Further progress on application of ADAPT-VPA to Antarctic minke whales

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University of Cape Town

The ADAPT-VPA assessment methodology originally developed by Butterworth et al. (1999) has been appreciably advanced by taking into account various comments made during a series of IWC-SC meetings and is applied here to abundance estimates (from both IDCR/SOWER and JARPA surveys) as well as catch at age data (both commercial and scientific) for the I and P-stocks of Antarctic minke whales. The improvements to the methodology allow account to be taken of various further aspects, primarily: 1) inter-annual differences in the distribution of the population between different management Areas, 2) a stock-recruitment relationship, 3) the effects of possible ageing-error, and 4) the effects of possible change in age-at-sexual maturity over time as indicated by analyses of readings of transition phases in ear plugs. Furthermore sensitivities to various functional forms for selectivity and natural mortality with age are explored. The general pattern shown by analysis for both stocks is of a minke whale abundance trend that increased over the middle decades of the 20th Century to peak at about 1970, and then stabilized or declined somewhat for the next three decades. The recruitment trend is similar, though with its peak slightly earlier. The annual natural mortality rate, M, is estimated to be 0.056 with a CV of 0.16 for the I-stock, and 0.069 with a CV of 0.15 for the P-stock for the “Reference case” assessments. When only the JARPA abundance estimates are used for tuning, M is estimated as 0.037 and 0.060 for the I- and P-stocks, respectively. The estimation of M is fairly robust to the various assumptions of the model. This analysis is considered preliminary, as conclusions relating to estimates of M and trends in abundance and recruitment obtained using this approach await further revisions of 1) abundance estimates obtained from IDCR and JARPA surveys, and 2) error-models for the catch-at-age data, particularly those obtained from the commercial harvests.