High growth and silvering status of cultured Japanese eels (Anguilla japonica) stocked into Lake Biwa, Japan

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.