Recent advances in artificial production of glass eels for conservation of anguillid eel populations



Remarkable progress in techniques for maturation of adults and rearing of larvae has been achieved for the Japanese eel Anguilla japonica over the past 50 years, but recent efforts have not yet succeeded in the mass production of glass eels. This article reviews recent advances in techniques for artificial production of A. japonica glass eels. Successful new protocols for obtaining viable eggs and larvae of A. japonica are largely resulting from using artificially feminised eels instead of wild silver eels. The feminisation technique by administration of 17β-estradiol to glass eels not only provides an opportunity for using females as experimental broodstock throughout the year, but also accelerates their oocyte development or even increases the competence of females to respond to salmon pituitary extract. Induced spontaneous spawning with paired eels can improve the success rate for obtaining good-quality eggs compared with the traditional stripping-insemination method. As a result of the accumulation of field data on natural environmental conditions, such as an optimum temperature, where eel eggs and larvae were collected, the survival and growth rate of captive eel leptocephali have been much improved under these conditions. However, 50% diluted sea water (17.5 psu) that is far from the natural condition can result in better growth and survival performance in A. japonica leptocephali, possibly because it reduces the energy necessary for osmoregulation. Starvation can be a cue for triggering the onset of metamorphosis in A. japonica leptocephali, reducing the prolonged duration of the larval stage in captivity. To reach the mass production of glass eels at a commercial level, further improvement in quality of an artificial larval diet not containing eggs of the endangered shark Squalus acanthias is needed. The control of deformities in captive leptocephali and glass eels is also an important task. New techniques reducing the excessive use of exogenous hormones with parent eels should also be developed, to avoid public perception of an unhealthy product. If these techniques are completely established, we can exchange artificially produced glass eels with some part of the wild glass eels that are heavily exploited and provided to the eel aquaculture industry, and this can help to reduce the impact of eel consumption, which will contribute to the conservation of eel species worldwide.