マハタの種苗量産技術の開発

摘要

The sevenband grouper is a protogynous hermaphrodite which matures first as a female then changes to a male in the later stage of its life cycle. Techniques for accelerating the sex reversal to obtain male brooders were required for the, success of induced breeding in this species. Administration of methylteststerone (MT) was reported to induce sex reversal effectively in groupers. However the optimum dose and treatment method have not been determined for the sevenband grouper. Thus, we examined the effects of oral administration (group A) and implantation (1 mg (group B) or 4 mg (group C) MT in a silastic capsule/fish) to immature females. The total quantity of MT administered to gruop A was estimated to be 1.5 mg/fish. Gonads exhibited various transitional phases at the end of treatment. The effect of oral administration was incomplete and varied with the feeding response of individual fish. Ten months after the treatment termination, all fish of group A reverted to resting females. In contrast, group B and C transformed into active males two months after the implan-tation, and some of them spermiated three months after the implantation. The group C remained resting males, while the group B reverted to a transitional phase. In conclusion-, MT-implantation at a dose of 2 mg/kg fish is an effective and sustainable treatment for induction of sex reversal in the sevenband grouper. Survival Activity Index (SAI) was measured to judge the activity of the larvae in the mass production of seedlings process of sevenband grouper. SAI was examined for the effects of the water temperture, illumination and feed oil addition to the breeding water for the purpose of survival rate improvement of larvae and juveniles of sevenband grouper. A clear correla-tion was recognized between SAI and the survival rates at the time of mouth opening. SAI was concluded as an effective index of the activity of the larvae. As for the larval survival rate at 10 days after hatching larvae reared at 25.0℃ had a higher survival rate than the natural water temperature (19.5 – 20.5℃) in the water temperature examination. The 24 hour continuous lighting treatment was higher than the natural day lighting treatment in the lighting experiment. The addition treatment was higher than the non-addition treatment in the feed oil addition experiment. Survival rate at 10 days after hatching in the mass production with larvae showing higher SAI values (> 10) in these good conditions were 36.4 - 68.7 %. These are about 2 times higher than 11.7 - 35.0 % in the reports from other seedling production establishments. Selection of spawners by PCR-based detection of the nodavirus gene, disinfection of fertilized eggs with residual oxidants in seawater and rearing of larval and juveniles using ozonated seawater were done to prevent VNN in mass production of seedlings of sevenband grouper in the hatchery. As a result, the occurrence of VNN was not observed in all 7 trials of the production. When juveniles were reared in sand filtered seawater, high mortalites by VNN occurred in all 4 trials. However, when the juveniles were reared in ozonated seawater, VNN did not occurred in all 7 trials. These countermeasures were judged to be effective as VNN control measures for the sevenband grouper. The sevenband groupers were cultured in indoor tanks in 2004 and 2005. The tanks used for experimental treatments were maintained under conditions of low water temperature (14.5 – 17.0℃) and short day-length (6 h:18 h, L:D) from April to the end of July. Afterwards, the the water temperature and photoperiod were gradually increased to reach 18.5℃ and long day-length (14 h:10 h, L:D) by September. A control tank was assumed to be at natural ambient water temperature and photoperiod. At the end of August, pre-vitellogenic oocytes and over-ripened eggs were collected from fish cultured under control conditions and vitellogenic oocytes were collected from the fish cultured under treatment conditions.