Fluctuation in spawning behavior and larval transport of the Japanese eel associated with global oceanic change

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Spawning ground of the Japanese eel is located in the North Equatorial Current (NEC) in the Pacific Ocean. However, several unanswered questions about the distribution of the eel and their larvae downstream of the NEC remain and are unlikely to be resolved by the observations alone. Three such questions are about 1) previous mis-determination of spawning ground, 2) large larval catch off the west coast of Taiwan, not off the east coast, 3) low abundance of larvae in Korean coastal waters. In this research, we have demonstrated that answers to these questions can be explained by the water circulation in the North Pacific by using numerical simulations. Movement of the salinity front associated with ENSO (El Nino and Southern Oscillation) controls abundance of larval transport from the spawning ground in the NEC to the nursery ground in the East Asia. The salinityfront has moved from 13°N to 17°N during last three decades. Since condition of larval transport north of 15°N is worse, this northward displacement of the salinity front probably can explain decadal scale linear decreasing of the Japanese glass eel catch from the 1970s. In this research, decadal scale ocean-climate change charecterized by the NAO (North Atrantic Oscillation) index was also proposed to explain decline of glass eel catch of the American and European eels. Associated with large regime shift in 1975, the Southern Oscillation Index and NAO index changed to another phase simultaneously and the decline of the glass eel catch in both the Pacific and Atlantic oceans was started in the mid 1970s. This correspondence suggests that the global ocean-climate changes afect migration behavior of the genetically close-related species in the Pacific and Atlantic Oceans through similar physical processes, and finally control their fish stocks.