- Objectives
- Improve the bigeye tuna stock assessment
- Background
- The assessment of bigeye is conducted every year, using Stock Synthesis
- The apparent regime shift in recruitment when the floating-object fishery expanded in the 1990s indicates that the assessment model is misspecified
- Management quantities are highly sensitive to the longline CPUE data
- The current assessment is no longer considered reliable for management advice, and stock status indicators are used instead
- Recent advances in stock assessment modelling allow several important improvements of the assessment model, with regard to a spatial stock assessment model, growth curves, time-varying selectivity, recruitment assumptions, data weighting, and diagnostics
- A benchmark assessment is scheduled for 2020
- Relevance for management
- The stock assessment is used to provide management advice
- The duration of recommended seasonal closures is based on the multipliers of fishing mortality (F) estimated in the bigeye and yellowfin assessments
- Improvements in the bigeye assessment will make the staff’s management advice more accurate and precise
- Duration
- 2018-2020
- Workplan and status
- 2018: Create a spatial model, integrate the new growth curve into the assessment, and implement time-varying selectivity
- 2019: Explore different recruitment assumptions, apply data weighting, conduct diagnostic tests
- 2019: Conduct a workshop to finalize the improvements to the longline CPUE and length composition data (Project H.1.f)
- 2020: Re-evaluate the model assumptions
- External collaborators
- Work conducted under the MSE project will contribute to this project
- Deliverables
- Reports for SAC-10 and SAC-11 in 2019 and 2020
- Updated date: 01 Oct 2020
- Progress summary for the reporting period
- Identified stock and spatial structure
- Developed spatial stock assessment model
- February 2018: CAPAM workshop on the development of spatio-temporal models of fishery CPUE data to derive indices of relative abundance.
- October 2018: CAPAM workshop on the development of spatial stock assessment models.
- January 2019: workshop to evaluate bigeye and yellowfin tuna ageing methodologies and growth models in the Pacific Ocean.
- February 2019: workshop to improve the longline indices of abundance of bigeye and yellowfin tunas in the EPO.
- Analyses for the external review, including exploring different recruitment assumptions, applying data weighting, and conducting diagnostic tests
- March 2019: External review of IATTC staff’s stock assessment of bigeye tuna in the EPO.
- March 2020: Benchmark assessment of bigeye tuna in the EPO
- Challenges and key lessons learnt
- The operational level longline data essential for improving the assessment are not permanently available to the staff
- An additional workshop to finalize the work on improving the longline CPUE and lengthcomposition data is needed (Project H.1.f), but not currently funded.
- The results used in the risk analysis produced a bimodal probability distribution making their interpretation in respect management advice complicated.
- See links above for workshop reports and presentations