Aquaculture Production of Glass Eels As a Possible Conservation Measure for Freshwater Eels

Japan consumes 70% of the freshwater eels eaten worldwide, but is also striving to conserve the Japanese eel in East Asia. Artificial production of Japanese eels is one unique effort now being intensively conducted. If trials succeed in mass production of glass eels, the human impact on wild glass eels can be reduced to help its future population recovery. The first larvae were obtained from artificially matured adults in 1973, and the first glass eels were produced artificially in 2003. Second generation eels were produced in 2010 and the resultant juveniles are now being reared for further breeding to produce domestic strains. At present, one experimental hatchery in Japan can produce about 1000 glass eels per year, possibly 2000 at most, which cost tens of dollars each to produce. Although recent nitrogen isotopic ratio analyses indicate that food for wild eel leptocephali in the ocean is abundant mid-water marine snow, the problem in the aquaculture process is the semi-liquid slurry-type diet for larvae that mainly includes Squalus acanthias shark egg yolk, which pollutes rearing tanks. Intensive research is developing a new type of larval diet to minimize tank pollution for achieving high-density culture and mass production of glass eels.