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The yellowfin tuna (YFT, Thunnus albacares ) is a circumtropical/subtropical marine species that supports major fisheries throughout its range. Tuna ranching – an activity that currently relies on fish captured from the wild – has rapidly expanded, thus placing even greater pressure on wild tuna stocks worldwide. Although captive propagation of tunas has proven feasible, hatchery technologies for YFT need improvement to increase larval survival. The onset of exogenous feeding is a critical phase during which digestive processes develop rapidly. Therefore, the present research was undertaken to examine size-dependent changes in YFT fertilized eggs and in 23, 25 and 34 mm (total length) early juveniles with respect to the development of digestive capacity, biochemical composition and amino acid (AA) profiles as indicators of larval nutritional requirements for AAs. To this end, the activities of alkaline and acid phosphatases, pepsin, trypsin, chymotrypsin, aminopeptidase, lipase and α-amylase were determined spectrophotometrically. Activities of the analyzed enzymes were greater in the YFT than in similar-age/size marine fish of other species. In YFT, enzyme activities progressively increased as fish grew, resulting in significantly different ( P P ≤ 0.001–0.05) from that of eggs. Histidine, taurine and alanine were the most prevalent AAs in the free pool; whereas, arginine's free pool concentration increased substantially from egg to juvenile stages (2.2 to 12.7 nmol/mg, respectively). These results support the hypothesis of precocious digestive capacity for scombrid fish. Observed AA patterns suggest that strategies for feeding larval and juvenile YFT in aquaculture should incorporate higher levels of specific AAs during early development to support rapid growth and diminish the threat of starvation or cannibalism. Although this information is useful for understanding feeding processes in juvenile YFT, more research is necessary to fully characterize these processes in the various stages from egg to juvenile, leading to the development of balanced diets that would allow for early weaning from live prey.

A land-based culture facility for research on yellowfin tuna, Thunnus albacares , was developed at the Achotines Laboratory in the Republic of Panama. Six concrete tanks, and seawater and life support systems were built to maintain a yellowfin broodstock. On average, 50% of the yellowfin caught survived capture and handling, and approximately 30% became broodstock in Tank 1 (17 m diameter, 6 m depth) or Tank 2 (8.5 m diameter, 3 m depth). Each fish was tagged with a microchip implant tag, then weighed, measured, and injected with oxytetracycline (OTC) prior to stocking. Daily rations of primarily market squid, Loligo opalescens , and Pacific thread herring, Opisthonema spp., were regulated based on the feeding activity and energy requirements of the fish. Feeding activity of the broodstock decreased when the water temperatures decreased, and the fish ate decreasing daily rations and increasing calories with increasing size. Spawning occurred in both tanks within 6–8 months of capture. Spawning first occurred in Tank 1 when 24 females ranged in size from 6 to 16 kg and 65 to 93 cm fork length (FL). Spawning was intermittent during the first 2 months and occurred near daily thereafter. Tank size appeared to affect survival rates, the types of mortalities that occurred, and the growth of the fish. Survival rates after 1 year in captivity were higher, and the fish were larger, on average, in Tank 1 than in Tank 2. Most of the mortalities in Tank 1 were the result of wall strikes, which occurred more frequently after the fish reached their highest density of 0.64 kg m −3 and sizes greater than 96 cm FL and 19 kg. Non-linear growth models were fitted to the initial stocking sizes and final sizes of fish that died or were removed from Tank 1 during 1996–1999. Estimated growth rates in length (11–48 cm year −1 ) for fish between 51 and 150 cm FL decreased with increasing length. Estimated growth rates in weight ranged from 9 to 19 kg year −1 for fish less than 19 kg and 20–23 kg year −1 for fish greater than 19 kg. The results of this work demonstrate that the stable environment of a land-based culture facility may be the preferred system for long-term maintenance of a yellowfin broodstock.

Mitochondrial DNA genotypes of captive broodstock of yellowfin tuna (Thunnus albacares) were compared with those of their offspring in order to monitor spawning frequency and periodicity. Among 38 broodstock individuals, 27 genotypes were observed, 18 of which established a single individual’s identity. Spawned eggs and hatched larvae were collected on 48 sampling days over a period of 1 year. Among 538 eggs and larvae analyzed, 10 genotypes were observed; eight of them established a single female’s identity, and two types were shared by two females. The spawning profiles of these females were determined by observing the occurrence of these genotypes in the offspring. Based on the dates when genotypes first occurred and on growth trajectories estimated for individual fish, the size of a female at first spawning was estimated to be 12–28 kg and 75–112 cm. Usually, multiple females spawned on a given date. The same genotypes were observed on almost any sampling day throughout the year. The results indicated that some individual females were capable of spawning almost daily for extended periods of time as long as they remained in the appropriate range of water temperatures and had sufficient food. Crown Copyright D 2003 Published by Elsevier Science B.V. All rights reserved.