On-going investigation of Japanese longline CPUE for yellowfin tuna in the Indian Ocean standardized by vector-autoregressive spatiotemporal model

Japanese longline CPUE for yellowfin tuna in the Indian Ocean (IO) was standardized for 1975-2020 by vector-autoregressive spatiotemporal model (VAST). Two types of abundance indices, size-aggregated index and size-specific index of yellowfin tuna in the IO were developed to address the shrinking fishing ground of Japanese longline fishery in the IO using VAST. Cluster analysis was conducted before standardization, and cluster number by sub area was used as a catability covariates in some models. The information about the cluster number was influential in the CPUE standardization process, which affected the time series. The cluster number (targeting effect) makes the decreasing trend of the time series of indices calm (flatten). In the eastern part of the IO, the smaller fish (age 2 and 3) distributed, while in the western part all size fish distributed, and the largest fish (age 5+) distributed southern part of the Mozanbique channel.