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 mosquit ofish, Gambusia affinis, sampled at seven sites along a 10 km reach of the upper San Antonio River in San Antonio, TX. Mosquitofish populations were s ampled from the downtown area, known as the ''Riverwalk'', where the river is concrete lined, deeply pooled, and receives heavy tour boat traffic. Pop ulations were also sampled from sites upstream and downstream from the Rive rwalk. Allozyme electrophoresis was performed on approximately 50 individua ls from each site. Five of 12 loci surveyed were polymorphic (95% criterion ). Exact tests over all loci for all populations indicated that allele freq uencies differed significantly among sites. Allele frequencies of the upstr eam population were significantly different from the Riverwalk populations. Differences resulted primarily from significant decreases in frequencies o f rare alleles at MDH-2* and two GPI* loci in the downtown area. Mean disso lved 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 dow ntown area.