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    UPDATE OF THE INTEGRATED STOCK ASSESSMENT FOR THE PATAGONIAN TOOTHFISH (DISSOSTICHUS ELEGINOIDES) FOR THE HEARD AND MCDONALD ISLANDS USING CATCH-AT-AGE DATA AND TWO YEARS OF SURVEY ABUNDANCE-AT-AGE DATA

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    Document Number:
    WG-SAM-09/09
    Author(s):
    S.G. Candy (Australia)
    Abstract

    The integrated assessment of Patagonian toothfish, Dissostichus eleginoides, for the Heard and McDonald Islands (Division 58.5.2) was updated by replacing catch-at-length proportions from commercial catches with catch-at-age proportions using age length keys (ALKs) where the ALK for each combination of fishery and year had available at least 50 aged fish. For the trawl fisheries that were divided into periods within each year, the same ALK for the year was applied to the length frequency (LF) samples for each fishing period within that year. For years where insufficient fish were aged the catch-at-length proportions were retained but for a given fishery the same age-specific selectivity function and parameter values were logically applied to both types of data. For 2006 and 2007 random stratified trawl surveys, there were sufficient aged fish to convert abundance-at-length to abundance-at-age data. Effective sample sizes for the commercial catch-at-age proportions, assuming a multinomial distribution, and the coefficient of variation (CV) for the abundance-at-age, assuming a lognormal distribution, each took into account uncertainty due to haul-level variability in catch-at-length proportions, ALK sampling error (sampling fraction of the LF samples that were aged ranged from 0.8% to 18%) and random ageing error. CASAL allows a single ageing error matrix to be defined and applies this matrix to predictions of numbers-at-age and proportions-at-age. In other work, this matrix was found to depend on the readability score of the otoliths used for ageing, and sensitivity of the assessment results to the assumed readability score was investigated for readability scores of moderate (3), good (4), and excellent (5). The median score for all aged fish in the assessment was 3 but some fishery-by-year combinations had a higher value of 4. The output from the integrated assessment of most interest in this study is the CV of the estimated historical recruitment series, since this parameter strongly influences the effect of the depletion rule on the allowable catch. Compared to the assessment that did not incorporate catch-at-age or abundance-at-age data, the aged-based assessment dramatically lowered the CV for the recruitment series, from around 1.5 to 1.8 down to approximately 0.3 to 0.4, if a readability score 5 was assumed or if for a score of 4 the most stable subset (1986-2000) of the full historical series (1984-2006) was used to estimate the CV. There was no reduction in CV for either series if a score of 3 was assumed. The difference between a readability of 3 and 4 in ageing error is that zero ageing errors are relatively less prevalent (e.g. for age 8 the percentage of errors that were zero was estimated from previous work at 40% for score 3 and 48% for score 4, the corresponding +/- 1 yr errors had prevalence of 46% and 45%, respectively). A +/- 1 year error may seem minor relative to the complete age range modelled of 1 to 35 yr, however most fish caught are in a more restricted, younger age range. For example, the upper age of fish in the main survey that have an upper selectivity greater than 0.2 was approximately 12 yr while the corresponding values for the trawl and longline fisheries were 15 and 20 yr, respectively. The results presented suggest that future ageing work would give a greater improvement to the integrated assessment if otoliths with readability score of at least 4 can be obtained in sufficient numbers to allow ALKs to be constructed using only the ages obtained from these otoliths.

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