Ecological determinants and temporal stability of the within-river population structure in Atlantic salmon (Salmo salar L.)

A gene diversity analysis was performed using microsatellite loci in order to (i) describe the extent and pattern of population structure in Atlantic salmon (Salmo salar L.) within a river system; (ii) establish the importanc e of quantifying the signal:noise ratio in accurately estimating population structure; and (iii) assess the potential usefulness of two evolutionary m odels in explaining within-river population structure from the ecological a nd habitat characteristics of Atlantic salmon. We found weak, yet highly si gnificant microscale spatial patterning after accounting for variance among temporal replicates within sites. Lower genetic distances were observed am ong temporal samples at four sampling sites whereas no evidence for tempora l stability was observed at the other three locations. The component of gen etic variance attributable to either temporal instability and/or random sam pling errors was almost three times more important than the pure spatial co mponent. This indicates that not considering signal:noise ratio may lead to an important overestimation of genetic substructuring in situations of wea k genetic differentiation. This study also illustrates the usefulness of th e member-vagrant hypothesis to generate a priori predictions regarding the number of subpopulations that should compose a species, given its life-hist ory characteristics and habitat structure. On the other hand, a metapopulat ion model appears better suited to explain the extent of genetic divergence among subpopulations, as well as its temporal persistence, given the reali ty of habitat patchiness and environment instability. We thus conclude that the combined use of both models may offer a promising avenue for studies a iming to understand the dynamics of genetic structure of species found in u nstable environments.