Effective population size and genetic conservation criteria for bull trout

Abstract

Effective population size (N_e) is an important concept in the management of threatened species like bull trout Salvelinus confluentus. General guidelines suggest that effective population sizes of 50 or 500 are essential to minimize inbreeding effects or maintain adaptive genetic variation, respectively. Although N_e strongly depends on census population size, it also depends on demographic and life history characteristics that complicate any estimates. This is an especially difficult problem for species like bull trout, which have overlapping generations; biologists may monitor annual population number but lack more detailed information on demographic population structure or life history. We used a generalized, age-structured simulation model to relate N_e to adult numbers under a range of life histories and other conditions characteristic of bull trout populations. Effective population size varied strongly with the effects of the demographic and environmental variation included in our simulations. Our most realistic estimates of N_e were between about 0.5 and 1.0 times the mean number of adults spawning annually. We conclude that cautious long-term management goals for bull trout populations should include an average of at least 1,000 adults spawning each year. Where local populations are too small, managers should seek to conserve a collection of interconnected populations that is at least large enough in total to meet this minimum. It will also be important to provide for the full expression of life history variation and the natural processes of dispersal and gene flow.

Citation

Rieman, Bruce E.; Allendorf, F. W. 2001. Effective population size and genetic conservation criteria for bull trout. North American journal of fisheries management. 21(4): 756-764