Bacterial hemolytic jaundice (BHJ) was first described in populations of cultured yellowtail, Seriola quinqueradiata in 1980 in Japan. ​BHJ is now one of the most important diseases of farmed yellowtail. ​The taxonomic position of the causative agent, Ichthyobacterium seriolicida, was recently determined as a single clade within the order Flavobacteriales and was distinct from the known clades of the family Flavobacteriaceae, Blattabacteriaceae and Cryomorphaceae. ​Whole genome analysis of the type strain JBKA-6^T revealed that this bacterium harbors a single circular chromosome. ​Polymorphic analyses of I. seriolicida identified four major clusters among I. seriolicida isolates, and all belonged to clonal complexes; thus, this bacterium is considered to be recently introduced in Japan. ​In passive immunization tests, significantly lower mortality was observed in fish that received the serum from a convalescent fish, indicating I. seriolicida is neutralized by the humoral component of the immune system. ​However, since this bacterium does not grow vigorously in vitro, control measures such as the production of an inactivated vaccine is under development.

The temperature of water used to rear juvenile stages of ayu affects the development of the thymus. ​However, subsequent differences in the thymus in adults after rearing juveniles at different temperatures are unclear. ​We analyzed thymus development of ayu grown at a constant water temperature of 15°C from juveniles reared at different water temperatures. ​Juvenile ayu weighed at 0.6 g and 1.4 g were reared at different temperatures (10°C, 12°C, 15°C, 18°C and 22°C) for 64 days and 54 days, respectively, and then held at 15°C for analysis of the thymus development. ​Thymus volume was measured by computed tomography and thymus development of experimental fish was evaluated as the ratio of thymus volume to body length. ​The thymus volume ratios of juveniles immediately after rearing at lower water temperatures (10°C, 12°C) were higher than in those reared at the higher temperatures (18°C, 22°C). ​These levels of thymus volume ratio were preserved in grown ayu continuously reared at 15°C. ​After the summer solstice, the thymus began to atrophy and no longer exhibited differences among the experimental groups. ​Thus the thymus formed at the juvenile stage retains its morphological properties until the summer solstice.

Cryptocaryon irritans has clear daily rhythms in protomont detachment from fish and theront excystment from tomonts. ​While the rhythms seem to be regulated by photoperiods, it has not been confirmed. ​We investigated whether the daily rhythms of the parasite would be controlled by giving different photoperiods of 12 h light and 12 h dark (06:00–18:00 L and 18:00–06:00 D; 15:00–03:00 L and 03:00–15:00 D) to infected fish and tomonts. ​Protomont detachment and theronts excystment mostly occurred in the last 3 h of the dark period and from 6 h to 3 h before the end of dark period, respectively, indicating that the rhythms are controlled by photoperiods.

Flavobacterium psychrophilum, which causes bacterial cold-water disease, was detected in the ovarian fluid of female chum salmon used for mating in a hatchery in Iwate Prefecture in 2018. ​The detection rate of F. psychrophilium in chum salmon was 0.0–91.7% in individuals captured from a river; 0.0–100.0% in individuals captured from the sea and stocked in fresh water before testing; and 21.4% in individuals captured from the sea and not stocked in fresh water. ​Pre-water-hardening disinfection of fertilized eggs was performed for 21,165 thousand eggs in the hatchery, where the average eyed-egg ratio was 92.4%. ​Intra-ovum infection was not detected in the disinfected eggs, but 23.3% of non-disinfected eggs became infected.