Indian Ocean Bigeye Tuna Management Procedure Evaluation Update March 2021

This working paper briefly describes developments on the Indian Ocean Tuna Commission (IOTC) bigeye (BET) Management Procedure (MP) evaluation project, since the 2020 Working Party on Tropical Tunas (WPTT) and Working Party on Methods (WPM). Some concerns about the BET OMs were identified and discussed in 2020, but the issues were of a generic nature affecting multiple IOTC species, with potential solutions being explored by multiple parties (e.g. the yellowfin stock assessment and OM are affected to a greater and more urgent extent). No changes were proposed for the bigeye reference set OM grid presented in 2020, while one additional robustness test was added (5% reported overcatch, on top of 5% unreported overcatch during the projection period). This paper describes:
• further development of Management Procedures (MPs) based on the Pella-Tomlinson Random Effects (PTRE) model with joint process and observation error (implemented with TMB software). A new MP was added which involves fitting the PTRE model, but then calculating the TAC recommendation by conducting internal projections to identify the constant catch quota that will result in the depletion hitting a target level in a certain number of years. i.e. The approach parallels the manner in which the Commission appears to interpret the K2SM.
• A suite of nine MPs, spanning a range of structural forms, were tuned to the TCMP tuning objectives with the reference set OM, and are compared with the standard IOTC MSE graphical outputs.
• Three representatives of the tuned MPs were evaluated against the 6 Robustness tests identified by the WPTT and WPM.
• Additional tests were conducted to determine whether the bigeye MP evaluations were sensitive to some nuisance assumptions related to spatial refugia and the specific catch equation implementation within the OM. Unlike the Indian Ocean yellowfin situation, the evaluations appear to be robust.
• The report concludes with a list of issues for the Task Force to consider, including:
o What to present to TCMP 2021 (we recommend two specific MPs at the two existing tuning levels).
o Further OM development considerations, notably the use of plausibility diagnostics
o Further MP development options
o Timeline and funding support toward BET MP adoption