Contrasting physical mechanisms of yellowfin tuna fluctuations between the western and eastern Indian Ocean

As an economically critical pelagic migratory species, yellowfin tuna (Thunnus albacores, YFT) is very sensible to physical and environmental conditions, such as sea surface temperature (SST), ocean heat content (OHC), and the mixed layer depth (MLD). We investigated the impact of SST, OHC, and MLD on fluctuations of YFT catch in the western/eastern Indian Ocean using the long time series of 63-year environmental and YFT datasets. We found that the impact of SST on YFT was heavily overestimated in the past, and MLD plays a more critical role in the YFT catch fluctuation. When the MLD deepens (>34.8 m), SST was more influential in predicting the catches of YFT than OHC in the western Indian Ocean, and OHC was more critical to YFT than SST in the eastern Indian Ocean. However, when the MLD shallows (<34.8 m), MLD was more vital to predict the catch per unit effort (CPUE) of YFT than SST/OHC in the western. After 2000, there was an asynchronous pattern of YFT CPUE induced by higher frequency variations and ocean hiatus of SST/OHC signals in the western and eastern Indian Oceans basins. The impact of the subsurface hiatus may induce the decrease of YFT in the eastern Indian Ocean. The above findings clarified a non-stationary relationship between the environmental factors and catches of YFT and provided new insights into variations in YFT abundance.