Effective Population Size and Genetic Conservation Criteria for Bull Trout

—Effective population size (Ne) 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 Ne 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 Ne 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 Ne 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. The concept of effective population size (Ne) plays an important role in conservation management of fishes (Waples in press). The Ne is a measure of the rate of genetic drift and is directly related to the rate of loss of genetic diversity and the rate of increase in inbreeding within a population (Wright 1969). Conservation of populations large enough to minimize such effects has become an important goal in the management of threatened or endangered salmonids, including Pacific salmon Oncorhynchus sp. (McElhaney et al. 2000) and bull trout Salvelinus confluentus (USFWS 1998). The ongoing fragmentation and isolation of habitat of species like bull trout have led to reductions in population size (Rieman and McIntyre 1993; Rieman et al. 1997) and presumably in Ne for many populations. A resulting loss of genetic variation can influence the dynamics and persistence of populations through at least three mechanisms: inbreeding depression, loss of phenotypic variation and plasticity, and loss of evolutionary potential (Allendorf and Ryman in press). The loss of ge* Corresponding author: [email protected] Received September 12, 2000; accepted February 19, 2001 netic variation in populations with small Ne may reduce fitness through the so-called inbreeding effect of small populations and lead to an accelerating decline toward extinction in a process termed an extinction vortex (Soulé and Mills 1998). Because of the importance of Ne, the so called ‘‘50/500’’ rule has emerged as general guidance in conservation management (Franklin 1980; Soulé 1980; Nelson and Soulé 1987; see Allendorf and Ryman [in press] for a recent consideration of these criteria). The generally accepted view is that an Ne of less than about 50 is vulnerable to the immediate effects of inbreeding depression. Although populations might occasionally decline to numbers on this order without adverse effects, maintenance of adaptive genetic variation over longer periods of time (e.g., centuries) probably will require an Ne averaging more than 500 (Allendorf and Ryman in press). These numbers have been generally applied as criteria for determination of conservation status among taxa (Mace and Lande 1991) and within the salmonids in particular (e.g., Allendorf et al. 1997; Hilderbrand and Ker-