Demographic and genetic estimates of effective population size (Ne) reveals genetic compensation in steelhead trout

Estimates of effective population size (N-e) are required to predict the impacts of genetic drift and inbreeding on the evolutionary dynamics of populations. How the ratio of N-e to the number of sexually mature adults (N) varies in natural vertebrate populations has not been addressed. We examined the sensitivity of N-e/N to fluctuations of N and determined the major variables responsible for changing the ratio over a period of 17 years in a population of steelhead trout (Oncorhynchus mykiss) from Washington State. Demographic and genetic methods were used to estimate N-e. Genetic estimates of N-e were gained via temporal and linkage disequilibrium methods using data from eight microsatellite loci. DNA for genetic analysis was amplified from archived smolt scales. The N-e/N from 1977 to 1994, estimated using the temporal method, was 0.73 and the comprehensive demographic estimate of N-e/N over the same time period was 0.53. Demographic estimates of N-e indicated that variance in reproductive success had the most substantial impact on reducing Ne in this population, followed by fluctuations in population size. We found increased N-e/N ratios at low N, which we identified as genetic compensation. Combining the information from the demographic and genetic methods of estimating N-e allowed us to determine that a reduction in variance in reproductive success must be responsible for this compensation effect. Understanding genetic compensation in natural populations will be valuable for predicting the effects of changes in N (i.e. periods of high population density and bottlenecks) on the fitness and genetic variation of natural populations.