Isolation by distance within a river system: genetic population structuring of Atlantic salmon, Salmo salar, in tributaries of the Varzuga River in northwest Russia

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Abstract
An important issue for designing any conservation programme aimed at preserving genetic diversity is estimation of the scale at which genetic structuring occurs. Additional relevant factors include distinguishing whether or not population structuring is expected to be stable as predicted by the member‐vagrant hypothesis, or alternatively, whether populations are more prone to local extinction–recolonization processes, as predicted by the metapopulation evolutionary model. In this study, the population genetic structure of Atlantic salmon from 11 locations within or nearby the Varzuga River tributary system was assessed using 17 microsatellites. Mantel tests and spatial autocorrelation analyses revealed a significant isolation‐by‐distance signal within the tributary system as well as a negative association between the level of genetic diversity and waterway distance from the river mouth, indicating that dispersal is less likely to occur to populations deep in the tributary system. Individual‐level spatial autocorrelation analyses indicated that the majority of migration occurred between populations situated within 20 km of each other. The relatively high level of genetic structuring and significant isolation‐by‐distance signal observed in the Varzuga tributaries are concordant with the predictions of the member‐vagrant evolutionary model. However, one subpopulation in particular revealed signs of instability which may be due to its location in the tidal zone, or due to the fact that it is more affected by human impacts. The results suggest that preservation of a number of spawning sites spaced throughout the tributary system is recommendable for ensuring sustainable fishing tourism in the river.