Browsing by Author "Sakai, Osamu"

Now showing 1 - 11 of 11
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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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    A check of operating model predictions from the viewpoint of the management procedure implementation in 2016
    (2016) Takahashi, Norio; Kurota, Hiroyuki; Sakai, Osamu; Itoh, Tomoyuki; Butterworth, Doug S
    Values of the core vessels’ longline CPUE and aerial survey (AS) indices (two required inputs to the Bali management procedure) are compared to projection results obtained from the operating model (OM). Recent observations for the CPUE index fall within the 95% probability envelope predicted by the Base case OM in 2011. The AS index values for 2012, 2014 and 2016, however, are outside the range predicted by this Base case. The 2016 AS index value remains outside the 95% probability envelope predicted even for the Robustness Test which assumes higher variability for the projected AS index, though the index values for 2012 and 2014 fall within this envelope. This constitutes a possible indication of Exceptional Circumstances. Through discussion of the Exceptional Circumstances reflected by the 2016 AS index, together with the other elements contributing to Exceptional Circumstances and consideration of their severities, the ESC needs to formulate management advice for the action required to calculate TAC for 2018-2020 fishing seasons. Regarding the TAC to be recommended for the 2017 season, we consider that no modification of the TAC value is required given that there has been no unexpected change in the fisheries indicators examined and there are no indications of any decline in recruitment indices for 2016.
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    A check of operating model predictions from the viewpoint of the management procedure implementation in 2017
    (2017) Takahashi, Norio; Sakai, Osamu; Kurota, Hiroyuki; Itoh, Tomoyuki; Butterworth, Doug S
    Values of the core vessels’ longline CPUE and aerial survey (AS) indices (two required inputs to the Bali management procedure) are compared to projection results obtained from the SBT operating model (OM). The most recent observations for the CPUE index and the AS index fall within the 95% probability envelopes predicted by the Base case OM in 2011. As regards a decision on implementation of the recommended TAC (calculated by the MP in 2016 for the 2018-2020 fishing seasons) for the 2018 season, it is considered that no modification of the value of this TAC is required because: 1) there is no evidence to support a declaration of Exceptional Circumstances from the viewpoints of a check of the OM predictions, this year’s in-depth stock assessment/projections, and other potential reasons (Indonesian small fish catch, overcatch of reported global TAC, unaccounted catch mortality); 2) no unexpected change has been detected in the fisheries’ indicators examined; and 3) there are no indications of any appreciable decline in recruitment indices for 2017.
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    Conditioning of the SBT operating model to inform projection specifications
    (University of Cape Town, 2011) Sakai, Osamu; Kurota, Hiroyuki; Takahashi, Norio; Butterworth, Doug S
    The effects of updates on the operating model are examined. Because of the new growth schedule and the most recent two years’ data, the operating model prefers somewhat higher steepness compared to the previous version. This difference has little influence on the historical trajectories of spawning stock biomass, but it leads to more optimistic projection results.
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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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    Exploration of empirical management procedures based on longline CPUE index and aerial survey index
    (2010) Kurota, Hiroyuki; Fujioka, Ko; Sakai, Osamu; Butterworth, Doug S
    We evaluated performance of four Management Procedures (MPs) with empirical algorithms to determine TACs using information from the longline CPUE series and the aerial survey index. This MP exercise shows that TAC levels in the future are quite different depending on six management targets (tuning levels for spawning biomass recovery) that were proposed by the commissioners in April 2010. It also indicates that MPs with larger TAC reduction in the early years, which might not be preferred from a socio-economic viewpoint, enable quicker stock rebuilding and greater TAC increases in later years, even though they achieve the same long-term management target for spawning biomass recovery.
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    Further evaluation of empirical management procedures based on longline CPUE index and aerial survey index
    (2010) Kurota, Hiroyuki; Fujioka, Ko; Sakai, Osamu; Takahashi, Norio; Butterworth, Doug S
    Based on recommendations made during the third Operating Model and Management Procedure Technical Meeting (June 2010, Seattle), we have revised and evaluated “HK” Management Procedures (MPs) using empirical algorithms to determine TACs using information from the longline CPUE series and the aerial survey (AS) index. The exploration of HK variants showed that this MP can behave in a variety of ways as its control parameters and sub-algorithms are changed. As evident also from previous trials, MPs with larger TAC reduction in the early years, which might not be preferred from a socio-economic viewpoint, enable quicker stock rebuilding and greater TAC increases in later years, while still achieving the same long-term management target for spawning biomass recovery (though this comparison is complicated by transient effects).
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    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.
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    Performance of the empirical management procedure (MP2) under the updated operating models
    (2011) Kurota, Hiroyuki; Takahashi, Norio; Sakai, Osamu; Butterworth, Doug S
    We evaluated the performance of the empirical management procedure (MP2) using the updated operating model (reference set and robustness trials). In order to meet management targets, TACs need to be increased steadily and substantially under the updated reference set, and the MP behavior is different from that under the previous reference set used at the 2010 ESC meeting. This is because the updated operating model estimates a very productive stock at present due to higher steepness and recent good recruitments. Tuning options, particularly regarding the maximum allowable change of TAC, would need to be reconsidered when using this optimistic operating model for performance evaluation of MPs.
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    SBT stock assessment and projection under overcatch scenarios using the Operating Model
    (2006) Kurota, Hiroyuki; Butterworth, Doug S; Sakai, Osamu
    Stock assessments and constant catch projections under several overcatch scenarios using the operating model developed by CCSBT SC are conducted. The main factors influencing the assessment results are (1) the period over which the longline overcatch took place and (2) assumptions about the extent to which the longline overcatch necessitates CPUE adjustments. We also consider the appropriateness of the criterion used previously by the SC to evaluate short term risk.
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    A scientific alternative to moratoria for rebuilding depleted international tuna stocks
    (Wiley, 2015) Hillary, Richard M; Preece, Ann L; Davies, Campbell R; Kurota, Hiroyuki; Sakai, Osamu; Itoh, Tomoyuki; Parma, Ana M; Butterworth, Doug S; Ianelli, James; Branch, Trevor A
    There is considerable international concern and scientific debate about the current state and future of tuna stocks worldwide and the capacity of Regional Fisheries Management Organisations to manage the associated fisheries effectively. In some cases, this concern has extended to predictions of imminent collapse with minimal chances of recovery, even under a commercial catch moratorium. As a viable alternative to a full fishery closure, the Commission for the Conservation of Southern Bluefin Tuna (CCSBT) has adopted a scientifically tested, adaptive rebuilding strategy for the depleted southern bluefin tuna (Thunnus maccoyii) stock. The management procedure (MP) adopted involves a harvest control rule that fully specifies the total allowable catch as a function of key indicators of stock status, adjusting future harvest levels every three years so as to meet the rebuilding targets agreed by CCSBT. It was chosen from a subset of candidate MPs selected following extensive simulation testing. This involved first selecting a wide range of plausible scenarios for stock status and input data, ranging from pessimistic to optimistic, against which the alternative candidate MPs were tested to ensure that they were robust to important uncertainties. This is the first time that a comprehensively evaluated MP has been adopted for an internationally managed tuna stock. Both the process and the outcomes have broad applicability to other internationallymanaged stocks.