LAUSR

Abstract The risk of onset of vertebral deformity, including kyphosis, in cultured yellowtail increases during spawning season. Farmers suffer an economic loss due to decreased product value as a result of kyphosis. The purpose of this study is to provide a basis for developing measures to reduce the risk of kyphosis occurring by clarifying what causes it. We focused on the relationship between sexual maturation and kyphosis, and the mechanism of kyphosis onset using two-year-old yellowtail collected in February, April, May, and June. We analyzed the gonadosomatic index (GSI), the gonadal stage, serum levels of sex steroid hormones, X-ray images of the caudal vertebrae, the strength of the vertebrae, bone density, and the osteoblast and osteoclast activity of the vertebrae. We found that sexual maturation occurs from April to May. Moreover, the decrease in vertebrae strength and bone density, thinning of compact bone, increase in bone porosity relative to the total spongy bone area of vertebrae, and increase in osteoclast activity were also observed from April to May. To clarify the mechanisms responsible for the weakening of vertebrae, yellowtail juveniles bred in the tank were used. Juveniles were implanted with 17α, 20β-dihydroxy-4-pregnen-3-one (DHP), a hormone that has the ability to induce sexual maturation. In the group of DHP-implanted fish, thinning of the bone and compact substance, and decrease in osteoblastic activity were observed. These results indicate that DHP induced weakening of vertebrae. Our study revealed that changes in physiological conditions such as hormone levels during the breeding period compromised the vertebral structure and increased the incidence of kyphosis.