Microgeographic Variation and Inter-Riffle Migration of Rhinichthys atratulus (Pisces: Cyprinidae) in a Small Connecticut Stream, United States

We examined the genetic diversity on a microgeographic scale of Rhinichthys atratulus (Eastern Blacknose Dace) in Allyn Brook, a small tributary in the upper Coginchaug River drainage in Connecticut. By looking at gene flow on a microgeographic scale among populations that had no physical barriers to migration, we tested the null hypothesis that the populations should be homogeneous. We resolved seven polymorphic microsatellite loci and one mitochondrial gene, nd2, in three adjacent populations (5 km) in the Coginchaug River. A dam from the 1920’s in lower Allyn Brook has isolated Allyn-Brook populations from Coginchaug-River populations. Allyn Brook was selected because there are only three riffle habitats in the brook and, therefore, there can be no immigration from upstream populations. Each population has private (i.e., unique) alleles and haplotypes, and there are significant genetic differences between all sites. The Allyn Brook populations are almost as different from one another as they are from the distant populations in the Coginchaug River from which they have been isolated for more than 80 years. These results point to in situ evolution and little migration or gene flow among populations on a microgeographic scale. This raises interesting questions for conservation of genetic diversity of stream fishes.