Trials of several vaccines were conducted to identify a promising candidate vaccine for controlling the infectious disease nocardiosis caused by Nocardia seriolae in Japanese amberjack Seriola quinqueradiata. We investigated the pathogenicity and vaccine efficacy in an indoor aquarium facility with fish weighing 50 to 100 g. A live vaccine using a low-virulence isolate of N. seriolae screened from several N. seriolae isolates derived from infected fish conferred significant protection against the disease. However, neither an inactivated vaccine using formalin-killed cells, ultraviolet-treated bacteria, nor a DNA vaccine encoding the Ag85 gene provided significant protection. Sixty weeks trial demonstrated the long-term effectiveness of the live vaccine. These results indicate that a live vaccine is the most promising approach for controlling nocardiosis in Japanese amberjack.

Ovigerous females of the cymothoid isopod Anilocra clupei were found around the opercula and dorsal fin of the Japanese sardine Sardinops melanostictus captured in Aso-kai Lagoon, Kyoto Prefecture, Japan. The attachment sites showed loss of scales and pigmentation, along with ulceration. The estimated infection prevalence, intensity, and mean intensity of A. clupei, including fish possessing presumably parasitic wounds, were 41%, 1–2, and 1.01, respectively. The infection prevalence, including individuals possessing only parasitic wounds, tended to be higher in the smaller-sized fish and they were also significantly less obese. When assessing economic impact of A. clupei, fish without isopod infection but showing parasitic wounds should also be considered.

A mass mortality event with a mortality rate of approximately 30% occurred among juvenile ayu Plecoglossus altivelis in a hatchery in Japan that uses pure seawater for rearing. Bacteria isolated from the kidneys of dead fish were identified as Tenacibaculum maritimum using 16S rRNA sequencing and PCR analyses. Immersion infection experiments using this isolate confirmed a high mortality rate in ayu at a bacterial concentration of approximately 10⁶ CFU/mL. Rearing affected ayu in half-strength seawater for 7 days significantly reduced deaths in the hatchery, and this effect was reproduced in the infection experiment. It was shown that rearing infected fish in half-strength seawater effectively treated this condition.

Sea-caged coho salmon Oncorhynchus kisutch in Tottori Prefecture, Japan, afflicted by microsporidiosis, exhibited clinical symptoms that include cyst formation and partial myoliquefaction of the trunk muscle. Analysis of small subunit ribosomal RNA gene sequences revealed that the isolated microsporidium showed high similarity to Microsporidium spp. of marine fishes, including M. seriolae, the causative agent of beko disease in Seriola fishes. Phylogenetic analysis placed the isolated microsporidium in the same clade as M. seriolae, indicating that the microsporidium infecting O. kisutch is an undescribed species closely related to M. seriolae. This is the first report of beko disease in salmonids.