The Japanese eel, Anguilla japonica, is a catadromous fish native to East Asia, found across the western Pacific Ocean. It inhabits regions from Japan in the north to Hainan Island in the south, including Korea, Taiwan, China, and the northern Philippines. It extends inland up to 600 kilometers upstream in the Yangtze River. The Japanese eel is known for its unique life cycle and cultural importance.
Physical Characteristics and Habitat
Anguilla japonica possesses an elongated, cylindrical body, compressed towards the tail. It is plain-colored, with variations in pigmentation throughout its life stages. The mouth corner extends to the posterior margin of the eyes, and the lower jaw is slightly longer than the upper.
It exhibits a migratory pattern, adapting to various habitats and salinities. It lives in freshwater rivers, lakes, estuaries, and coastal areas. Adults feed on benthic crustaceans, bony fish, and insects.
Unique Life Cycle
The Japanese eel’s life cycle begins in the tropical Pacific Ocean west of the Mariana Islands, near the West Mariana Ridge. Spawning occurs in deep ocean waters during new moon periods, at depths of 150 to 180 meters. The season extends from April through August.
After hatching, larvae (leptocephali) are leaf-like and transparent. They are passively transported by the North Equatorial Current and Kuroshio Current, traveling thousands of kilometers towards East Asian continental shelves. As they approach coastal waters, leptocephali metamorphose into “glass eels,” a translucent, juvenile stage.
Glass eels migrate into estuaries and freshwater rivers, becoming “elvers.” These elvers continue their upstream migration, growing into “yellow eels” in rivers and lakes. Yellow eels reside in these freshwater environments for several years, between 4 and 17 years, with an average of 8 years for females. They hide in refuges like holes, crevices, and mud burrows.
Finally, upon reaching sexual maturity, yellow eels transform into “silver eels,” returning to the spawning grounds. Silver eels undertake a long oceanic journey back to the West Mariana Ridge, where they spawn and complete their life cycle. This migration takes approximately six months.
Cultural and Economic Significance
The Japanese eel holds cultural and economic importance, particularly in East Asia. It is a valued food fish, considered a delicacy. In Japan, it is known as “unagi” and is a central part of the food culture, with many restaurants specializing in grilled eel dishes called “kabayaki.”
It is valued for its high nutritional quality, including fat, vitamins, and calcium. It has historically been believed to prevent summer fatigue and maintain stamina. The aquaculture of Japanese eel began in Japan around 1880, expanding to other regions like China, Taiwan, and South Korea due to its profitability. Japan remains the largest global consumer of eel products, accounting for 50-70% of world consumption. However, the aquaculture industry relies entirely on capturing wild glass eels for stocking, as artificial breeding remains challenging despite recent advancements.
Conservation and Threats
Anguilla japonica is classified as endangered by the International Union for Conservation of Nature (IUCN) and was listed by the Japanese Ministry of the Environment in 2013. The population has declined, with glass eel recruitment dropping by 90% since the 1970s. Several factors contribute to this trend.
Overfishing is a threat, particularly the intensive harvesting of glass eels for aquaculture, creating unsustainable pressure on wild populations. Habitat degradation also plays a role, with 76.8% of effective habitat area lost in 16 East Asian rivers between the 1970s and 2010s due to pollution and shore protection works. Dams and other instream structures obstruct migratory routes, impeding journeys between freshwater and marine environments.
Climate change further exacerbates these threats by affecting ocean currents and temperatures, important for larval development and migration. For example, changes in water temperature can delay glass eel recruitment. Ongoing conservation efforts include population management, habitat restoration, and international cooperation among countries where the Japanese eel is found. Despite these efforts, the complex life cycle and numerous uncertainties surrounding the species’ ecology make population assessment and effective conservation challenging.