The ecology of eels is not fully understood, but recent research has revealed that eels are born in the Pacific Mariana Islands, 2,000 kilometers south of Japan and return to Japan.
However, the life history of eels is still a mystery and has not been fully elucidated. Therefore, artificially hatched eel fry (called Shirasu eels) cannot be used for aquaculture as an industry. Therefore, eel farmers are 100% dependent on natural sources for their supply of glass eels, which are harvested from rivers and coastlines and placed in ponds during the period from December to April.