DEMOGRAPHIC GENETICS OF BROWN TROUT (SALMO-TRUTTA) AND ESTIMATION OF EFFECTIVE POPULATION-SIZE FROM TEMPORAL CHANGE OF ALLELE FREQUENCIES

We studied temporal allele frequency shifts over lj years and estimate d the genetically effective size of four natural populations of brown trout (Salmo trutta L.) on the basis of the variation at 14 polymorphi c allozyme loci. The allele frequency differences between consecutive cohorts were significant ill all four populations. There were no indic ations of natural selection, and we conclude that random genetic drift is the most likely cause of temporal allele frequency shifts at the l oci examined. Effective population sizes were estimated from observed allele frequency shifts among cohorts, taking into consideration the d emographic characteristics of each population. The estimated effective sizes of the four populations range from 52 to 480 individuals, and w e conclude that the effective size of natural brown trout populations may differ considerably among lakes that are similar in size and other apparent characteristics. In spite of their different effective sizes all four populations have similar levels of genetic variation (averag e heterozygosity) indicating that excessive loss of genetic variabilit y has been retarded, most likely because of gene flow among neighborin g populations.