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This paper summarizes progress on phase 2 of the development of Operating Models (OMs) and evaluation of candidate Management Procedures (MPs) for IOTC yellowfin (YFT) tuna (a separate paper describes parallel progress for bigeye tuna). This report summarizes progress toward the cumulative list of development goals from WPTT and WPM 2016 and 2017.
The main focus has been the development of a new YFT reference case OM. The OM proposed by the IOTC working parties in 2016 (in the absence of results to review) was based on a number of independently reasonable assumptions, but many of the models had implausible dynamics due to assumption interactions (most evident in non-stationary recruitment) and very different inferences from the assessment. Models which excluded the tags tended to be very optimistic, and the CPUE and size composition data were not very informative about model plausibility. Tags were very helpful in constraining the model to a plausible parameter space, but there are reasons to doubt the tag inferences (i.e. questionable tag mixing assumptions). We propose a new approach for the OM. It involves i) expanding the grid of models with additional uncertainty dimensions, and ii) sampling the expanded grid (with replacement) to create an OM that has central tendencies for SB/SB(MSY) and MSY that are consistent with the assessment, but with CVs that are inflated by an arbitrary factor to be determined by the broader IOTC MSE community (results are shown for a factor of 3 inflation, CV ~ 13% for both quantities). Additionally, two dimensions were sampled to be equally representative - inclusion/exclusion of tags, and CPUE catchability trends of 0 and 1% per annum.
