictor variable. These sample processes are (i) the actual fishing Read More
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A version of the confidence interval calculating program TrawlCI was recompiled to enable it to be run within the DOS emulator of recent versions of the Microsoft Windows operating system. The performance of the recompiled version was compared with that of the original version.
The recompiled version of TrawlCI produced very similar, though not identical, results to the original version. Read More
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Three Operating Models (OMs) reflecting an optimistic, an intermediate, and a pessimistic current status for the toothfish resource in the Prince Edward Islands are developed. These are planned for use for initial trials of candidate Management Procedures (MPs) to provide future TAC recommendations for this resource. These will aim to provide an appropriate balance between avoiding risk of Read More
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This paper describes a possible method for implementing the precautionary approach in assessments of Dissostichus stocks when a number of different datasets are being integrated for estimating current biomass and age structure. Consideration is given to its application to the assessment of Dissostichus eleginoides in Division 58.5.2. CASAL and GYM are key elements in the implementation Read More
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We investigate the spatial and temporal (i.e. fishing season) variation both systematic and random in haul-by-haul catch and effort data from the longline fishery for Dissostichus eleginoides in Subarea 48.3. We concentrate on presenting the results of preliminary work on the potential effects of changing fishing patterns on the standardisation of the CPUE time series. We consider whether the Read More
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The ASPM model used by Agnew and Kirkwood (2004, WG-FSA-04/82) has been modified through the introduction of a new annual selectivity function, modelled as a unimodal, algebraically decaying function of age, with three easily interpretable parameters.
When only a single selectivity curve was fitted for all years, the change to the new selectivity function improved the fit to length Read More
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A short simulation study has been carried out to investigate the size of the bias in estimates of growth parameters resulting from fish of different sizes having different probabilities of being included in catch samples. Fish were included in catch samples from the population with probability equal to an assumed relative selectivity at length. Both the mean growth parameters and the Read More
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Quantifying the catch rates and biomass of by-catch species on CCAMLR’s fishing grounds is an essential component of the assessment advice prepared by WG-FSA. However, such analyses are problematic because the CCAMLR datasets are incomplete and have a high occurrence of ‘missing catch values’.
A method to treat ‘missing catch values’ using estimates derived from the mean weights of by- Read More
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ASPM have been proposed and applied for Patagonian toothfish stock assessment at CCAMLR Subarea 48.3. Last results obtained from this model, discussed at WG-FSA (2004), do not show acceptable fit with standardized CPUE series and observed length proportions in the catches.
In this paper, we discuss some of the problems related with available CPUE data and estimate new vulnerability Read More
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This paper investigates the influence of mixing of fish, and the uneven distribution of tag placements and recapture effort, on bias in the Petersen estimate. It does so by constructing a linear model of the South Georgia toothfish fishery, simulating fish movements within this system and overlaying various combinations of tagging and recapture effort to investigate bias. The fishable grounds Read More
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The mark-recapture method of estimating toothfish population size described last year, which uses a modification of the Petersen estimator to take account of mortality and selectivity, is implemented here in Splus code.

This paper presents a new approach to the stratification of catch-at-length data of Antarctic toothfish (D. mawsoni) in the Ross Sea.
Tree based regression techniques were used to stratify the sampled catch based on the median length of Antarctic toothfish for each set using the observer length frequency data. The median lengths were weighted within the regression by the inverse of the Read More
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This paper describes an approach, using CASAL, to undertake operating model/estimation model experiments for Dissostichus spp., and methods for calculating CCAMLR yields using CASAL. We present an example model for the fictitious fishery on a fictitious species Dissostichus spurious (Everson, 2004) and investigate model performance from alternative types of observations. In general, most Read More
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