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

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