Impacts of stocking and its intensity on effective population size in Brook Charr (Salvelinus fontinalis) populations

Effective population size (Ne) is a measure of the genetic size of a population and a crucial parameter for wildlife population management since it is strongly related to retention of genetic diversity in time and/or to inbreeding levels. Many exploited fish populations are stocked with the purpose of increasing population sizes to sustain important fishing pressures. However, stocking hatchery-reared fish could at the same time increase population census size and decrease Ne. Our study aimed at characterizing how stocking affected Ne in supplemented populations of Brook Charr (Salvelinus fontinalis) in Quebec and at assessing how this relationship varied with the intensity of stocking (e.g. number of stocking events, number of fish stocked/ha, proportion of domestic genetic background). We estimated Ne with the linkage disequilibrium method in 54 populations (3361 sampled individuals analyzed at 20 microsatellites) with various levels of stocking intensity. We found that stocked lakes have significantly lower Ne than unstocked lakes. However, we found little evidence of an additional effect of stocking intensity on Ne of stocked lakes. Our results suggest that stocking may have a negative impact on Ne but that more intense stocking does not necessarily translate into lower Ne. However, even though low Ne in stocked populations could be attributed to an effect of stocking, it is also likely that stocked lakes consist of poor environments that translate into low Ne.