Influence of prolonged cultivation on sexual characteristics of sterilized female tilapia, Oreochromis mossambicus, induced by high-temperature treatment

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Tilapia represent an important commercial fish for aquaculture. The sterilization of this species is expected to not only promote efficient cultivation and improve flesh quality, but can additionally reduce negative impacts on local environments resulting from the unintentional escape of stock. Although the induction of triploidy is widely used for sterilization in tilapias, triploid tilapia retain their germ cells, which has led to concerns that fertility may persist. A recent study successfully induced sterility in tilapias by implementing only a high-temperature water treatment (HT) in fry stages for a period of 45 days or longer. The present study extends this work by verifying the continued sterility following treatment when cultivation is continued for more than 19 months. We confirmed that high-temperature water treated females had germ cell-deficient gonads based on the histological observation and vasa transcript expression. Although immunohistochemistry against steroidogenic enzymes in the treated gonads revealed that each steroid-producing cell functioned normally, plasma sex steroid hormone levels in treated tilapia were significantly lower than control. Furthermore, the genital papillae in treated females were noticeably undeveloped, suggesting their possible use as an indicator of sterility. Overall, we concluded that sterilization in female tilapia via HT is “permanent”, and HT represents a potential alternative to induced triploidy for enforcing sterility in the aquaculture industry.