A critical assessment of estimating census population size from genetic population size (or vice versa) in three fishes

Technological and methodological advances have facilitated the use of genetic data to infer census population size (N(c)) in natural populations, particularly where traditional mark‐and‐recapture is challenging. The effective number of breeders (N(b)) describes how many adults effectively contribute to a cohort and is often correlated with N(c). Predicting N(c) from N(b) or vice versa in species with overlapping generations has important implications for conservation by permitting (i) estimation of the more difficult to quantify variable and (ii) inferences of N(b)/N(c) relationships in related species lacking data. We quantitatively synthesized N(b)/N(c) relationships in three salmonid fishes where sufficient data have recently accumulated. Mixed‐effects models were analysed in which each variable was included as a dependent variable or predictor term (N(b) from N(c) and vice versa). Species‐dependent N(b)/N(c) slope estimates were significantly positive in two of three species. Variation in species slopes was likely due to varying life histories and reinforce caution when inferring N(b)/N(c) from taxonomically related species. Models provided maximum probable estimates for N(b) and N(c) for two species. However, study, population and year effects explained substantial amounts of variation (39%–57%). Consequently, prediction intervals were wide and included or were close to zero for all population sizes and species; model predictive utility was limited. Cost‐benefit trade‐offs when estimating N(b) and/or N(c) were also discussed using a real‐world system example. Our findings based on salmonids suggest that no short cuts currently exist when estimating population size and researchers should focus on quantifying the variable of interest or be aware of caveats when inferring the desired variable because of cost or logistics. We caution that the salmonid species examined share life‐history traits that may obscure relationships between N(b) and N(c). Sufficient data on other taxa were unavailable; additional research examining N(b)/N(c) relationships in species with potentially relevant life‐history trait differences (e.g., differing survival curves) is needed.