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.

Abstract The global warming-induced rise in seawater temperatures has reduced survival rates and sustainability in coastal aquaculture. In this study, we aimed to perform rearing experiments to assess the seedling survival rate and harvest size of kuruma shrimp ( Penaeus japonicus ), an important species for seafood production in Japan, under high-temperature conditions. In addition, we evaluated the reduction in aquaculture-suitable days predicted for future temperature scenarios. Our results demonstrate that kuruma shrimp seedling exposure to 36 °C for 1 day and 34 °C for 10 days reduced the survival rates to 50% and 55–60%, respectively. Furthermore, under the Representative Concentration Pathways 8.5 scenario, the number of aquaculture-suitable days is projected to decrease by 6 and 60 days (median) by 2050 and 2100, respectively, highlighting the significant challenges posed by climate change to the sustainability of shrimp aquaculture. The results of this study underscore the urgent need for adaptive measures to address climate change to ensure kuruma shrimp aquaculture sustainability.

Abstract When groundwater rich in dissolved ferrous ion [Fe(II)] is pumped from wells, iron-oxidizing bacteria can grow and clog the well’s screens, pipes, and pumps, making it difficult for the pump to operate fully. In this study, we employed 16S rRNA gene amplicon sequencing to investigate the types of microorganisms that adhere to a well’s pipes and identified the main bacterial species involved in the microbial mat and clogging process. Deposits from the sampling equipment were collected daily for seven consecutive days and subjected to 16S rRNA gene amplicon sequencing, revealing the continuous presence of a Gallionella sp. We speculated that this species was the initial factor in the microbial mat’s formation and hypothesized that Gallionella forms long thread-like "stalks” and attaches to the pipe’s inner surface; as the stalks accumulate and become intertwined, they create a mat. As the mat continues to grow without detaching from the pipe’s inner surface even under rapid water flow, the pipe eventually clogs.