Induction of final oocyte maturation and ovulation in Jack Mackerel, Trachurusjaponicus, temporarily reared in captivity

It has been difficult to obtain a stable seed supply of fertilized eggs in jack mackerel in aquaculture because adult female fish do not have well-developed ovaries. Wild fish caught by purse seine usually have atretic ovaries caused by fishing stress, while long-time reared fish often fail to undergo vitellogenesis, possibly due to captivity-induced stress. In the present study, we used wild-caught females that were then reared in sea pens for about 1 month to obtain fish for inducing final oocyte maturation (FOM). A total of 20 females with oocytes of >500μm diameter were selected, and they were divided into two groups: 16 fish with normally developed oocytes (Group A) and 4 with atretic oocytes (Group B). All fish were injected intramuscularly with human chorionic gonadotropin (hCG; 500 IU/kg). In Group A, in which FOM was induced, germinal vesicle migrating oocytes were observed at 25 h post-injection. Oocytes at the germinal vesicle breakdown, hydration, and ovulation stages were observed at 32, 39, and 39-42 h post-injection, respectively. The water temperature during the experiment was 16.7-17.0°. When hCG was injected into fish of Group B, FOM did not occur. Thus, FOM and ovulation could be reliably induced by hCG injection, in temporarily reared females, and the detailed time course of FOM and ovulation of jack mackerel were clarified.