Reiji Masuda

Incorporation of Dietary Docosahexaenoic Acid into the Central Nervous System of the Yellowtail Seriola quinqueradiata

In order to show the involvement of docosahexaenoic acid (DHA) in the development of the central nervous system (CNS) in carangid fish, we conducted tracer experiments by feeding radioactive DHA to larval yellowtail (Seriola quinqueradiata). Artemia nauplii were enriched with 14C-labeled DHA and fed to larval yellowtail for eight or ten days. Autoradiography of frozen sections, using both electric imaging plates and X-ray sensitive film, clearly showed that DHA was incorporated into and retained in the brain, spinal cord, and eyes. The brain, eyes, gill raker, liver, guts, and other muscle and bone structures were dissected, and radioactivity was measured in each organ by liquid scintillation counter. The results of this study suggest the incorporation of DHA into the brain. Considering our previous results indicating that DHA-free fish cannot form schools, we conclude that the incorporation of DHA into the brain might be a critical factor in the ontogeny of schooling behavior.

Feeding behaviour of hatchery-reared juveniles of the Japanese flounder following a period of starvation.

Abstract The effect of starvation on feeding behaviour was studied in hatchery-reared juveniles of the Japanese flounder in the context of improving stock enhancement strategies. Fish (54.0±5.9 mm in total length) were deprived of food (mysids) for 1, 2, or 4 days, and their subsequent feeding behaviour was observed using a video camera. The phase of feeding action, the foraging pattern, the off-bottom duration, and the swimming speed were defined and analyzed. The feeding action was divided into four phases: aim, creep, attack, and return. The foraging patterns were classified into four types. The typical foraging pattern, with a single attack and a quick return to the initial position (type A), became less common as the fish were starved, while there was a greater incidence of slower returns (type B), returns to different positions (type C), and multiple feeding (type D). The frequency of the feeding action and the duration of off-bottom swimming increased as the food deprivation period increased. The swimming speed for creeping to the mysid prey increased, but that of the return decreased as the fish were starved. These changes in feeding behaviour under starvation may prolong the time flounder juveniles spend in the water column and may result in a higher risk of predation. In the stock enhancement program of Japanese flounder, starvation should be avoided to reduce predation and improve survival.

Effect of fish size, handling stresses and training procedure on the swimming behavior of hatchery-reared striped jack: implications for stock enhancement.

The marine ranching of striped jack (Pseudocaranx dentex) in Japan is a new type of fishery enhancement that releases cultured juveniles and keeps them in coastal waters without cages, utilizing their behaviour of associating with floating objects. To improve the release strategies, the behaviour of released fish under different conditions was observed directly using SCUBA. We found that factors of fish size, the amount of handling stress and trained feeding behaviour had an impact on the potential loss of the juveniles from the release site. Large juveniles (115 mm) showed a greater potential for loss from the release site than smaller juveniles (59 mm) due to diving deeper immediately after release (an average of 5.4 vs. 4.1 m, respectively). Stressed fish dived deeper than fish provided with a vertical underwater structure (average of 8.5 vs. 3.8 m, respectively). Fish trained to respond to sound for feeding dived to a more shallower depth (3.3 m) and stayed near the release site. Handling stress before release may be the main cause of the loss of fish from the release site, while training showed great potential for improving fish retention at the release site.

Morphological development in relation to phototaxis and rheotaxis in the striped jack, Pseudocaranx dentex

The development of sensory and swimming organs in striped jack Pseudocaranx dentex was morphologically and histologically analysed and compared with the development of phototaxis and rheotaxis. A major inflection of allometric growth occurred at 9 mm TL and a minor inflection occurred at 20 mm. At about 10 mm, fin ray formation was complete. Red muscle layers first appeared at 4.3 mm and increased to more than 10 layers at 12 mm. These morphological changes were considered to be essential for cruise swimming, which might help higher behaviours such as predator evasion, schooling and migration. Strong phototaxis showing a preference for a light intensity of 105 lux appeared at 3.5 mm (3 days old), corresponding with the development of pigment in the retina. The light intensity preference changed from 10 to 10 lux at 10–12 mm (23–25 days), when the rod formation was suggested. Rheotactic behaviour was apparent in 4.5 mm larvae. Canals of buccal lateral lines were formed from 8–18 mm, while those of trunk la...