24 SEPTEMBER 2024 • WORLD AQUACULTURE • WWW.WAS.ORG to YFT, the ELHG of the IATTC has been conducting the majority of research at the Laboratory, but important research collaborations have also been conducted with scientists from Kindai University of Japan, University of Miami, USA, and other research organizations. Different areas of research have been developed at the laboratory (Figure 3), including ecology, physiology and biology (the core of this research), applied aquaculture, genetics, climate change effects, and bycatchreduction technologies for tuna fisheries. Ecological Studies Since the establishment of a YFT broodstock population at Achotines Laboratory in 1996, the broodstock management program has continuously provided year-round spawning for the past 28 years, and YFT eggs and larvae have been studied in multiple ecological investigations. The spawning behavior of YFT was described by Margulies et al. (2007b), and the IATTC was the first institution globally to characterize and record courtship and spawning events of this species1. The influence of physical and biological factors on spawning, hatching, and egg development have also been studied and described (Margulies et al. 2016). Broodstock YFT are captured off the coast of Achotines Bay and transported to the laboratory, where the fish are injected with a microchip identification tag, weighed and measured, and placed in quarantine tanks. After receiving prophylactic treatment, the fish are transferred to mid-size reserve tanks (80 m3 or 170 m3) where they reside for a few months, until final transfer to T1. A diet of 50 percent squid and 50 percent fish, including mackerel or sardines, with the addition of vitamins, is given to the fish at a daily ration of 2.5-4.8 percent body weight. Broodstock growth has been welldocumented (Wexler et al. 2003; Margulies et al. 2016) and average growth rates (AVG) were 3 cm/month in fork length (FL), and 1.5 kg/month in weight (WT). Spawning occurs following natural fluctuations in the water temperature, almost on a daily basis, and water temperature is the dominant physical factor that controls occurrence of spawning events. Since 1996, spawning has occurred over 80 percent of the time in which broodstock fish were held, when water temperatures in T1 ranged from 23.3° C to 29.7° C (Buchalla et al. 2024 In Press). Development, survival and growth dynamics of YFT larvae up to juvenile transformation (3 weeks of age) have also been studied (Figure 4), and the ELHG has experimentally investigated the influence of biological and physical factors on pre-recruitment life stages. These experimental studies have focused on developing forecasting tools to predict fluctuations in YFT recruitment and their use in stock assessment models. The influence of physical factors such as dissolved oxygen, water temperature, ocean acidification and wind-induced microturbulence, as well as biological factors such as food type, food abundance and density-dependent growth have been studied experimentally and described for YFT larvae (IATTC, 2021). At hatching, YFT yolksac larvae (YSL) average 2.5 mm in notochord length (NL). Mean larval length at hatching is positively correlated with mean egg diameter, and negatively correlated with mean incubation temperature (Margulies et. al. 2007 b). YFT larvae exhaust their yolk-sac reserves rapidly and exhibit pigmented eyes and open mouths, 225 to 350 µm width, at 3.3 mm NL (first-feeding). In the laboratory, YFT larvae can successfully consume different types of copepods as initial prey, but enriched s-type rotifers Brachiounus rotundiformis also have proven to be a suitable source of food during first-feeding (Margulies et al. 2001, 2016, Buchalla et al. 2024 In Press). YFT larvae become piscivorous at approximately 6.5 mm SL (post-flexion stage), when they start to forage for larger prey items, and microzooplankton (enriched rotifers and Artemia) become insufficient in supporting growth and survival. The use of newly-hatched fish larvae (YSL of YFT broodstock) as prey is obligatory during this phase to assure optimum survival for the post-flexion larvae. Experimental studies of density-dependent growth have indicated an early onset of density effects during the first 2.5 weeks FIGURE 4. YFT larval and juvenile stages studied by the ELHG at the Achotines Laboratory. FIGURE 5. Laboratory-reared juvenile YFT ready to be transferred to sea cages. Since the establishment of a YFT broodstock population at Achotines Laboratory in 1996, the broodstock management program has continuously provided year-round spawning for the past 28 years, and YFT eggs and larvae have been studied in multiple ecological investigations.
RkJQdWJsaXNoZXIy MjExNDY=