Effective number of breeders provides a link between interannual variation in stream flow and individual reproductive contribution in a stream salmonid

The effective number of breeders that give rise to a cohort ( N b ) 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 N b in two long‐term studies of brook trout populations. We found no supporting evidence for our initial hypothesis thatN ^ b reflectsN ^ c (defined as the number of adults in a population at the time of reproduction).N ^ b was stable relative toN ^ C 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 N b . We observed an intermediate optimum autumn stream flow for bothN ^ b ( R 2  = 0.73, P  =   0.02) and full‐sibling family evenness ( R 2  = 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 N b and N C (so‐called genetic compensation). This work provides evidence that N b is a useful metric of population‐specific individual reproductive contribution for genetic monitoring across populations and the link we provide between stream flow and N b could be used to help predict population resilience to environmental change.