Do tributary dams pose a greater conservation threat than main-stem dams for nonmigratory freshwater fishes?

Objective Riverine fish conservation commonly focuses on restoring connectivity for migratory fishes by removing old dams that fragment otherwise large swaths of free-flowing river. For nonmigratory fishes, the removal of these dams may not elicit the same positive response because they can complete their life cycle in small stretches of river. Many nonmigratory fishes exhibit a bell-curve-shaped distribution in density: low at their upstream and downstream extents and high at their core. We hypothesized that Redbreast Sunfish Lepomis auritus in tributaries to the lower Hudson River, New York, United States, would show lower population density and mean effective population size (Ne) in smaller streams and would show increased genetic drift when these smaller streams were impounded. Methods We developed a microsatellite genotyping panel to test for genetic isolation by distance, identify different measures of Ne across the watershed, and identify landscape features that drive pairwise genetic differentiation between populations. Results We found genetic isolation by distance and inhibition of upstream movement across dams and a waterfall. Further, Ne and catch per second of electrofishing were significantly lower in upstream reaches. Redbreast Sunfish genetic differentiation was primarily driven by river distance and intact dams, with dams on smaller streams being associated with greater differentiation than dams on larger streams. Conclusions Our findings suggest that removal of dams at the upstream fringes of a nonmigratory species’ within-basin range could alleviate a greater degree of genetic differentiation than the removal of dams at the core of the species’ distribution. The conservation implications of upstream barriers for imperiled nonmigratory taxa have potentially been overlooked and may lend more options to managers balancing the complex social, financial, and ecological criteria guiding the dam removal decision-making process.