Browsing by Author "Kitakado, Toshihide"

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    Application of ADAPT-VPA to Antarctic minke whales in the JARPA research area
    (2006) Mori, Mitsuyo; Kitakado, Toshihide; Butterworth, Doug S
    The ADAPT-VPA assessment methodology originally developed by Butterworth et al. (1999) has been greatly improved 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, and 3) the effects of possible ageing-error. Furthermore sensitivities to various functional forms for selectivity and natural mortality with age are explored. The general pattern shown by the results 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 0.038 and 0.060 for the I- and P-stocks, respectively. The estimation of M is fairly robust to the various assumptions of the model. The CVs of these M estimates for the “Reference case” assessments, when compared with those of typically 0.35 for the Areaspecific assessments of Butterworth et al. (1999) which were based on fewer data, indicate an improvement in the precision of these estimates due to the accumulation of data over the long-term of the JARPA surveys. The fits of the stock-recruitment model generally require a carrying capacity for minke whales that first increased and then stabilized or declined somewhat during the last century, and suggest MSYR(1+) values in the 4-6% range. The improved precision in the estimation of M may contribute in the improvement of management and assessment of this species on a stock-specific basis, since it can reduce the uncertainty concerning the value of M and can provide an improved prior distribution for MSYR. The latter in particular, in the context of providing a measure of the productivity of which the species is capable, is essential information for effective RMP implementation through reduction of the range of plausible scenarios which need to be considered in Implementation Simulation Trials.
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    Further progress on application of ADAPT-VPA to Antarctic minke whales
    (2007) Mori, Mitsuyo; Butterworth, Doug S; Kitakado, Toshihide
    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.
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    Performance review of simple management procedures
    (International Council for the Exploration of the Sea, 2015) Carruthers, Thomas R; Kell, Laurence T; Butterworth, Doug S; Maunder, Mark N; Geromont, Helena F; Walters, Carl; McAllister, Murdoch K; Hillary, Richard; Levontin, Polina; Kitakado, Toshihide; Davies, Campbell R
    Using a management strategy evaluation approach, we compare a range of new and established management procedures (MPs) for setting catch-limits in fisheries. Performance is evaluated with respect to fish life history type, level of stock depletion, data quality, and autocorrelation in recruitment strength. We quantify the robustness of each MP with respect to the various observation processes. Methods using observations of absolute biomass or stock depletion offer the best overall performance and this is consistent across life history types, data qualities, and stock depletion levels. Simple MPs can outperform conventional data-limited methods and data-rich assessments that use time-series of catch and effort data. MP performance is most sensitive to biases in catch data. Our results indicate that often tuning MPs for specific stocks is important, though this may not be viable in data-poor assessment scenarios because of insufficient data and analysis resources.
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    Structure and estimation framework for Atlantic Bluefin tuna operating models.
    (International Commission for the Conservation of Atlantic Tunas, 2016) Carruthers, Tom; Kimoto, Ai; Powers, Joseph E; Kell, Laurence; Butterworth, Doug S; Lauretta, Matthew V; Kitakado, Toshihide
    A preliminary spatial, multi-stock statistical catch-at-length assessment model is developed as a basis for defining operating models for Atlantic bluefin tuna. The modifiable multi-stock model (M3) aims to improve upon previous multi-stock models such as MAST (Taylor et al. 2011) in three core areas. The first iteration of the model: (1) makes use of indices of abundance specific to time-area strata (e.g. for a given ocean area and month of the year), (2) does not use conventional tagging data to inform exploitation rates, (3) is fitted to samples of length composition data and therefore avoids established problems related to ageing individuals based on a growth curve and length data only. In this paper we provide a full account of preliminary M3 model equations and discuss the results of simulation evaluations of model estimation performance. Limitations of the current approach and future areas for model development are also discussed.