Induced spawning and larval rearing of Japanese eel Anguilla japonica in Taiwan.

Abstract

In Taiwan, broodstock cultivation is a premise of the artificial propagation of Japanese eel Anguilla japonica due to the depletion of seaward migrating wild eel brooders. Aged Japanese eels reared in low salinities (5-10 ppt) and mud-based ponds were found with advanced gonads as well as high condition factor. The strategy of induced maturation of pond-reared female eel is commenced by injection of human chorionic gonadotropin (HCG) at the first month in order to increase the hormonal response of eel to the subsequent weekly injection of catfish pituitary extracts. At the final maturation stage, the full matured females were injected with catfish pituitary extracts and 17 α , 20β-dihydroxy-4-pregnen-3-one (DHP) to induce their ovulation and spawning in 100 l aquaria. By this way, fertilized eggs can be obtained year around. However, the problems of steadiness in fertilization and hatching rates as well as the survival rate of yolk-sac larvae are not yet solved. Eel eggs are buoyant and with multiple oil droplets. Potency of yolk-sac larvae to adapt wide range of water temperature and salinities were astonishingly strong, but they were typically sensitive to the disturbances such as aeration, oil film and water current. Darting behavior of yolk-sac larvae was often induced by mentioned disturbances and led to a crooked body shape and even death. The larvae started to exhibit a negative phototactic behavior since the forming of eye pigment. Taking together these observations, we speculate that the eel larvae may prefer a deeper, dim, serene and even low-temperature water layer at the onset of feeding. Despite evidences indicating that the eel larvae may depend on soluble organic matter in the water as their nutrient source, they could not survive beyond 25-day old in dissolved organic matter (DOM)-enriched rearing water or in green water. Based on the unique characteristics of eel- larvae, finding appropriate foods and developing a suitable larval rearing system to meet their peculiar characteristics may be the key-point strategies for the successful larval rearing of the Japanese eel. The impediment of larval rearing of eel is due to the rudimentary and fragmented biological information of eel larvae. Undoubtedly, the mysterious life cycle of eels is considered as an attractive topic in the research field and remains to be overcome.