Effects of temperature and genotype on sex determination and sexual size dimorphism of bluegill sunfish Lepomis macrochirus

Much interest has been generated concerning the development of monosex male populations of bluegill Lepomis macrochirus due to their more rapid growth capacity relative to females. The methods involved to develop monosex population require a comprehensive understanding of the underlying basis of sex determination and gonadal function with the development of monosex male populations. In this study, effects of genotype by temperatures on sex determination, and sexual size dimorphism and growth were tested on two batches of fry from different geographic populations. In the first batch, sex ratios significantly deviated from 1:1 in 29 °C and 34 °C groups, in which a significantly higher proportion of males (70.64% and 66.67%) were found (P < 0.05). The proportion of males in the 29 °C and 34 °C groups were significantly higher than in the 17 °C and 23 °C groups (P < 0.05). In the 2nd batch, sex ratios were not significantly different from 1:1 in all groups (P > 0.05). The pooled sex ratios were compared and this showed that temperature had significant effects on sex ratios in the first batch of fish (P < 0.001), but no significant effects on the second batch (P > 0.05). Through histological examination, intersex fish were identified in 17 °C and 34 °C groups. Rearing temperature strongly affected the growth of bluegill. Fish reared at the temperature of 29 °C performed best, followed by fish at 34 °C, 23 °C and 17 °C. No significant differences (P > 0.05) were detected in the growth of juvenile bluegill (< 8.15 cm) between the two sexes for any thermal treatments. It was concluded that genotype-temperature interactions existed on bluegill sex determination and their coexistence suggests the interesting possibility of selecting thermo-sensitive genotypes in breeding programs for mostly male populations.