Approximately 1.6 billion chum salmon Oncorhynchus keta fry are released annually into rivers of northern Japan. Hatcheries adhere to several objectives and easy-to-use methods as required for fish health assessment of salmon fry. In this study, we investigated the usefulness of a high-salinity tolerance test (HSTT) as a measure of the readiness of the fry to be released. Groups of fry of body weight 0.3 - 1.6 g were transferred to seawater with salinities of 33 to 51 psu (differing in increments of 3 psu), and their survival rates were determined over the next 24 h. Although chum salmon fry can potentially survive for 24 h at 51 psu, survival varied among the experimental groups. Most groups showed a stable survival rate exceeding 95% for up to 5 h at a salinity of up to 45 psu. Tolerance tests were similarly conducted with fry whose health had been artificially compromised by Ichthyobodo spp. infection, high-density rearing, or fasting. In these groups, survival was < 85% within 5 h of the transition, suggesting a reflection of the fish health status. We assert that the HSTT is a sufficiently simple and practical method for the health assessment of hatchery-reared chum salmon fry prior to release.

To develop an efficient artificial insemination technique for Pacific oysters, we investigated the effects of adding ammonia to seawater on the maturation of testicular spermatozoa and ovarian oocytes. Additionally, we examined the optimal conditions for artificial insemination using a 24-well microplate as the insemination vessel. The addition of ammonia to seawater increased the percentage of motile testicular spermatozoa and the rate of germinal vesicle breakdown (GVBD) of ovarian oocytes. However, this treatment did not significantly improve fertility. This is likely because testicular spermatozoa exhibited low motility even in ammonia-free seawater, and oocytes can be fertilized during both the prophase and metaphase stages of meiosis I. The appropriate conditions for artificial insemination using a 24-well microplate were a volume ratio of 0.5 µl or more of semen to 20,000 oocytes per 2 ml of seawater containing 0.75 mm ammonia ([spermatozoa oocyte] ratio > 813:1). The duration of contact between spermatozoa and oocytes during insemination should be approximately 30 minutes.

The survival and growth of larval Pacific bluefin tuna were examined at three temperatures of 22, 25, and 28 ℃ in 200 l tanks. Twelve days after hatching, the survival rate Pacific bluefin tuna larvae at 22, 25, and 28℃ were 36.1 ± 5.1%, 43.2 ± 4.7% and 11.3 ± 4.3%, respectively. During the rearing period, growth of larvae were 5.58 ± 0.06 mm, 6.04 ± 0.06 mm and 5.97 ± 0.29 mm, respectively, from their initial size of 3.61 ± 0.07 mm. These results suggest that a temperature around 25°C may be suitable for culturing Pacific bluefin tuna larvae.