Yellow perch (YP, Perca flavencens) exhibit sexually dimorphic growth, with females growing faster, maturing later, and reaching a larger size than males. Production of monosex fish using sex control technology has become essential for the improvement of aquaculture productivity. Multiple studies indicate that hormonal treatment for sex reversal in most of fish species need to initiate before 35 days post-hatching (dph). The aim of this study was to determine the latest labile period or time during gonad development for the treatment success of the synthetic steroid hormone 17a-methyltestosterone (MT) in the yellow perch older than 35 dph.
Juvenile fish were subjected to two dietary concentrations of methyltestosterone (20 and 50 mg MT/g feed) for 60 days in three (3) age groups of 38, 46, and 67 dph, where control group were fed with standard commercial feed. Following a 10-month on-growing period, sex phenotypes were determined by gross and histological gonad morphology. Results showed the juvenile YP responded to the exogenous hormone when it was applied at 38 dph for both low and high dose of MT resulting in 100% males. At 46 dph, only high dosage of MT resulted in 100% males. Both MT-treated at 38 and 46 dph significantly differed (P<0.01) from the expected normal population of male: female (1:1). MT-treated at 67 dph resulted in 37% and 25% intersex fish for both the low and high dosage group, respectively. MT-treated at 38 and 46 dph promoted growth and showed significantly heavier mean body weight (P<0.05) compared to control. The gonadosomatic index (GSI) of MT-treated at 38 and 46 dph was significantly lower than control. This study provides the first evidence that juvenile YP can be successfully masculinized when the treatment is initiated at the age of up to 46 dph.
In conclusion, the dosage and time of MT-supplemented feed initiation influences the percentage of sex-reversed males and promotes growth. Where using higher dosage MT at 50 mg/kg feed could effectively broaden the efficacy of sex-reversal up to 46 dph initiation in yellow perch, nevertheless the higher concentrations of MT have evidenced a negative effect on the gonadal development of fish. Hence, the initiation of low dosage at 20 mg/kg of MT-supplemented feed at 38 dph for 60 days could be suggested in an attempt to produce all-male fish stocks of yellow perch, as it had effectively produced 100% sex-reversed males, promoted growth, and at the same time reduced the total amount of MT used. The identification of the labile period of sex determination that is manipulated by MT dosage and age of treatment would be a valuable progression toward optimizing commercially viable regimes for sex reversing YP for aquaculture industry. The findings are important for sex control in aquaculture and assessment of potential effects of environmental pollution on sex ratios of wild populations.