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Project H.1.a - Improve the bigeye tuna stock assessment

01 Jan 2018 - 31 Dec 2020

Program(s) in charge: Stock Assessment Program
Improve the bigeye tuna stock assessment
  • 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
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
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