Genetic Diversity and Structure of Two Hybridizing Anadromous Fishes (alewife and blueback herring) Across the Northern Portion of Their Ranges

Alewife (Alosa pseudoharengus) and blueback herring (Alosa aestivalis), collectively known as river herring, are anadromous clupeids that display considerable spatiotemporal overlap during riverine spawning migrations. Both species have experienced severe population declines. Evidence that they home to their natal rivers to spawn suggests the likelihood of extensive population structure, yet this hypothesis has largely remained untested. We examined genetic diversity, differentiation and population structure in 34 alewife and four blueback populations spanning a 2500 km portion of their northern range, using 14 microsatellite loci. Significant differentiation was detected among most rivers, and eight genetically defined alewife population clusters that largely corresponded to hydrographic regions were identified. Similar population structure was seen for blueback. Genetic isolation by distance (IBD) was not significant among alewife populations in regions that have been historically influenced by dams, and/or stock transfers, but IBD was highly significant in two regions that have not been subject to these influences. Genetic differentiation of alewife populations was strongest in the Bay of Fundy. Bottleneck tests suggest the majority of alewife populations have experienced past demographic bottlenecks severe enough to produce detectable effects on genetic variation. Lastly, our results showed evidence of widespread, relatively common hybridization between alewife and blueback.