Recently, artificial reproduction of chub mackerel was attempted by applying hormone injection to seedling production. Shiraishiet al. (2005) induced the production of eggs by administering gonadotropin releasinghormone (GnRH). However, the requirement for regular injections using this method makes it difficult to maintain spawning behavior over an extended period. The Fisheries Laboratory atthe Kinki University succeeded in closing the life-cycle of the chub mackerel in land-based tanks without hormone injection (Murataetal.,2005). However, ensuring consistency with respect to spawning frequency, egg quality, and mass production has been difficult (Murata et al., 2005; pers. comm.). Matsuyama (2006) clarified that the condition of the rearing environment, especially the rearing space and the process
of acclimation, has a marked effect on stress and reproductive success in captive fish. In addition, artificial reproduction that may be facilitated by hormone administration or performed in land-based tanks may limit the genetic diversity of the cultured fish. It is therefore important, to develop natural spawning in net-cages that do not require artificial treatment methods in order to ensure stable egg production and better quality eggs. Surprisingly, even though the spawning grounds, maturation, and larval ecology of the chub mackerel are well known, relatively little is known about the in situ spawning behavior of this species (Dickerson et al., 1992; Yukami et al., 2009).