Effect of dietary tryptophan on maturation of ayu Plecoglossus altivelis

Two preliminary rearing experiments were conducted to investigate the effects of oral doses of tryptophan on fish maturation. Young ayu born in Lake Biwa were used as the experimental fish. In 1991, the experiment was started one month before the usual spawning season using ayu with gonadosomatic index (GSI) of 8.0% in male and 9.0% of female, but in 1992 entirely immature ayu with GSI below than 0.1% in both sexes were used. They were fed three different test diets: i.e., 0.1% tryptophan (TRP), 0.1% p-chlorophenylalanine (PCPA; depleter of serotonin) diet or control. In the group fed the diet containing TRP, the peak of serum testosterone level and timing of spermiation were advanced in males and the final maturation for females was accelerated. Whereas, in ayu fed the diet with PCPA, suppression of serum testosterone levels and the delay in the timing of spermiation were noted in males, and in females lowering of serum estradiol-17 beta and the delay in the final maturation were observed. Thus, it is estimated that dietary TRP is involved in the maturation, being intermediated by serotonin. Moreover, these findings indicate the possibility that fish maturation can be controlled by not only injection or implanting of sexual hormones but also by oral administration of TRP, one of essential amino acid in fish.