Our study demonstrated that the GSI of hybrids in spring was closer to the GSI of gibel carp (especially evidenced for females), suggesting a heritable maternal effect on hybrid reproduction. The GSI reflecting gonad development in common carp × gibel carp hybrids may suggest that these inter-generic hybrids have the biological capacity to reproduce.
However, the increase in calcium concentration in spring with higher calcium in females compared to males was observed only in parental species (this phenomenon was much more evident in gibel carp) and no obvious difference for calcium was observed between spring and autumn or between males and females in hybrids. Svobodová et al. (2009) demonstrated higher calcium concentrations in females compared to males in common carp especially near spawning time and suggested the importance of calcium for egg development. Therefore, we highlight the need to analyze sperm and egg quality and concentration for determining reproductive performance in common carp × gibel carp hybrids.