Browsing by Subject "SBT operating model"

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    Brief examination of conditioning results of the SBT operating model for management procedure evaluation
    (2010) Kurota, Hiroyuki; Sakai, Osamu; Butterworth, Doug S
    Updated conditioning results for the operating model used for evaluation of management procedures are examined briefly. Although no serious problems are found, the examination shows that: (1) higher steepness is preferred in the grid sampling for the reference set, which leads to slightly more optimistic future projections, and (2) based on the results of grid sampling for future projections under the current catch level, robustness trials relating to longline CPUE series often show features that differ from those for the reference set.
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    Examination of the SBT operating model to inform conditioning and projection specifications
    (2009) Kurota, Hiroyuki; Sakai, Osamu; Butterworth, Doug S
    Stock assessments and constant catch projections have been conducted using the Operating Model (OM) specified at the CCSBT technical meeting held in Seattle in July 2009. The current analysis shows that: (1) for the base case, higher steepness and lower M10 (natural mortality at age 10) are preferred in the grid sampling based on the likelihood (in contrast to the prior-based weight for steepness), and this leads to more optimistic future projections despite lower current spawning biomass (3.7% of the unfished biomass for the likelihood-based and 4.9% for the prior-based approaches), (2) when incomplete mixing of fish tagged is taken into consideration, the model fit, particularly to tag recaptures, is improved, and projection results are somewhat more optimistic, (3) the low recruitment estimate in 2006 seems to be primarily a consequence of LL1 catch-at-size data from 2008, (4) when CPUE is not adjusted for overcatch (i.e., S = 0), lower steepness and higher M are preferred, and higher S scenarios generally lead to more optimistic projection results despite worse fit to observed CPUE series, and (5) several sensitivity trials which accord less reliability to the Japanese longline CPUE favour higher M values and lead to more pessimistic results.
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    Open Access
    Further examinations of the SBT operating model to explore new tagging model and grid specifications
    (2009) Kurota, Hiroyuki; Sakai, Osamu; Takahashi, Norio; Butterworth, Doug S
    Stock assessments and constant catch projections were conducted using new Operating Models (OMs; sbtmod21 and sbtmod22, which have different tagging models) developed by the CCSBT ESC. The current analysis showed that: 1. a new candidate for the tagging model (incorporated in sbtmod22) led to higher M0 (natural mortality at age 0), lower M10 (natural mortality at age 10) and lower omega (non-linearity of the CPUE-abundance relationship) estimates than the previous tagging model which is used in sbtmod21, and estimated lower current stock abundance relative to the virgin unfished biomass, 2. a high S (S=0.5; S is the proportion of longline overcatch attributed to the reported effort) led to a lower M10, but the overall results were scarcely different from those for the base assumption (S=0.25), and 3. a slight change of assumptions regarding the Indonesian fishing selectivity impacted on M estimates substantially (leading to low M0 and high M10), which indicates poor ability to explain the Indonesian catch-at-age data when using a low M10 as pointed out during the 2008 SAG meeting.