Proceedings of the Third World Fisheries Congress: Feeding the World with Fish in the Next Millenium—The Balance between Production and Environment
Status and Prospects of Artificial Propagation of Japanese Eel Anguilla japonica in China
Jun Xie, Guangjun Wang, Debo Pan, Gang Xie
In recent years, the demand for eel for human consumption has been increasing, especially in China. In China, adult eel output in 1997 was about 170,000 tons; glass eel demand was about 70 tons. The demand can no longer be met by using local natural resources, so the price of glass eel has risen drastically. For example, the glass eel price per individual was 0.7 Chinese yuan (CNY; around US$0.09) in 1990 and 10.5–18.2 CNY (around US$1.5) in 1995.
To meet market demand, China imports increasing quantities of glass eel from Europe. In the long term, this situation may not be stable; for the past 20 years, the yearly arrival of elvers in the European rivers has declined drastically, increasing speculation that the production of European eel Anguilla anguilla may not be sustainable. To worsen the situation, the number of glass-eel Japanese eel Anguilla japonica entering Chinese rivers and streams is decreasing. We hope to solve these problems through advances in eel aquaculture.
In Japan, attempts to induce maturation began in the 1960s. In 1973, researchers succeeded for the first time in obtaining fertilized eggs and larvae from eels, the maturation of which had been hormonally induced. Preleptocephalus larvae then reared for 2 weeks reached approximately 7 mm in total length. Since, many researchers have succeeded in obtaining eel larvae but not in raising the larvae beyond the time of yolk depletion and oil droplet stores, because suitable larval feeds have not been identified. As a result, eels remain long and slender, failing to reach the transparent, leaflike leptocephalus stage. In one study, embryogenesis in the Japanese eel was reported, but at 23°C, the larvae died at the end of their yolk sac period, on day 6 (Yamamoto and Yamauchi 1974); later, such larvae survived 14 d at 19°C (Yamauchi et al. 1976). The larvae were fed on endogenous nourishment only (yolk containing an oil droplet) in both experiments.
Successful artificial maturation of the European eel with egg hatching has been described (Bezdenezhnykh et al. 1983), but the larvae survived for only 3.5 d. The development and morphology of such larvae were described only until day 4 after hatch, when the yolk supply was exhausted and larvae were dying. Minor morphological differences were found between European and Japanese eels; the most important one appeared to be that the mouth and anus were still unopened in the oldest European eel larvae (Prokhorchik 1986), whereas these structures were open in Japanese eel larvae 3 d after hatching (Yamamoto and Yamauchi 1974). Russian experiments with European eel have stopped, but work continues on Japanese eel in the Far East.
In 1998, researchers at the Japanese National Research Institute of Aquaculture discovered that eel larvae would eat freeze-dried shark egg yolk. In preliminary feeding trials, researchers confirmed growth up to 10 mm in total length and survival up to 30 d after hatching of eel larvae fed freeze-dried shark egg yolk.
In 1999 trials, larvae were given feed 8 d after hatching and began feeding as soon as the feed was placed on the bottom of the tank. These larvae grew continuously until 50 d after hatching. After 30 d, the body depth gradually increased, and larvae began to take on a willowlike leaf form. On day 209 after hatching, the larvae had reached 30 mm in total length and had developed into typical leptocephali.