Controlled feeding alleviates the reduced growth associated with spawning in farmed yellowtail (Seriola quinqueradiata)

Growth initiates sexual maturation in fish but the onset of maturity also leads to a marked loss in body weight. To avoid the growth loss associated with spawning, yellowtail (Seriola quinqueradiata) farmers have attempted to restrict the diets of their fish stocks from winter to spring, the period just before the breeding season. This prevents reductions in body weight. In our present study, we show through endocrinological and physiological analyses of harm-cultured yellowtails that the growth of these fish can be regulated through dietary control. Our results indicate that the body weights of a diet restricted yellowtail group gradually increases through the breeding season compared with the controls. Both the body weights and folk lengths of the diet restricted group surpassed the control groups. In the spawning season, the diet restricted group had smaller gonads than the controls in both sexes. However, the gonad somatic index (GSI) rapidly increased in the control group compared with the restricted group and histological observations of the controls further indicated that the testes contained sperm, and that the female gonads contained numerous oocytes that had accumulated yolk. These findings confirm that a restriction of the diet suppresses gonad development during the yellowtail breeding season. The sex steroid hormone, 17α, 20β-diOHprog, was found to be associated with this dietary control of gametogenesis as it showed decreased levels in the restricted groups compared with the controls. To further investigate the functions of 17α, 20β-diOHprog on fish growth, this hormone was administered intraperitoneally to juvenile yellowtails. Implanted fish showed a remarkable increase in body weight compared with the control group and the lipase activities of the dorsal and ventral muscles of 17α, 20β-diOHprog-injected fish were also higher than those in the control group. We conclude from our data that the regulation of the yellowtail diet can control their growth and gametogenesis.