Abstract

As wild populations become increasingly small and vulnerable, conservation managers typically must make quick decisions based on limited resources. Two crucial parameters affecting management decisions are the census population size (N) and the effective number of breeders (Nb). However, measuring N and Nb is often difficult, making it of interest to generalize one from the other. We assessed the relationship between N and Nb from 2008-2015 in twelve brook trout populations varying greatly in N (49-10032) and Nb (3-567). Most of the variability in Nb could be explained by N (R2m=0.54, p<0.001) or stream length (R2m=0.44, p<0.001) alone. The ratio Nb/N increased at small N or following an annual decrease in N (R2=0.49, p<0.01), suggesting density-dependent constraints on Nb/N (genetic compensation). We did not find any evidence for consistent differences invariability in Nb and/or Nb/N between small and large populations; however, small populations had more varying temporal variability in Nb/N ratios than large populations. Nb and Nb/N were respectively 2.5-fold and 2.3-fold more variable among populations than temporally within populations. Collectively, our results suggest that conservation resources could be saved by using N or Nb to infer the other to assess relative population sizes. However, using one variable to infer the other to monitor trends within populations is less recommended, perhaps even less so in small populations given their less predictable Nb vs. N dynamics.