Efficient Artificial Fertilization Method Based on the Ovulation Cycle in Barfin Flounder Verasper moseri.

Barfin flounder Verasper moseri is a teleost belonging to Pleuronectidae, inhabiting cold seawater basins. This species is suitable for aquaculture and proliferation in the north sea basin of Japan due to its high commercial value and growth rate at low temperatures. Natural spawning under rearing and artificial fertilization were repeatedly tried for several years. However, a stable supply of fertilized eggs could not be obtained since the fertilization rate was too low. It is essential to develop a stable suppy of fertilized eggs for fry production. The low fertilization rate is thought to be due to the quality of the egg and/or sperm. In the egg, the fertilization rate decreases with time after ovulation in the rainbow trout Oncorhynchus mykiss,1) and several Pleuronectiformes species such as Japanese flounder Limanda yokohamae,2) turbot Scophthalmus maximus,3,4) and Atlantic halibut Hippoglossus hppoglossus.5) On the other hand, it is known that the sperm in the seminal duct acquires motility due to dilution with seminal plasma in rainbow trout and chum salmon 0. keta.6,7) Therefore, the low fertilization rate in the barfin flounder was assumed to be caused by deterioration in egg quality by over-ripening or from using the sperm before obtaining motility. The present study summarizes the natural spawning and artificial fertilization results obtained so far in barfin flounder during rearing for the first time. The ovulation cycles, changes in fertilization rate with over-ripening of eggs and semen quality were investigated to clarify the cause of the low fertilization rate in the conventional artificial fertilization method. An efficient artificial fertilization method for barfin flounders was then discussed.