The production of sterile fish via triploid induction is an effective method for preventing the unauthorized duplication of breeds developed through breeding. It can also help prevent genetic contamination or ecological problems caused by the escape of farmed fish from the natural world. Moreover, triploids are utilized in fishery resource management to genetically separate native populations from artificially produced resources and to prevent the settlement of invasive species. In addition to these benefits, it is also possible to reduce male aggression that appears when sexual characteristics are expressed and to prevent problems that may occur when fish in breeding escape to the natural world.
In this study, we examined sexual maturation in diploid and triploid starry flounder, and diploid and triploid hybrids. Changes in gonad tissues and the gonadosomatic index (GSI) were evaluated from December to April, and sperm activity and volume were measured for a total of 12 weeks, from early February to late April. Male GSI was significantly lower in triploid than in diploid fish, and decreased sharply in March in the diploid hybrid, and in April in the diploid starry flounder.
Analyses of gonad tissue showed that spermatozoa were first observed in December in the diploid hybrid, and in January in the diploid starry flounder. Spermatozoa were observed in triploid fish beginning in March, with a rapid progression then seen in April, indicating a shorter spawning season in triploid than in diploid fish. Sperm volume and fertility were evaluated using sperm collected from each experimental fish. Sperm volume was > 3-fold higher in diploid than triploid fish. Sperm activity in triploid fish was < 50% that in diploid fish. To evaluate sperm fertility, we performed artificial insemination on diploid fish eggs, and observed the floating eggs. Artificial egg fertilization using the sperm of triploid fish was unsuccessful.