AbstractThe effective number of breeders that give rise to a cohort (Nb) is a promising metric for genetic monitoring of species with overlapping generations; however, more work is needed to understand factors that contribute to variation in this measure in natural populations. We tested hypotheses related to interannual variation in Nb in two long‐term studies of brook trout populations. We found no supporting evidence for our initial hypothesis that reflects (defined as the number of adults in a population at the time of reproduction). was stable relative to and did not follow trends in abundance (one stream negative, the other positive). We used stream flow estimates to test the alternative hypothesis that environmental factors constrain Nb. We observed an intermediate optimum autumn stream flow for both (R2 = 0.73, P = 0.02) and full‐sibling family evenness (R2 = 0.77, P = 0.01) in one population and a negative correlation between autumn stream flow and full‐sib family evenness in the other population (r = −0.95, P = 0.02). Evidence for greater reproductive skew at the lowest and highest autumn flow was consistent with suboptimal conditions at flow extremes. A series of additional tests provided no supporting evidence for a related hypothesis that density‐dependent reproductive success was responsible for the lack of relationship between Nb and NC (so‐called genetic compensation). This work provides evidence that Nb is a useful metric of population‐specific individual reproductive contribution for genetic monitoring across populations and the link we provide between stream flow and Nb could be used to help predict population resilience to environmental change.