Fine-scale differences in genetic and census population size ratios between two stream fishes

Abstract

Comparing the ratio of effective number of breeders (Nb) to adult population size (N) among closely related coexisting species can provide insights into the role of life history on Nb/N ratios and inform conservation programs towards limiting the loss of evolutionary potential in natural populations. We estimated Nb and N in two coexisting salmonid fishes (Brook trout and Atlantic salmon) for 3–4 consecutive years in two small, adjacent streams in Newfoundland, Canada, using mark-recapture (N), linkage disequilibrium (Nb(LD)), and sibship frequency approaches (Nb(Sib)). We found that Nb/N ratios were about 20-fold greater in Atlantic salmon than in brook trout (mean 0.20, range 0.06–0.56 vs. mean 0.02, range 0.01–0.05, respectively). This difference was consistent across Nb estimators. In addition, we found that removing migrants reduced Nb: the strength of the effect was weak for Nb(LD) and much stronger for Nb(Sib). Our results highlight the importance of subtle ecological differences and gene flow in shaping Nb/N. They also provide some evidence that the linkage between demographic and evolutionary processes varies between closely related taxa and suggest that a more complete understanding of the Nb/N range across various species is an important component of conservation genetics and management.