A Japanese eel or Anguilla japonica is a fish found around China, Vietnam, Japan, and the Philippines. These fish are commercially valuable, especially in Japan, where they are known as unagi and are a highly prized delicacy. Several fisheries produce Japanese eels and they are also raised in captivity to meet demands. Overall numbers are declining and some researchers have suggested that there may be several reasons for this drop in numbers.
Like other eels, Japanese eels spawn in the ocean. The Japanese eel spawning grounds have been pinpointed to a very small area within the Pacific. Ocean currents pick up the eels in their larval form and allow them to reach the coastline so they can travel inland to mature. After several years in freshwater, the eels are matured and can travel back to the spawning grounds to reproduce.
The spawning ground is believed to be small because of the limited number of currents available. The adult eels need to be able to lay their eggs in a region with ocean currents that will carry the Japanese eel larvae in the right direction and very few spots in the open water offer the right mixture of ocean currents. This may be one reason why eel populations are declining, especially because some researchers believe that changes in salinity in the ocean may be confusing Japanese eels and leading them to spawn in the wrong locations.
The eels eat crustaceans they find on the ocean floor, along with insects and bony fish. Some Japanese eels will spend most of their time in saltwater, while others prefer freshwater, and they are highly adaptable fish capable of coping with environments that have varying degrees of salinity.
Aquaculture operations for raising the Japanese eel count on trapping larvae or so-called “glass eels,” immature eels getting ready to move into freshwater, because fish farmers have not been able to breed eels in captivity. Some scientists are concerned that aquaculture may be harmful to eel populations, as fish are captured before they have an opportunity to breed and produce more eels. This, combined with harvesting adult eels for the commercial market, may be another reason for the fall in the eel population.
In Japan, unagi is a popular dish, especially during the summer months. Eel can be prepared and served in a variety of ways at restaurants and at home. The Japanese eel is believed to confer vitality and vigor on people who consume it and it is among a family of foods that people may eat to treat specific medical complaints.