Abstract Streptococcus iniae infects a wide range of fish species and causes significant losses in aquaculture. In Japan, commercial vaccines against S. iniae have been approved for use in marine fishes. In this study, we evaluated the efficacy of two commercial vaccines against S. iniae in three freshwater-farmed Oncorhynchus species—rainbow trout O. mykiss , masu salmon O. masou masou , and coho salmon O. kisutch —and examined the role of serum antibodies in vaccine-induced protection. All three species exhibited comparable susceptibility to S. iniae strain TB-0107. In each of the three species, the vaccinated groups showed significantly higher survival rates following S. iniae challenge than the control group, with a relative percentage survival exceeding 90%. The serum antibody titers were found to be significantly elevated in vaccinated fish across all of the species. Furthermore, passive transfer of serum from vaccinated fish conferred enhanced survival in recipients, which may suggest a contribution of antibodies to protection. These results indicate that commercial vaccines against S. iniae are effective in Oncorhynchus species, and that the induction of specific antibodies plays an important role in vaccine efficacy in these fish. Commercial vaccines are expected to prevent the mortality caused by S. iniae infection in Oncorhynchus species.

Abstract Tidal flats are dynamic ecosystems where bivalves play a key role in nutrient cycling through selective feeding on suspended and benthic organic matter. However, conventional bulk stable isotope analysis can have limited resolving power for distinguishing among multiple food sources when their carbon isotope ratios (δ 13 C) overlap, particularly with respect to identifying the dominant organic matter sources. In this study, we evaluated the applicability of fatty acid compound-specific isotope analysis (FA-CSIA) to identify the food sources of benthic organisms inhabiting tidal flat ecosystems. We collected bivalves and polychaetes from the Gamou tidal flat and analyzed their δ 13 C values using bulk isotope analysis, fatty acid biomarkers, and FA-CSIA. Principal component analysis of fatty acid biomarkers revealed differences in the characteristics of benthic organisms and their food sources. FA-CSIA improved the accuracy of source identification for essential fatty acids such as linoleic acid and α-linolenic acid. Bivalves showed isotopic signatures similar to those of particulate organic matter (POM), while polychaetes aligned more closely with those of sediment organic matter (SOM). These findings suggest that FA-CSIA can effectively resolve trophic relationships in complex environments where conventional methods are limited.

Abstract In the artificial seed production of Japanese eels, Anguilla japonica , the frequent occurrence of glass eels with jaw abnormalities has become a serious problem. To gain fundamental insights into this issue, we evaluated the effects of lower jaw abnormalities on survival during the glass eel stage and revealed that these abnormalities are lethal. Bone staining of glass eels and cartilage staining of leptocephali larvae showed that the abnormality was caused by mandibular dislocation in both stages. Focusing on the jaw opening angle (OA), we examined the relationship between OA and the occurrence of mandibular dislocation during the glass eel stage using logistic regression and receiver operating characteristic (ROC) analyses. The results indicated that the probability of mandibular dislocation increases when OA exceeds 69.3°. Therefore, individuals with an OA of 70° or greater were defined as having mandibular dislocation, and their appearance timing and body size were investigated. As a result, individuals with an OA of 70° or greater appeared when the total length reached approximately 35 mm, and their occurrence increased with age. These findings provide important insights into the mechanisms underlying mandibular dislocation and may contribute to its prevention in artificial eel seed production.

Abstract Triploid all-female salmonids are used in aquaculture to mitigate the negative effects of sexual maturation; however, little is known about how triploidy and sex influence aquaculture performance in masu salmon Oncorhynchus masou masou reared in seawater. In this study, 1 + -year-old diploid and triploid males and females produced from the same broodfish were individually identified using PIT tags and communally reared in a 30 m 3 seawater tank. Growth, gonadal development, and dorsal muscle proximate composition were compared among groups. From January to September 2023, total length and body weight were measured at four time points, and gonadosomatic index (GSI) and muscle composition were analyzed in May and September. Diploid females exhibited higher growth until May, whereas triploid females showed significantly greater weight gain than other groups by September. The GSI of triploid females remained near zero throughout the experiment, while that of diploid females increased to 28.2 ± 5.1% in September. Although no differences in muscle composition were observed in May, triploid females exhibited significantly higher crude lipid content (16.6 ± 1.9%) than diploid females (10.2 ± 2.4%) in September. These results indicate that, under the present rearing conditions, triploid females may maintain superior flesh quality during the maturation period.

Abstract In this study, we induced triploidy in the Japanese jack mackerel ( Trachurus japonicus ), an important species in Japanese fisheries, through cold-shock treatment, and evaluated the sterility of the resulting triploids. Eggs and sperm collected from wild-caught adults were fertilized, and the fertilized eggs were used to optimize two treatment variables: water temperature and duration of the cold-shock treatment. Based on the proportion of normally developed triploids that hatched from the eggs treated 4 min after fertilization, the most effective cold-shock conditions were 0 °C (65.0% ± 19.1%) and 4 °C (63.4% ± 9.7%) for 5 min. Triploid fish were reared to adulthood, and their gonadal maturation was compared with that of control diploids. Histological examination of control diploids during the spawning season at 1 and 2 years of age showed normal vitellogenesis in the ovaries and abundant sperm production in the testes. Conversely, the gonads of triploids were underdeveloped, with no vitellogenesis in the ovaries, and no spermiation was observed in males. These findings indicate that oogenesis and spermatogenesis are disrupted in triploid T. japonicus , suggesting that triploidization is an effective method for inducing sterility in this species.

Abstract Growth and silvering of cultured Japanese eels stocked in Lake Biwa, where recruitment of wild eels was inhibited by dams, were evaluated. Small and large cultured eels for stocking were sex-undifferentiated and males, respectively. Yellow eels mainly caught with long-lines in Lake Biwa were all females, suggesting sex-undifferentiated cultured eels transitioned into females following their being stocked. Considering ages of sex-undifferentiated cultured eels for stocking (1.4 ± 0.5 years) and captured eels (5.8 ± 2.8 years), stocking strategies in Lake Biwa should be designed with a 3–5-year post-release period in mind. Growth rates of yellow females in Lake Biwa (111.4 ± 38.4 mm year −1 ) were similar to or higher than those of wild individuals elsewhere. Most captured Y2 eels were classified as “typical Y2,” having ovaries with oocytes at the chromatin nucleolus or perinucleolus stage, although some were confirmed as having a fully pigmented black or dark-brown belly and ovaries at the early/late oil droplet or yolk vesicle stage, being characteristics of silver eels (“silver-like yellow”). These results demonstrated the potential role of lakes as habitats for stock enhancement of Japanese eels, providing valuable insights into the stocking strategy for this species.