Gavin Partridge

The current status of tuna fiseries, fattening and farming practices, as well as advances in closed-cycle tuna aquaculture, are summarized. The emergence and expansion of tuna fattening and farming activities during the last four decades have led to a shift from traditional fisheries toward aquaculture. This change is entirely reshaping the tuna fishery industry and the management of their stocks worldwide. Tuna fattening and farming operations still rely primarily on wild-caught juveniles that are fattened using small pelagic fish, blurring the line between fisheries and aquaculture and merging these activities to the point that it is no longer possible to analyze them separately. Progress in fattening operations has been limited to improved management and decreased mortalities during the capture, towing, transferring, and feeding stages of the tuna in cages. However, tuna aquaculture is now rapidly changing due to remarkable progress in closed-cycle tuna aquaculture production through advancements in broodstock maturation, spawning, larval rearing, and juvenile production technologies. Indeed, following the pioneering achievement of closing the life cycle of the Pacific bluefin tuna (PBFT) in Japan, researchers the world over are now making significant progress in hatchery technology. Closing their life cycle and the development of ecologically and economically efficient feeds that meet the specific nutritional requirements of tuna are required to ensure the future of tuna production and the conservation of tuna species. Collective efforts by researchers, academics, and the global industry are making it possible to achieve these goals.

Presently there are 23 identified tuna stocks in the world: 14 fully exploited, 8 overexploited or depleted, and only a single skipjack stock deemed to be under exploited. Wild juveniles of ABFT (. Thunnus thynnus), PBFT (. Thunnus orientalis), SBFT (. Thunnus maccoyii), and YFT (. Thunnus albacares) are used to stock tuna ranches for "fattening" (grow-out, from advanced juvenile to slaughter size). Ranched tunas are taken from diverse stocks managed under various legal regimes and monitoring agencies, but all serve the fresh and frozen sushi/sashimi-grade tuna market. Because Japan commands the largest part of this market, it sets fishing and ranching trends throughout the world. In any country, as the level of affluence rises, food preferences tend to shift from those centered on starches to ones centered on protein, and Asian countries have a strong predilection for this protein to be provided by seafood products. The rise of China as an economic power already has shifted its status from being a net exporter to that of net importer of seafood products, and this change is becoming evident in global tuna markets. Consequently, all available information points to a significant and growing impact of human consumption, especially in Asia, on global tuna resources. Catch restrictions and the banning of international tuna trade clearly have failed to achieve management of tuna stocks for sustainability; so, it is now incumbent to offer proactive strategies rather than reactive approaches. Such strategies need to be ones that academic institutions, international tuna markets, fishing fleets, and tuna farmers as well as regulatory agencies and environmentalists can endorse. Such is the justification for this chapter, which offers content relevant to both ecological and economic perspectives of tuna aquaculture. The solutions presented are straightforward and the science we now have in hand is strong enough to pursue them. As Roger Payne put it: "Act - that's what's so important.".

The reliance on wild-caught juvenile tuna is seen as the Achilles heel of tuna ranching industries worldwide and it is widely recognised that closed-cycle hatchery production is essential to sustain the demand for tuna and reduce pressure on wild stocks. The many features that make tuna such a unique group of fish also give rise to the many challenges associated with their culture and despite some forty years of effort, commercial scale hatchery production has yet to be fully realised. This chapter summarises the history of closed-cycle tuna production and discusses the recent and significant achievements that have been made towards the achievement of this elusive goal.

This chapter discusses the development of artificial diets for larvae of marine finfish. The typical feeding protocol for marine finfish larvae begins with rotifers, Brachionus plicatilis followed by brine shrimp (Artemia), and then larger Artemia. Marine microalgae are also added to the larval rearing tank and weaning is the phase when live food organisms are replaced with artificial or formulated food particles. The high cost of live food production in finfish hatcheries could be minimized by cheaper production of live food organisms and earlier weaning onto formulated feeds. The potential advantages of using artificial diets to replace live food organisms include reduced feed costs, convenience and short- to medium- term food storage. The most significant advantage is the size of the food particle and diet composition which can be adjusted to suit the exact nutritional requirements of the larvae. The majority of artificial diets for finfish larvae have been presented as either microencapsulated diets or microbound diets.