Problématique de la conservation, de l'exploitation halieutique et de l'aménagement des ressources ichtyologiques dans une grosse rivière de la zone à barbeau : l'Ourthe liégeoise

Since 1970, the Fish Research Unit of the University of Liege has been conducting ra esearch on the fish fauna of the River Ourthe, a medium size (20-30 m wide) "barbel zone river" in the vicinity of Liege. Data were collected on several parameters : structure, density and biomasss of the ichchyofauna, dynamic and production of the most representative populations (mainly running water cyprinids such as Barbus barbus, Chondrostoma nasus, Leuciscus leuciscus, Leuciscus cephalus), exploitation by angling and restocking. This paper examines the principal practical implications of the results obtained with regards to fishery management and conservation of the fish resources. On the whole, the ecological and fishery situation of the R. Ourthe is rather satisfactory : the 23 species recorded form an ichtyomass amounting to 315 kg/ha out of which 210 kg are available for angling, taking account of the legal sizes. Nevertheless, some alterations of the fish fauna are revealed, these are due to an eutrophication process or physical damages to the stream (these factors affecting mainly S. trutta, T. thymallus, and A. bipunctatus) and to a high angling pressure (major incidence on S. trutta, T. thymallus, P.fluviatilis, E. lucius, P. phoxinus and to a lesser degree on Barbus barbus. Because of the importance off those wild species populations for which there exist no restocking methods, most efforts in fishery management should be centered on the conservation of these populations through three kinds of actions: i) protection of the habitat against pollution and physical alterations; ii)modification of certain angling regulations (fixing a legal size for C. nasus, and L. leuciscus; legal size of 25 cm instead of 18 cm in L. cephalus; closing the fishing season for T. thymallus by November-December instead of January; limiting the number of fish allowed to be caught; iv) artificial propagation of wild species like T. thymallus, B. barbus, G. gobio, etc… As far as the other valuable species (S. trutta, R. rutilus, P. fluviatilis, E. lucius) are concerned there is a need for improving the efficiency of the restocking technics currently in practice at a large scale and, simultaneously, for protecting the habitat (spawning grounds) and reducing the growing angling pressure. The general conclusion stresses a special attention on the urgent necessity of protecting some endangered species or representative populations or ichtyocenosis by taking severe conservation measures (ichthyological reserves, total prohibition of catching certain fish species).