Takayuki Kogane

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.

Improvement of larval rearing technique for mass seed production of snow crab Chionoecetes opilio

Larval rearing experiments were conducted to examine the potential for mass seed production of the snow crab Chionoecetes opilio using a total of eight 20-kL tanks. Tanks were equipped with agitators, which move the water using a rectangular blade to prevent the zoeas sinking to the bottom of the tank. Larval rearing water was treated with sodium nifurstyrenate once a week to reduce the chance of larval infection by pathogenic bacteria. Zo eas were fed with rotifers and Artemia nauplii. A total of 122 830 megalops and 16 660 first-stage crabs were produced. Thus, the potential for mass seed production of snow crab was determined. Survival rates up to the megalopal stage were high in tanks with a feeding regime that fed rotifers to larvae through an entire zoeal stage. This study also describes the fatty acid composition of snow crab larvae. It revealed that the first zoeas had a high DHA content and DHA/EPA ratio, but these values significantly decreased in the second stage zoeas and megalops. Improving the DHA content and/or DHA/EPA ratio of larvae should be important in studies on mass seed production technology of the snow crab.

Temperature conditions enhancing resting egg production of the euryhaline rotifer Brachionus plicatilis O. F. Müller (Kamiura strain)

We examined the effect of low temperature treatment (12°C), followed by transfer to higher temperature (25°C), on resting egg formation of the rotifer Brachionus plicatilis, Kamiura strain. This strain has been mass cultured as live feed at Kamiura Station (Japan Sea Farming Association) for 9 years at 20°C without the appearance of sexual reproductive stages.