Effective size of an Atlantic salmon (Salmo salar L.) metapopulation in Northern Spain

The genetic diversity of metapopulations is influenced not only by the effective sizes (N e ) of individual subpopulations, but also by the total effective size of the metapopulation (meta-N e ). We estimated meta-N e of four neighbouring Atlantic salmon populations connected by gene flow using genetic estimates of subpopulation N e s and migration rates derived from capture–recapture data. The meta-[^(N)]e meta{\hbox{-}}\hat{N}_{e} was lower than the sum of [^(N)]e \hat{N}_{e} s of the subpopulations, suggesting that genetic diversity harboured by the four river salmon metapopulation is lower than what would have been expected by viewing individual subpopulations separately. In addition, meta-[^(N)]e meta{\hbox{-}}\hat{N}_{e} was found to be sensitive to changes in [^(N)]e \hat{N}_{e} of the subpopulation from which net emigration rate was largest, so as that the genetic diversity of the metapopulation would be best preserved by avoiding any reductions in N e of this subpopulation. Yet, this subpopulation is the one that has historically—and still is—experiencing the highest exploitation rate in the metapopulation system.