1. Reservoirs modify riverine ecosystems worldwide, and often with deleterious impacts on native biota. The immediate effects of reservoirs on native fish species below dams and in impounded reaches have received considerable attention, but it is unclear how reservoirs may affect fish species at larger spatial and temporal scales. Documented declines of stream fish populations in direct tributaries of reservoirs suggest reservoir pools may reduce gene flow among historically connected populations.
2. Because of increased predator densities in reservoirs and the extent of habitat alteration in impounded reaches, I predicted reservoir habitats would reduce gene flow among small-bodied fish populations separated by reservoir habitat. I used microsatellite markers to assess the spatial genetic structure of populations of the red shiner (Cyprinella lutrensis), in a reservoir-fragmented stream network (Lake Texoma, U.S.A.). I also tested the prediction that populations in two direct tributaries that have experienced population declines would have low genetic diversity. Individuals were collected from six sites upstream of the reservoir, three sites in the reservoir and three sites in direct tributaries of the reservoir during 2008 and 2009.
3. Results indicate that most populations were isolated by distance with little divergence among populations. In one direct tributary population, however, there was substantial genetic divergence, and genetic diversity was significantly lower than in other populations. Gene flow also seemed to be lower in reservoir habitats than in intact stream habitats, suggesting reservoir habitats may be reducing gene flow among the reservoir-separated populations. These results indicate that reservoirs may reduce gene flow among reservoir-fragmented stream fish populations, altering the evolutionary trajectories of fragmented populations.