Methods of selecting fish for breeding and optimum growth in conditions of various salinity are disclosed. Fish are selected for growth in specific salinity based on their prolactin 1 genotype. A simple sequence repeat polymorphism (microsatellite) in the tilapia prolactin (prl 1) promoter is associated with differences in prl 1 expression and differences in growth in nullsalt-challengednull or nullsalt-effectednull fishes. This discovery suggests that dinucleotide microsatellites may represent an under-appreciated source of genetic variation for regulatory evolution, and belie the textbook interpretation that non-coding microsatellite length variation lacks functional consequences. Thus, the methods of the invention include determining or selecting the salinity of the environment in which the fish will be grown; determining the prolactin genotype of at least one male and at least one female fish being considered for breeding; breeding male and female fish having the desired genotype(s) to result in offspring having known, predictable genotypes; and raising the fish in a salinity environment compatible with the fishes' genotype(s).