The Relationship Between Effective Number of Breeders, Census Population Size and Role of the Environment in Brook Trout

Understanding interannual fluctuations in the effective population size (Ne) and the relationship between Ne and adult census population size (N) is important to predict genetic and demographic changes for threatened populations. Difficulties in measuring Ne have spurred interest in a more practical surrogate: the number of breeders per cohort (Nb). We evaluated the relationship between the environment, Nb, and N, for five consecutive cohorts across 14 brook trout populations located in Cape Race, Newfoundland, Canada. These populations differ greatly in terms of their size, life history, and environmental characteristics. We found that only two variables, discharge and N, explained the variability in Nb. In particular, N was positively correlated with Nb and explained most of the variation. There was no evidence that years, population, or other stream variables affected this relationship. We also found a strong inverse relationship between Nb/N and N suggesting the occurence of genetic compensation in brook trout. Collectively, our results suggest that it might be possible to infer Nb from N, or vice-versa, saving resources and time.