Relationships Between the External Mortphology and Gonds of Androgen-Treated Oreochromis niloticu

Abstract

A population of 270 methyltestosterone-treated Oreochromis niloticus was described, based on the shape of the genital papilla and the nature of the gonads. The shape of the genital papilla suggested that 82% of the fish were male, but only 49% of the fish had male-shaped papilla and gonads completely composed of testicular tissue. Of the fish with male-shaped genital papilla, 59.9% had testes, 28.8% had ovotestes, and 11.3% had ovaries. Of the fish with female-shaped genital papilla, 60.4% had ovaries, 27.1% had ovotestes, and 12.5% had testes. Only 60% of the fish examined had gonads that corresponded to the sex suggested by the papilla. These results suggest the use of papilla shape alone to determine sex can be very misleading in evaluating the success of androgen treatment of tilapia to produce male populations. Microscopic examination of the gonads of a subsample is recommended for evaluating sex ratios.