Population genetic structure of the western mosquitofish, Gambusia affinis, in a highly channelized portion of the San Antonio River in San Antonio, TX

Population genetic assessments were performed on populations of the mosquitofish, Gambusia affinis, sampled at seven sites along a 10 km reach of the upper San Antonio River in San Antonio, TX. Mosquitofish populations were sampled from the downtown area, known as the ''Riverwalk'', where the river is concrete lined, deeply pooled, and receives heavy tour boat traffic. Populations were also sampled from sites upstream and downstream from the Riverwalk. Allozyme electrophoresis was performed on approximately 50 individuals from each site. Five of 12 loci surveyed were polymorphic (95% criterion). Exact tests over all loci for all populations indicated that allele frequencies differed significantly among sites. Allele frequencies of the upstream population were significantly different from the Riverwalk populations. Differences resulted primarily from significant decreases in frequencies of rare alleles at MDH-2* and two GPI* loci in the downtown area. Mean dissolved oxygen measurements were also significantly lower at Riverwalk sites. These results suggest that selective forces such as non-point source runoff or low dissolved oxygen, perhaps in combination with limited migration due to numerous dams, have reduced genetic diversity of populations in the downtown area.