Common Garden Experiments Confirm the Impact of Salinity on Reproductive Traits that is Observed in Wild Populations of the Back-Chinned Tilapia Sarotherodon melanotheron

The black-chinned tilapia Sarotherodon melanotheron is a very hardy species, particularly notable for its ability to tolerate a wide range of environmental salinities. The impact of environmental salinity on reproductive traits has been well documented in this species under natural conditions, but few studies have been experimentally conducted to prove such a relationship. This study assessed the effects of different salinities on the reproductive traits of S. melanotheron in experimental conditions. The number of spawning eggs as well as the mean number and frequency of incubation showed significant relationships with ambient salinity. The average number of incubations and spawning eggs were lower in hypersaline water (70 psu) compared the freshwater (0 psu) and seawater (35 psu). However the egg quality, as measured by the percentage of black eggs, was higher in hypersaline water. These variations in reproductive traits do not reflect genetic differences among individuals because fish used in common garden experiments were obtained by natural breeding from couples, the initial brood stock of which was from the same wild population. The results therefore confirm findings of previous studies in natural habitats, where variations in salinity are associated with differences in reproductive traits among populations. The lower number and frequent incubations recorded in hypersaline water could be an adaptive response to sustain the population dynamics despite the constraints imposed by extreme salinity.