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The Japanese eel, Anguilla japonica, is a species of eel found in Japan, Korea, the East China Sea and the northern Philippines. Like the all the eels of its family, it is catadromous, meaning it lives parts of its life in both freshwater and saltwater. The specific spawning grounds have been recently discovered amongst seamounts to the west of the Mariana Islands. Adult eels migrate thousands of miles from freshwater rivers in East Asia to this location. The larvae, called leptocephali, hatch in the open sea and are carried by the Kuroshio Current to areas close to land where they consume plankton. After reaching adequate size, they enter the headwaters of rivers and travel upstream where they eventually reach adulthood. They are known to sometimes leave water at night and crawl over land. Its diet consists mainly of shrimp, insects and small fish.^[1] The eels are eaten in Japan, where they are called unagi, and also have uses in Chinese medicine.

Populations of the Japanese eel, along with anguillid eel populations worldwide, have declined drastically in recent years. This is presumably due to changing water conditions, interfering with spawning and the transport of their young larvae, also called leptocephili. In the case of the Japanese eel, spawning is likely affected by the gradual northward shift of a salinity front created by the meeting of high and low salinity waters. The front is detected by the adult spawning eels and recognised as the spawning grounds. The northward shift has been documented over the past 30 years and has had an adverse impact on larval transport. This is because larvae that hatch too far north arrive at their nursing grounds in freshwater habitats at times when upstream migration is not favourable.^[2]

In 2006 a team of Japanese scientists at the University of Tokyo discovered the near precise location of a spawning ground of the Japanese eel. Based on genetically identified specimens of newly hatched pre-leptocephali only 2 to 5 days old, the site was pinpointed to a relatively small area near the Suruga seamount west of the Mariana Islands (14–17° N, 142–143° E). This area allows the young to be caught in the Kuroshio Current, which carries them north to their freshwater habitats in East Asia, rather than the Mindanao Current to the south, which would carry the eels southward.^[3]

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