抄録
Japanese eel, Anguilla japonica, is one of the most important aquaculture species in Japan. From the past to the present, elvers for stocking of farms are completely dependent on the wild catch of glass eels, however the glass eel catch drastically decreased after the 1960s and has maintained extremely low levels since 2010. In this severe situation, development of full-life cycle eel culture was completed in 2010. This article reviews the history of the development of artificial seedling production techniques in the eel. Owing to the fact that eels do not undergo vitellogenesis, in other words, sexual maturation spontaneously in captive conditions, the early stage of studies was focused on inducing vitellogenesis. In 1973, Yamamoto and Yamauchi succeeded in inducing vitellogenesis and oocyte maturation/ovulation using weekly salmon pituitary (SPE) injections to silver eels, achieving the first production of eel larvae in the world. However, the rate of success in inducing oocyte maturation/ovulation by SPE injections alone was still very low. In 1985, a maturation-inducing steroid was first discovered, DHP, from masu salmon for the first time in vertebrates. DHP was found to be very effective in inducing oocyte maturation/ovulation in the eel also. The combination of weekly SPE injections and DHP injection at the final step enabled them to obtain fertilizable eggs in the 1990s. Since then, due to the higher availability of fertilized eggs, feeding experiments were carried out for the artificial eel larvae. After much trial-and-error, Tanaka developed an initial feed formulated based on shark eggs, for the artificial eel larvae, resulting in the first production of glass eels under artificial conditions in 2002. Using these eels produced in Tanaka's laboratory, the second generation of artificial produced eels was accomplished, closing the full-life cycle of the eels in 2010. However, commercial production of artificial eel seedling for aquaculture industries is still difficult because of the high cost of production compared to wild glass eels. At present, The Japan Fisheries Research and Education Agency is focused on developing the composition of the initial feed to improve growth speed and survival rate of the larvae, improving the rearing system specialized for mass-production of glass eels. These continuous efforts will allow the commercial production of glass eels, that are not dependent on wild glass eel catch, resulting in the protection of eel stock in the wild.
