Keeping things local: Subpopulation Nb and Ne in a stream network with partial barriers to fish migration

Abstract

For organisms with overlapping generations that occur in metapopulations, uncertainty remains regarding the spatiotemporal scale of inference of estimates of the effective number of breeders Nb and whether these estimates can be used to predict generational Ne. We conducted a series of tests of the spatiotemporal scale of inference of estimates of Nb in nine consecutive cohorts within a long-term study of brook trout (Salvelinus fontinalis). We also tested a recently developed approach to estimate generational Ne from Nb and compared this to an alternative approach for estimating Ne that also accounts for age structure. Multiple lines of evidence were consistent with Nb corresponding to the local (subpopulation) spatial scale and the cohort-specific temporal scale. We found that at least four consecutive cohort-specific estimates of Nb were necessary to obtain reliable estimates of harmonic mean Nb for a subpopulation. Generational Ne derived from cohort-specific Nb was within 7%–50% of an alternative approach to obtain Nb, suggesting some population specificity for concordance between approaches. Our results regarding the spatiotemporal scale of inference for Nb should apply broadly to many taxa that exhibit overlapping generations and metapopulation structure and point to promising avenues for using cohort-specific Nb for local-scale genetic monitoring.