Effects of relatedness and inbreeding on reproductive success of Nile tilapia (Oreochromis niloticus)

It is well established that progeny of inbred matings are less fit than those of outbred matings, and that inbred individuals suffer from reduced viability and fertility. Inbreeding can be avoided by dispersal of progeny or by actively avoiding mating with kin (mate choice). We investigated the effects of sex ratio, relatedness (kinship coefficient) and level of inbreeding on the reproductive success in Nile tilapia under semi-natural mass spawning conditions in two net enclosures (hapas). Fry were collected from the mouth of incubating females on a weekly basis and parentage assignment was done on 1115 offspring from 56 spawnings using seven polymorphic microsatellite markers. Results show that the degree of pairwise relatedness, measured by kinship coefficient, did not have any significant effect on the reproductive success of males. Female total fecundity was significantly affected by the level of inbreeding but not by body weight and gonadosomatic index. Male reproductive success, calculated as the proportion of offspring sired within a single spawning, was significantly affected by the level of inbreeding, body weight, GSI of males and sex ratio. Effects of inbreeding on male reproductive success were higher in the presence of higher number of males suggesting that the effects of inbreeding are magnified under stronger male–male competition. Our results show that there is no inbreeding avoidance via kin recognition and that accumulation of inbreeding in tilapia populations is counterbalanced by reduced reproductive success of inbred males and females.