Most aquatic ecosystems in Flanders are disturbed by human activities. We can distinguish 2 categories of human-induced alterations: (1) physical disturbance such as modifications of river channels, and (2) disturbance of the water quality. Bothof them have direct effects on aquatic life in the river. However, indirect effects may also occur: physical disturbance and water pollution can act as barriers for aquatic organisms from tributaries. Fish populations in undisturbed river trajects (with very good water quality) can become completely isolated when the water quality downstream is too poor to survive or when physical barriers such as weirs or dams are present. Fragmentation of the habitat of organisms leads to isolation ad a decrease in the size of populations. As a consequence this results in a decreased genetic variability or genetic erosion. Genetic erosion makes populations highly vulnerable towards environmental stress (decrease in tolerance and adaptation capacity with respect to changing environmental conditions). Despite several efforts of the Flemish government with respect to water quality improvement (such as wastewater treatment and regulation of toxic discharges), the distribution area of many native fish species in Flanders has diminished. There are several indications that the loss of genetic variability of native fish populations is an acute threat for their conservation. The aim of this proposal is to investigate the extent of genetic erosion, usinggenetic markers, for some representative native fish species. Simultaneously the condition and tolerance of isolated populations towards environmental stress (such as water pollution) will be assessed using non-invasive techniques. The proposed research is a prerequisite to provide insights on the minimal viable size of a fish population, and will make it possible to formulate management directives. (1) to determine the degree of isolation in different populations of four native fish speciesin Flanders, and the population size in each of the populations; (2) to examine using molecular genetic markers if a decreased population size and an increasing isolation leads to a loss of genetic variation; (3) to examine if a loss of genetic variation leads to a decrease of individual fitness, and if so, to examine if this affects the survival chance of populations; (4) to compare the condition of individual fishes between isolated and less isolated populations (at different levels of pollution) using morphological measurements and selected biomarkers; (5) to document how the genetic structure affects the responses of individuals and populations to stressfactors; (6) to develop measures to prevent (the consequences of)genetic pauperization; (7) to identify populations that might provide the source material for augmentation or reintroduction programs (in the case of Bullhead and Dace).