Annual changes in the gonadal histology of captive reared jack mackerel, Trachurus japonicus

The aim of this study was to establish a suitable experimental system for inducing maturation and spawning of captive reared jack mackerel, Trachurus japonicus. Specifically, we examined annual changes in the gonadal histology of captive jack mackerel. A total of 285 adult fish (147 females and 138 males) were sampled regularly from rearing tanks (3 ton) in the months of May through July (2006). We found that the gonadosomatic index (GSI) values in both sexes increased during May and June. Spermiation was observed in 60 to 80% of males; however, the GSI values were lower for captive males than for wild males of the same stage. In females, vitellogenic ovaries were recorded in only four fish in the annual sampling period. All of the fish sampled were 2 to 4 years old, with fork lengths of at least 200mm. Condition factors (CF) were between 13 and 17. From the above results, we can conclude that in the present experimental set up female jack mackerel fail to proceed into vitellogenesis, even though they are of reproductive age and their size and body condition have developed to mature values. Further studies on the development of reliable techniques for inducing vitellogenesis based on endocrinological dysfunction mechanisms in the gonadal development of captive jack mackerel are necessary.