WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2013 59 throughout its life in captivity and injection with OTC establishes a temporal mark in otoliths and vertebrae, useful for studies of fish growth. The 1300-m3 main tuna broodstock tank was designed to be sufficiently large to reduce the stress of captivity and enhance the chances of yellowfin tuna spawning. Since 1996, multiple groups of yellowfin tuna have been maintained in the main broodstock tank. The average fish residence time in the main tank has been approximately 2 years, although individual fish have been held nearly 6 years. Broodstock yellowfin tuna (Fig. 4) are fed a diet of squid, herring, and anchovy, supplemented with vitamins and minerals, at approximately 2.0 to 4.5 percent of body weight daily. Growth rates for 2 to 3-year-old yellowfin in the main broodstock tank have ranged from 11 to 62 cm/y or 7 to 33 kg/y. Over 16 years, the broodstock population in the main tank has ranged from 5 to 45 fish. In March 2013, the tank contained 12 yellowfin, ranging from about 13 to 75 kg each. Approximately half of the yellowfin tuna caught offshore for use as broodstock in 1996 survived capture and transport and about a third of these were sufficiently healthy following a quarantine period that they were added to the main broodstock tank. Smaller fish (2 to 3 kg) survive capture and handling better than larger fish. Most deaths in the tank are due to wall strikes, when a fish collides with the tank side-wall, dying instantly or shortly thereafter from spinal injury or secondary infection from sustained injuries. Incidence of wall strikes is more frequent when fish density in the tank reaches 0.65 kg/m3, so a lower target density is maintained. Growth rate of captive yellowfin tuna is lower overall than the estimated growth rates of wild yellowfin in the eastern Pacific, likely due to reduced caloric intake, daily spawning, and confinement in the tank (Fig. 5). Yellowfin tuna spawning. Yellowfin tuna in the main broodstock tank have been spawning almost daily since October 1996. This represents the only successful long-term spawning of yellowfin tuna in land-based tanks anywhere in the world. Spawning generally occurs from early afternoon to late evening. Spawning events are usually preceded by several hours of courtship behavior (paired swimming, chasing). Spawning can be intermittent during February and March, when offshore upwelling, induced by northerly winds during the dry season, brings cooler water near shore and into rearing tanks. The numbers of fertilized eggs collected after each spawning event in the main broodstock tank range from several hundred to several million (Fig. 6). Eggs are collected by several methods, including siphoning and dip netting at the surface and seining with a fine-mesh surface egg seine. Fertilized eggs are hatched in 240-L cylindrical incubation tanks. In a series of publications, the IATTC’s ELH group has described methods of broodstock development, reproductive biology, spawning dynamics, courtship and spawning behaviors and genetics of captive yellowfin tuna. Studies of yellowfin tuna eggs, larvae, and juveniles. Larvae hatched from eggs spawned in captivity at the Achotines Laboratory are used in a variety of laboratory experiments (Fig. 7). In several publications, the IATTC’s ELH group described growth, survival, and early development of yellowfin tuna. Additional publications have described effects of key physical variables (water temperature, dissolved oxygen, microturbulence, and light intensity) on growth and survival of yellowfin tuna larvae. Results from several recent experimental studies at Achotines appear to have potential as forecasting tools for understanding pre-recruit survival of yellowfin tuna in the eastern Pacific Ocean (EPO). These forecasting tools include the use of wind speed data (representative of wind-induced microturbulence and its effect on larval feeding success) and a larval growth index to predict pre-recruit survival of yellowfin tuna. The experimental program at the Achotines Laboratory has led to advances in rearing larval and early-juvenile tunas. The IATTC’s ELH scientists routinely rear early-juvenile yellowfin tuna for 30 to 60 days after hatch for research purposes (Figs. 8 and 9) and have reared yellowfin tuna to 100 days after hatch to139 mm in length. Although IATTC research objectives are to understand more of the biology and ecology of tuna in the open ocean, research results related to optimal conditions for growth and survival would obviously be quite useful for potential mariculture applications. A summary of research findings from studies conducted at the Achotines Laboratory through 2006 is presented in IATTC Special Report No. 16 (2007), available on the IATTC website. FIGURE 6. Yellowfin eggs shortly before hatching (Photo: IATTC). FIGURE 7. Yellowfin larvae at first-feeding stage (3.5 mm in length) (Photo: IATTC). (CONTINUED ON PAGE 58)
RkJQdWJsaXNoZXIy MjExNDY=