However, the percentage of fish caught during December-January and July had a complete ring close to the edge, and therefore the growth rings found on the opercular bones were probably due to growth interruption caused by the gonad maturation and the spawning activity (Gómez-Márquez et al. 2003). This is because O. niloticus is a mouth brooder and it is possible that in the females, this breeding habit would prevent feeding and thus lead to growth mark formation.