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Management strategy evaluation for Greenland halibut (Reinhardtius hippoglossoides) in NAFO Subarea 2 and Divisions 3 LKMNO David C. M. Miller, William B. Brodie, Brian P. Healey, Peter A. Shelton and M. Joanne Morgan Northwest Atlantic Fisheries Centre, DFO Science Branch St. John’s, Newfoundland, Canada
Stock assessment & management n n n Data from commercial fisheries and research surveys are used to assess the status of stocks. XSA Population model is used. Stock assessments are done in the NAFO Scientific Council (SC). SC provides advice on TACs and other management measures to NAFO Fisheries Commission
2007 SC assessment results 5+ Biomass low 1975 -1999 Mean SSB at low levels F on ages 5 -10 very high F 0. 1 in 2007 Recent recruitment at low levels
FC Rebuilding Plan –main points NAFO Contracting Parties shall implement a fifteen-year rebuilding program for the GH stock in Subarea 2 and Divisions 3 KLMNO. Objectives: - Attain a level of exploitable biomass (5+) of 140 000 tons on average - Allow for a stable yield over the long term in the Greenland halibut fishery. Initial TACs set at: 2004 - 20, 000 tons 2005 - 19, 000 tons 2006 - 18, 500 tons 2007 - 16, 000 tons
FC Rebuilding Plan –main points The Scientific Council shall monitor and review the progress of the rebuilding and the TAC for subsequent years shall be established taking this into account. The TAC from 2008 onwards shall not be set at levels beyond 15% less or greater than the TAC of the preceding year. In reviewing the implementation of this rebuilding plan, the Fisheries Commission may decide on additional measures to ensure the effective attainment of its objective.
Background – Study Group n n n The 2006 and 2007 stock assessments indicated that the stock is no nearer to achieving the goals set out under the rebuilding plan. An exploratory MSE using FLR was presented at the 2007 Scientific Council meeting proposing this method be used as a means to design and test a robust management strategy capable of achieve the rebuilding plan goals. SC considered that in order to investigate and advise on appropriate management strategies for Greenland halibut, a study group (SG) should be formed to consider a comprehensive analysis of the performance of rebuilding strategies, including the one currently in place.
Background – Study Group n n n Primary work to be done by DFO scientists with input from other members of the NAFO scientific council (Spain in particular). Aimed to involve stakeholders in the development of the MSE: resource managers and industry representatives from Canada, Spain, Portugal and Japan. Only partially successful – involved representatives NAFO member countries, but not First Nations and ENGO’s who also have an interest.
Background – Study Group Involve the stakeholders at the building stage rather than present them with the finished product
Study Group Wiki http: //nafo-mse-ghal. wikidot. com
Greenland Halibut OMs n Three “reference subsets” of Operating Models (OMs).
Greenland Halibut OMs Stock-Recruit Function Fishery selectivities Projected Rec XSA SSB ‘Starting point’ • Current indices • Optimistic indices Natural Mortality • M=0. 1 (90% annual survival) • M=0. 2 (82% annual survival).
Management Strategies (MSs) n n - - Exploratory, need to be ‘tweaked’ Basic types: Model-free Fixed TAC (no feedback) Index based e. g. : TACy = TACy-1(1+λ*slope) Model-based Use (e. g. ) XSA results Potentially use PA reference points
FLR n n n Started on v 1. 4; later began using parts of v 1. 99 FLCore, FLXSA, FLAssess, (FLSTF)… No economics or complicated fleet dynamics at this point
Basic Simulation Structure 1. 2. 3. 4. N Specify simulation settings Set up ‘past’ according to OM Create bootstrap replication of OM ‘past’ Project population for ‘future’ by applying MS OPERATING MODEL (OM) LOOP REPLICATE LOOP MANAGEMENT STRATEGY (MS) LOOP YEAR LOOP 24 25 /100 8 20072030
OM Replicates n A bootstrap technique was used for the initial Numbers at Age in each replicate: (i) Resample XSA residuals (within age and index) to create new psuedo -indices: Ia, y = Na, y*qi*exp(ra, i) (ii) Refit XSA to generate new starting numbers at age. (iii) Extend plusgroup numbers out to age 20
Levels of Error n Deterministic “P” (process) error only: - MSs use ‘perfect’ data from ‘True Pop. ’ (N, wt, commercial selectivity etc. ) n “PO” (process-observation) error: - Indices are generated using Observation error - MSs use these indices. n If using “POM” (process-observation-model) error: - Indices are generated using Observation error - MSs use number at age data from a model assessment using these indices. n
Performance measures Average TAC to 2010 (3 year projection) Average TAC to 2018 (rebuilding plan) Average TAC to 2030 (long term) CV in TAC to 2010 (3 year projection) CV in TAC to 2018 (rebuilding plan) Fishery Related CV in TAC to 2030 (long term) Weighted mean age in the catch Catch as Fraction of MSY Ratio of 5+ biomass in 2010 to average 1975 -99 Ratio of 5+ biomass in 2018 to average 1975 -99 Ratio of 5+ biomass in 2030 to average 1975 -99 5+ biomass growth rate to 2010 5+ biomass growth rate to 2018 5+ biomass growth rate to 2030 Weighted mean age in the population At or above rebuilding target Biomass as fraction of Bmsy Stock Related
OM 1 Means
OM 17, MS 5 From the bootstrap Short term No error in catch Rebuilding plan long term
2018 OM 1 OM 9 OM 17 Avg. Catch AAV (absolute change) B 2018/B 2007 CV of F to 2018
Main conclusions n n n Management strategy evaluation for the Greenland halibut stock is a useful exercise and should be continued to completion. Further investigation of operating models is required – particularly w. r. t. conditioning. Management strategies investigated were generally acceptable – some minor modifications could be explored. Stability in TACs important, annual adjustments not necessary. n Performance statistics evaluated were acceptable n The Study Group format (Scientists, managers, industry) worked quite well.
Main recommendations n n The Study Group should be continued, but only if appropriate expertise is available. The work on Greenland halibut MSE could be regarded as a 3 -year project, currently at the end of year 1. Funding to continue the MSE work should be pursued by Contracting Parties of NAFO. Ideally, the project could have 2 dedicated researchers housed at a host institute with a critical mass of expertise. Scientific Council should examine whether a useable index of abundance can be derived from catch rate data. The Wiki site used by the Study Group in advance of the Vigo meeting should be maintained.
Lessons learnt n n Many steps to the process Stakeholder involvement is crucial - feedback required at each step of the process - more likely to buy in to the idea if they are involved in the process - problems of short term pain vs long-term gain still persist (i. e. how fast to rebuild? )
Lessons learnt n - Operating Model conditioning important: Alternative assessment models Must cover a range of perceptions Stock structure Stock-Recruit relationship must include compensation Various types of error require consideration: Correlated errors in future indices Error in catches over the historic period – also going forward Implementation error
Lessons learnt n - Management strategies: Decided on basic set Test: Deterministic -> Process error -> Full error Final candidates need to be tweaked Data can be overwhelming - Performance stats need to be clear, descriptive and understandable for all parties involved - Suite of performance measures should be decided on early and used to evaluate all MSs n
FLR impression n Very useful in the development of the MSE n Useful structure to experiment with n n n Expertise in coding not essential – accessible method Work would progress easier and quicker if main developer embedded in an institution with a critical mass of expertise Need a ‘stable’ version
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