Replicate divergence between and within sounds in a marine fish : the copper rockfish (Sebastes caurinus)

Understanding the factors that influence larval dispersal and connectivity among marine populations is critical to the conservation and sustainable management of marine resources. Whereas marine populations were once thought to be panmictic through extensive larval exchange, recent studies are increasingly uncovering fine-scale substructure driven by contemporary features of the seascape such as coastal topography, bathymetry, and nearshore currents. In this study I tested the degree of genetic subdivision among ten populations of copper rockfish (Sebastes caurinus) representing paired samples collected from inlets and adjacent outer coast habitats in five replicate sounds on the west coast of Vancouver Island, British Columbia, using 17 microsatellite DNA loci. Overall, subdivision (Fst) was low (Fst = 0.031), but consistently higher between paired inlet and coast sites (mean Fst = 0.047) compared to among the five coast sites (Fst = -0.001) or among the five inlet sites (Fst = 0.026). Heterozygosity, allelic richness, and estimates of effective population size were also consistently lower in inlet sites, suggesting local inbreeding due to limited larval delivery from coastal habitats. Bayesian analysis of population structure identified two genetic groups across all samples, a single genetic group amongst only coast sites, two genetic groups amongst only inlet sites, and two genetic groups within each sound analysed separately. Copper rockfish collected from inlets were also consistently shorter, had lower condition factors, and grew more slowly than fish collected from coast sites. My results implicate inlet-coast seascape transitions in driving the evolution of population structure, likely resulting from inlet topography and estuarine circulation patterns that act to retain pelagic larvae within inlets and reduce physical connectivity with coastal habitats. Phenotypic differences between inlet and coast fish suggest differential growth environments across the inlet-coast seascape transition, which may promote genetic differentiation through selection against maladapted immigrant genotypes. Coast sites appear to be well served by the existing Rockfish Conservation Areas (RCAs), which have a coastally biased distribution. By contrast, inlet sites may require more specific local conservation measures as they appear to be less well connected to adjacent coast sites and may be more susceptible to overexploitation.Science, Faculty ofZoology, Department ofGraduat