Effects of mesterolone on the sex ratio, physiology and liver function of juvenile tambaqui (Colossoma macropomum)

In fish, sex dimorphism generates benefits for farming when one gender displays better production traits than the other. In such cases, farming exclusively one sex becomes more profitable. The tambaqui (Colossoma macropomum) is the Brazilian native species with the highest production in the Brazilian fish industry. In tambaqui farming, it is more lucrative to produce females because they grow faster than males. As the species has the XX/XY genetic sex determination system, the use of XX neomales (masculinized genetic females) as breeders to produce solely all-female populations is the most sustainable approach. With the aim of obtaining tambaqui neomales, we tested three doses of mesterolone (1-α-methylandrosterone; at 10, 20 and 40 mg kg−1) through the diet of pre-differentiated juveniles for three months, to masculinize the females. The sex ratio and immediate and long-term effects on the physiology of the fish were evaluated by hematological and biochemical analyses of the blood plasma at the end of the treatment, and three months later. Possible hepatotoxic and mutagenic effects of mesterolone were assessed via liver histopathology, hepatic enzyme production and the micronucleus test (fragmented DNA) in the erythrocytes. The results indicate that 20 and 40 mg kg−1 of mesterolone produced over 76% phenotypic males, have no influence on growth and on the biochemical parameters, do not cause DNA damage, nor do they have toxic effects in the liver of tambaqui. A decrease in the hepatosomatic index and an increase in some hematological parameters were observed during the treatment, but all values were similar to the control three months after the end of the experiment. •20 mg kg−1 mesterolone in the feed of tambaqui results in 77% of phenotypic males.•Mesterolone does not affect tambaqui growth.•Hematocrit, hemoglobin, erythrocyte, MCH and MCHC are immediately affected by mesterolone.•Mesterolone added to the diet does not cause mutagenic effects in tambaqui.