Identifying the spatial scale of population structure in anadromous rainbow smelt (Osmerus mordax)

Identifying conservation units based on the scale of biological processes is important in fisheries management. In anadromous fish, the spatial scale of population structure results from the opposing effects of homing and dispersal and varies among species by life history and geographic location. We used genetic data to evaluate the population structure of anadromous rainbow smelt (Osmerus mordax), a species of concern in U.S. waters. A total of 2211 smelt were genotyped at 10 microsatellite loci from 18 river systems and 11 bays along the northeastern Atlantic coast, spanning the entire U.S. range of the species. Across the study area gene flow was relatively high (FST = 0.017) and genetic variation followed an isolation by distance model, consistent with dispersal occurring most frequently among nearby rivers. Bayesian clustering approaches identified 4–6 genetically distinct population clusters, which varied in their geographic extent according to coastal circulation patterns. Genetically divergent populations were identified in topographically structured bays with local hydrography favoring larval retention. Conversely, gene flow was high across long stretches of topographically indistinct coastline where circulation patterns maximize passive larval dispersal. Overall, our results suggest that larval retention patterns, driven by hydrography, influence the population structure of rainbow smelt, and homing occurs to broad coastal zones of retention. The genetic structure identified in this study can be used in conjunction with ecological data to inform management at the appropriate spatial scale.

► We studied genetic variation in anadromous rainbow smelt from 18 rivers in the northeastern U.S.
► Broad-scale population structure consisted of 4–6 distinct population groupings.
► Gene flow was consistent with patterns of hydrography driving larval retention.
► Spatial scale of population structure varies with coastline topography and coastal circulation.
► River-specific management units may not be warranted, given the connectivity among neighboring rivers that feed the same bay.