Fragmentation by weirs in a riverine system: A study of genetic variation in time and space among populations of European grayling (Thymallus thymallus) in a Danish river system

Human induced habitat alterations affect the genetic structure of many fish populations. Weirs in particular have caused fragmented populations previously connected by gene-flow. We studied the effects of weirs on the distribution of genetic variation within European grayling (Thymallus thymallus) populations from the Skjern River, Denmark. We compared microsatellite data from DNA extracted from historic scale samples collected 60 years ago with data from contemporary samples. Pairwise multilocus F-ST estimates between all contemporary population samples were significant as well as exact tests for population differentiation. Assignment tests of individuals to a set of baseline samples showed correct assignment to the population of origin of between 54 and 79%. Assignment of individuals from recent samples to the historic population samples showed highly variable results (3-83%) of correct assignment suggesting different population histories. Pairwise multilocus F-ST estimates were significantly correlated with the number of intervening weirs but not with waterway distance. A simulation procedure was used to estimate differences in relative population sizes, which indicated that the main river population was approximately three times larger than those of the tributaries. There were no signs of any loss of genetic variation for the river system as a whole during these 60 years. The results show that weirs can be an important factor for creating the current distribution patterns of genetic variation among grayling populations, most likely by only allowing passive downstream drift of fry and obstructing active upstream migration.