AQUACULTURE DEPENDENCY ON WILD SEED

In the 1980s, the wild catch of Mojako started to decline. In order to protect this natural resource the Japan Fisheries Agency required the Marine Aquaculture Association of Japan to regulate both the catch season and numbers (Figure 2) and since 2003 the Prefectural Fisheries Cooperatives are allowed to regulate this fishery and allocate quotas to each prefecture (Inagaki, 1990).

However, when the number of Mojako caught has been insufficient, yellowtail fingerlings have been imported mainly from the Republic of Korea. Amberjack juveniles have been imported from China and Viet Nam, including some 20 million imported in 2000 which were cultured from wild amberjack seed. These are acclimated and reared to 50 to 300 g. Several viral and parasitic diseases have entered Japan with the imported juveniles. In 2005, the nematode worm Anisakis,
which was introduced with juvenile amberjack from China. Because Anisakis larvae in fish can be transmitted to humans, the Japanese Ministry of Health, Labour and Welfare prohibited sales of the infected farmed fish for raw meat consumption. Concerns over the health of imported juveniles have persuaded some fishing cooperatives to send workers to China to conduct quality control on the health of the amberjack juveniles. Furthermore, all imported amberjack juveniles are also checked by custom officials as they are often fed with high levels of antibiotics prior to shipping.

A further and important seed source is aquaculture itself. The Marine Aquaculture Association of Japan and several prefectural experimental stations have established techniques for the artificial production of about 60 marine fish species. Significant quantities of yellowtail, amberjack, gold-striped amberjack and striped jack juveniles have been produced by aquaculturists (Kawabe et al., 1996; Arakawa et al., 1987; Tachihara, Ebisu, and Tukasima, 1993; Kawanabe et al., 1997). Viable eggs are obtained from both wild spawners and cultured broodstock fed high quality formulated feeds, with maturation being stimulated by hormone injections in many instances (Mushiake et al., 1993; Nagasaki Prefectural Fisheries Experimental Station, 1998). Healthy fry are fed on mass-produced food organisms such as rotifers and brine shrimp nauplii fortified with n-3 highly unsaturated fatty acids (HUFA) as well as formulated feeds (Verakunpiriya et al., 1997a; Verakunpiriya et al., 1997b; Fukuhara, Nakagawa and Fukunaga, 1986). Farm-raised juveniles have been released into the wild and used as seed for aquaculture.

Production benefits from aquaculture

For more than 30 years, the annual yellowtail capture fisheries harvest in Japan has been around 50 000 tonnes, while 160 000 tonnes are produced from aquaculture. The fishery is carried out using different fishing techniques and takes place in numerous fishing grounds along the extended coast of Japan. At present, the fisheries stations in the prefectures, along with the Japan Fisheries Agency monitor the wild resources and make recommendations to prevent excessive harvest of Mojako, juveniles and adults.
If wild Mojako harvesting was limited to what is needed for aquaculture, wild yellowtail fisheries could increase to more than 100 000 tonnes, however the wild yellowtail might then consume more than 1 000 000 tonnes of prey fish, which could otherwise be harvested for human food. It is suggested that wild Mojako and small jacks should be fully utilized as a resource for aquaculture, as they have very high natural mortality rate in the early life stages. Farm feeds could include artificial granulated feed, moist pellets, and extruded pellets made from trash fish and vegetable matter as well as human food left overs.