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Barfin flounder, Verasper moseri, are large flat fish inhabiting cold sea water basins in the northern Pacific off Japan. This species is suitable for aquaculture and resource enhancement in Hokkaido due to its high commercial value and growth rate at low temperatures. Recently, a developmental program for brood stock management of barfin flounder was proposed by the Hokkaido government ; one of the program objectives was to establish a reliable method for seed production of this species. At the Hokkaido Institute of Mariculture, technique for seed production of barfin flounder have been developed since 1990. However, the annual output of seedlings has been highly variable, and a consistant supply of seedlings could not be realized. This was primarily due to the very limited knowledge about reproduction, growth and habits of this species. Therefore, in the present study, the author firstly examined the mechanism of sexual maturation, spawning and larval development in cultured barfin flounder. This knowledge was subsequently applied to determine the most suitable system by which a large amount of seed can be consistently produced. When barfin flounder were reared in culture from the larval stage, both male and female fish matured normally. However, there was an obvious difference in the timing during which eggs or sperm could be obtained, because testicular development progress faster than ovarian development. Therefore, the effects of water temperature on gonadal development, and the potential to synchronize maturation among sexes by water temperature manipulation were subsequently examined. In experiments conducted in 1997 and 1998, 50 cultured barfin flounder aged3years were reared from October to April under two water temperature regimes. As reflented by the gonadosomatic index (GSI) and by histological observation, males reared at ambient seawater temperature reached functional maturation of testes in February. For females in this group, both GSI and oocyte diameter linearly increased from October and first ovulation was observed in April. These results indicated that there was a2-month difference in the timing of spawning between male and female flounders reared at ambient seawater temperature. However, in males reared at 18°C from October to December, functional maturation did not occur until April, because spermatogenesis was inhibited at 18°C. While in females vitellogenesis proceeded normally at 18°C and ovulation commenced in April, which is the same period as females reared at ambient seawater temperature. By maintaining the rearing temperature of barfin flounder at 18°C from October to December, and then gradually shifting to ambient seawater temperature over one month, it is thus possible to delay the maturation of males and thereby synchronize the spawning periods among sexes. Obtaining eggs with good quality is very important for seed production of fish, because a deterioration of egg quality results in a decrease in fertilization rate or low vitality of larvae. Therefore, we examined the cause of a deterioration of egg quality in barfin flounder. Female barfin flounder has a regular ovulation rhythm that is 3.5 北水試研報 69,1-116(2005) Sci.Rep.Hokkaido Fish.Exp.Stn.

Studies on techniques for reliable production of seed of barfin flounder Verasper moseri