Demographic and Genetic Characteristics of Dispersal in the Mosquitofish, Gambusia affinis (Pisces: Poeciliidae)

Analysis of 3,271 mosquito fish was conducted during a single dispersal event in a small thermally-impacted reservoir to characterize both demographic and genetic traits of dispersing fish. Males dispersed in much larger numbers than females or juveniles. Mean lengths of fish were greatest at distances farthest from homesites. Pregnant females which dispersed carried significantly more fetuses than non-dispersers, a direct result of their larger size. Genotype frequencies of four enzyme loci of dispersing fish were non-randomly distributed throughout the pond and significant clines of gene frequencies with distance from the major refuge were found. Associations of genetic distance values and geographical distances between collection sites were non-significant for some loci, indicating fish were not distributed in a pattern based on random intermixing of refuge groups. Refuge fish (N = 368) were also collected after dispersal. Males represented very small percentages of post-dispersal refuge populations. Changes in allele frequencies of refuge fish of ten percent and greater occurred during the dispersal event at some sites. Wright's F-coefficients indicated that increased genetic heterozygosity and population differentiation occurred both within and among refuge groups after dispersal. Dispersal, as documented here for Gambusia affinis, is proposed to be selectively advantageous for species which occupy temporally and/or spatially fluctuating environments.