Joint CPUE indices for the bigeye tuna in the Indian Ocean based on Japanese, Korean and Taiwanese longline fisheries data up to 2020

Joint CPUE standardization was conducted for the Indian Ocean bigeye tuna based on Japanese, Korean
and Taiwanese longline fisheries data up to 2020 to provide the WPTT with information on provisional
abundance indices for use in the coming stock assessment for this stock. The intention was to produce
combined indices by increasing the spatial and temporal coverage of fishery data. Due to the limitation
of remote data access, an approach adopted among the three counties for the previous analyses of
tropical tunas for IOTC and ICCAT was used to share only aggregated data. As an underlying analysis,
a clustering approach was applied to account for the inter-annual changes of the target in each fishery
in each region. For this purpose, a hierarchical clustering method with “fastcluster” was used, and the
outputs of the finalized cluster were then used to assign the cluster label on fishery target to each catcheffort
data. For standardizing the catch-per-unit-effort data, the conventional linear models and deltalognormal
linear models were employed for the shared aggregated data of monthly and 1° grid
resolution in each region. Basically, the trend of CPUE was similar to that for the previous stock
assessment. The models were diagnosed by the standard residual plots and influence analyses.