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

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.