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 ([Formula: see text]) 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 [Formula: see text] and compared this to an alternative approach for estimating [Formula: see text] that also accounts for age structure. Multiple lines of evidence were consistent with [Formula: see text] corresponding to the local (subpopulation) spatial scale and the cohort‐specific temporal scale. We found that at least four consecutive cohort‐specific estimates of [Formula: see text] were necessary to obtain reliable estimates of harmonic mean [Formula: see text] for a subpopulation. Generational [Formula: see text] derived from cohort‐specific [Formula: see text] was within 7%–50% of an alternative approach to obtain [Formula: see text] , 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 [Formula: see text] for local‐scale genetic monitoring.