The disruption of longitudinal connectivity by man-made obstacles and the stocking of fish communities with non-indigenous species or genotypes may threaten the fish fauna of Belgian rivers to various extents. Obstacles impede migrations between habitats that are vital for populations, and they may restrict the gene flow between populations, thereby reducing the effective size and genetic diversity of populations and increasing the risk of local extinction. Restocking programs often involve the introduction of non-indigenous genotypes in native populations. Moreover, although stocking programs lead to the temporal and superficial enrichment of local fish communities or gene pools, they generally result in a loss of biodiversity on a regional or international scale through the homogenisation of communities and the breakdown of genetic differentiation between populations.
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