Fine-Scale Population Genetic Structure of Bridle Shiner in a Low-Gradient Swampy Stream

We evaluated population genetic structure of Notropis bifrenatus along 5.5 km of the Shunock River, a low-gradient stream in southeastern Connecticut, USA. Bridle Shiner is a species of concern and considered declining throughout its range. Its habitat is associated with macrophytes in slow-flowing reaches and backwaters, including beaver impoundments and ponds. We genotyped individuals at 7 microsatellite loci. Genetic structure was evaluated separately for age-0 and adults using program STRUCTURE and a discriminant analysis of principal components.  We evaluated the influence of stream distance, the presence or absence of dams and culverts on the genetic distance among individuals, using mixed modelling of distance matrices.  Results revealed strong evidence of genetic structure within the river and age class difference in pattern.  Stream distance was the sole predictor of genetic distance among age-0 individuals; stream distance and the presence of dams were predictors of genetic distance among adults.  Dams were associated with upstream grouping of adults and a downstream signature of ‘washout’.  Further study will be needed to reconcile surprisingly high levels of age-0 dispersal with fine-scale spatial structuring of adults.