Essai d'évaluation des ressources ichtyologiques actuelles et potentielles dans la bassin de l'Ourthe (bassin de la Meuse) en Belgique

This report is a first attempt to assess the effective (1970-79) and potential fish resources in the Ourthe River basin (3672 km2; trout, grayling and barbel zones); it aims at providing an objective basis for an economical evaluation of recreative fishing and fishery incidences of the fish stock alterations caused by human activities. The data on the fish populations were collected during numerous electro-fishing surveys. Thirty-two fish species were recorded from the Ourthe River basin during the last decade. Their population make up together a stock of 127 tons for the whole basin. 80 % of this stock are contributed by S. trutta, T. thymallus, B. barbus, C. nasus, L. cephalus and L. leuciscus. The upper Ourthe (direct and indirect effects of a barrage), the Warche-Ambleve river system (strong organic and chemical pollution caused by a tannery+ hydroelectric impoundment + barriers to fish movements) and the Vesdre River (very strong urban and industrial pollution since 1835) are suffering major alterations of their fish fauna which result in a stock reduction of around 38,6 tons. This stock reduction mainly concerns S. trutta , B. barbus, C. nasus and T. thymallus; these sensitive species are replaced by more resistant ones, the stock of which either remains relatively the same (L. cephalus) or slighly increases (L. leuciscus). When considering these six species only, the loss of annual production is 16,1 tons/year.