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A preliminary operating model for Indian Ocean yellowfin tuna was successfully developed with the stock assessment package SPM. The model was spatially explicit at the 5° cell level with a quarterly (3-month) time step. The model was age-structured with many of the same biological characteristics as the 2018 IOTC Stock Synthesis assessment. Fish movements were estimated using preference functions based on distance and time-varying SST and chlorophyll, with independent preference functions for mature and immature fish. The model was fitted with catch, size, CPUE and tagging data. Initial biomass was fixed because biomass scale appeared to be confounded with movement rates.
This preliminary model can be considered a proof of concept for spatially explicit operating models of pelagic species and their potential utility.
As an example of its use, the MPD estimate of the SPM operating model was used to simulate randomised observational data for size, CPUE and tag recoveries, and these observational data were reformatted and loaded into a Stock Synthesis model based on the 2018 IOTC YFT stock assessment. Four versions of the SS assessment were trialled with different approaches to the tagging data, namely mixing periods of 1, 4, and 8 quarters and a version that omitted the tag likelihood.
The model with mixing period of 8 quarters estimated very similar biomass to the ‘truth’ represented by the operating model, while shorter mixing periods produced lower biomass estimates. Omitting the tag likelihood resulted in biomass estimates approximately twice the true value. These results should also be seen as preliminary, since full exploration and validation of the models was beyond the scope of the project.
There is potential to both further develop the SPM model for yellowfin and other tuna species, and to use the operating model to test and improve current assessments. Recommendations are made for further work.
