To estimate the survival process of Pacific bluefin tuna Thunnus orientalis during the larval period, estimated growth histories were compared between larvae collected in late spring and juveniles collected in the boreal summer of 2004, which were considered to be survivors of the larval cohorts. Larval tuna (3.3 to 9.6 mm standard length, SL) were collected from mid-May to early June around the Ryukyu Islands, northwestern Pacific Ocean, and juvenile tuna were collected offshore of Kochi and Nagasaki prefectures in July-August. Preflexion, flexion and postflexion larvae were collected, and their ages ranged from 4 to 18 d. Back-calculated SLs by the biological intercept method showed that larval tuna in the postflexion phase were larger-at-age than preflexion and flexion larvae, suggesting that only larger and faster growing larvae were able to survive to the postflexion phase. The logarithms of otolith radii (ln OR: proportional to SL) of larvae with slower growth and development were smaller than the minimum ln OR of surviving juvenile tuna, which indicated the smallest possible size required for larvae to successfully recruit to the fishery. These results indicate that the survival of larvae of Pacific bluefin tuna depends largely on size and growth rates during early life history.KEY WORDS: Growth · Survival · Recruitment · Otolith · Pacific bluefin tuna
Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 319: [225][226][227][228][229][230][231][232][233][234][235] 2006 higher tolerance to starvation and a greater ability to escape from predators than smaller larvae (Anderson 1988, Miller et al. 1988, Bailey & Houde 1989. The target species in these studies were flatfish, cod, coral reef fishes and clupeoids, which have relatively low growth rates and a long pelagic period in their early life stages, with the exception of larval bluefish Pomatomus saltatrix that has a relatively high growth potential (Hare & Cowen 1997). Scombrid fishes including Pacific bluefin tuna are generally considered to have survival strategies in the early life stages characterized by large prey and fast growth (Hunter 1981. Scombrids have a high growth rate in the larval stages, and this growth is potentially variable depending on water temperature and food availability, implying that small variations in larval growth may induce broad variations in cumulative mortality and recruitment of the stock. We examined the hypothesis that growth during the early larval period is a key factor in the survival process of Pacific bluefin tuna larvae.Growth history of individual fish is recorded in the otolith (Degens et al. 1969, Dunkelberger et al. 1980, Watanabe et al. 1982, Mugiya 1987. In this study, individual growth histories of larval bluefin tuna were back-calculated from the otolith increment width and compared to growth histories of juvenile bluefin tuna collected in the coastal zone of Japan. These juvenile bluefin tuna were considered to be survivors of the sampled larval pop...
The ontogenetic changes in the growth potential of larval and juvenile laboratoryreared Pacific bluefin tuna were examined based on RNA-DNA and protein-DNA ratios. Experimental fish were reared at the Ohshima Experiment Station of Kinki University Fisheries Laboratory in August 2002. Samples were taken from 13 to 35 days after hatching (DAH). Metamorphosis from larva to the juvenile stage was observed around 23 DAH. Somatic growth of Pacific bluefin tuna was accelerated after metamorphosis. The value of the RNA-DNA ratio from 13 to 19 DAH increased slightly from 3.77 Ϯ 0.58 (mean Ϯ SD) to 7.28 Ϯ 2.23. After that, the ratio markedly increased from 13.89 Ϯ 3.71 on 21 DAH to 19.11 Ϯ 4.27 on 23 DAH, which was the end of the metamorphic period. After 25 DAH, the ratio remained at a high level of 15-20. The protein-DNA ratio showed a similar tendency to the RNA-DNA ratio. These results suggest that the rapid increase in the RNA-DNA ratio in the metamorphic period supports the consequent rapid somatic growth in the juvenile stage. The high ratio after the metamorphic period could be because of the species-specific traits large prey exhibit for their survival and because of the tuna's fast -growth after the juvenile stage.KEY WORDS: early growth, metamorphosis, Pacific bluefin tuna, RNA-DNA ratio.
Mid-water trawl surveys were conducted from late August to late September in 1999 and 2004 in order to investigate the distribution pattern, hatch date, and growth of juvenile Pacific bluefin tuna Thunnus orientalis in the Sea of Japan. Juveniles were collected at the stations where ambient water temperature (mean water temperature from surface to 30 m deep, WT0-30) was 23.4-25.9°C, and most of them were found in waters where WT0-30 was higher than 24°C. Sampled juveniles ranged 108-280 mm fork length. Based on otolith analysis, they were estimated to grow to approximately 180 and 250 mm at 60 and 90 days old, respectively, and showed similar growth to that of Atlantic bluefin tuna in the Mediterranean Sea. The back-calculated hatch date of the samples was mostly in July and most juveniles spawned in the Sea of Japan.
In mass culture of Pacific bluefin tuna Thunnus orientalis, a marked growth variation is observed after they start feeding at 6-7 mm in body length (BL) on yolk-sac larvae of other species, and the growth variation in tuna larvae is a factor leading to the prevalence of cannibalism. To examine the relationship between prey utilization and growth variation, nitrogen stable isotope ratios (d 15 N) of individual larvae were analysed. A prey switch experiment was conducted under two different feeding regimes: a group fed rotifers (rotifer fed group), and a group fed yolk-sac larvae of spangled emperor, Lethrinus nebulosus (fish fed group) from 15 days after hatching (6.87 mm BL). The fish fed group showed significantly higher growth than the rotifer fed group. Changes in the d 15 N of the fish fed group were expressed as an exponential model and showed different patterns from those of the rotifer fed group. The d 15 N of fastgrowing tuna larvae collected in an actual mass culture tank after the feeding of yolk-sac larvae was significantly higher than those of the slowgrowing larvae, indicating that slow glowing larvae depended largely on rotifers rather than the yolk-sac larvae.
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