Contribution Rates Of Different Spawning And Feeding Grounds To Adult Pacific Bluefin Tuna (Thunnus Orientalis) In The Northwestern Pacific Ocean

Pacific bluefin tuna (Thunnus orientalis, PBF) is an economically important species, however, the population size has decreased for decades due to overexploitation. PBF has multiple spawning grounds and shows trans-Pacific migration for forage and reproduction. Therefore, a comprehensive understanding of the population connectivity and mixing rates among feeding and spawning grounds are required for effective fishery management measures. Otolith delta O-18(oto) values suggested that most individuals (82%) analyzed were hatched in the spawning grounds of the Western North Pacific Ocean (WNP) with a small proportion (18%) of the fish hatched in the Sea of Japan (SoJ). The greater contribution of the Pacific spawning grounds to the spawning stock may be due to the larger reproductive output per capita, higher larval growth, and survival rate in the Pacific spawning grounds. Approximately 69% of the fish examined showed trans-Pacific migration at age 1-3 inferred from otolith delta C-13(oto) profiles. This result suggested a relatively higher contribution of the trans-Pacific migrants than the whole-life residents in the WNP to the giant PBF landed in Taiwan. Besides, the analysis of muscle delta N-15 values suggested that most spawners (>90%) used the pre-capture feeding grounds in the WNP or the SoJ while fewer individuals might have recently migrated from the California Current Large Marine Ecosystem to the tropical spawning grounds in the WNP. This study supports the one-stock hypothesis and further suggests a life-stage dependent population connectivity of PBF among the different feeding and spawning grounds, which should be considered in the fishery management strategies.