Japanese Eel

The Japanese Eel (Anguilla japonica) is a catadromous fish in the family Anguillidae, native to East Asia, including Japan, Korea, China, Taiwan, and Vietnam. As with other Anguilla species, adults live in fresh and brackish rivers, lakes, and estuaries before making a long spawning migration to the ocean, in this case to spawning grounds near the West Mariana Ridge in the western Pacific. Its life cycle moves through leptocephalus larva, glass eel, elver, yellow eel, and silver eel stages. The species has experienced steep population declines across its range over recent decades due to river damming, coastal habitat loss, and overexploitation of juvenile glass eels, and is currently classified as Endangered, prompting regional monitoring and stock management efforts.

Japanese Eels have the typical elongated, cylindrical, snake-like anguillid body, with smooth-looking, scaleless-appearing skin and a single continuous fin wrapping from the back around the tail to the underside.

Key field marks:

• Dark olive-brown to blackish back grading to a pale yellow or white belly in the freshwater yellow eel stage
• Silvery flanks and a darker back develop in the migratory silver eel stage, with enlarged eyes
• Small, rounded pectoral fins set just behind the head; no pelvic fins
• Relatively short, blunt snout compared to some other Anguilla species

Adults typically reach 40-90 cm. It closely resembles other Anguilla eels and is best distinguished from them by geographic range and, where ranges overlap, by vertebral counts or genetic analysis.

Japanese Eel larvae hatch far offshore in the western North Pacific near the West Mariana Ridge and drift for months on ocean currents toward East Asia, entering coastal estuaries as glass eels. Juveniles and adults then live for several years to over a decade in rivers, lakes, irrigation channels, and brackish estuaries across Japan, the Korean Peninsula, mainland China, Taiwan, and northern Vietnam, favoring muddy or vegetated bottoms with cover. Some individuals remain in coastal or estuarine brackish waters rather than moving far upstream. When mature, silver-phase eels leave inland waters and migrate back out across the Pacific to their offshore spawning grounds.

Japanese Eels are mostly nocturnal, resting in burrows or under cover during the day and foraging at night on aquatic invertebrates, small fish, and other prey. They tolerate a wide range of salinities and can survive in low-oxygen water for periods by gulping air at the surface. After a long freshwater growth phase, mature eels transform into the silver migratory stage and undertake a single, one-way journey of well over 2,000 km to reach spawning grounds in the western Pacific, after which they die. Heavy historical harvest of glass eels for aquaculture stocking, combined with river barriers blocking upstream migration, has driven sharp population declines and made the species a priority for regional fisheries management.