Plan of trilateral collaborative study among Japan, Korea and Taiwan for producing joint abundance index with longline fisheries data for the tropical tuna species in the Indian Ocean

Three distant-water tuna longline countries, Japan, Korea and Taiwan, have started a collaborative study for improving the joint abundance index using integrated fishery data of these fleets for tropical tuna species in the Indian and Atlantic Oceans. In addition to some preliminary steps to confirm similarity and dissimilarity of fishery operation, nominal CPUE, length frequency and spatio-temporal coverage, we planned three tasks to produce the joint CPUE; 1) investigation of better approaches to account for changes in target within each country; 2) analyses using conventional regression models with geographical, environmental and fishery (including target) information; and 3) analysis using an advanced spatio-temporal model (e.g. VAST) for developing abundance indices with additional consideration of spatio-temporal correlations. Although we have started with some coding work for bigeye tuna in the Atlantic Ocean, we will also apply the methods to yellowfin tuna in the Indian Ocean in a parallel way. A final set of results on the IO yellowfin tuna will be submitted to the Working Party on Methods and Working Party on Tropical Tuna next year for use as inputs for the update of its stock assessment. The work can also be extended for the IO albacore for its future stock assessment.