Le repeuplement en barbeaux fluviatiles, Barbus barbus (L.) d'élevage dans les cours d'eau de Wallonie : analyse approfondie des résultats obtenus dans la Méhaigne et la Meuse liégeoise en 1983-1989

The paper deals with the 1983-1989 results of a barbel stocking programme carried on in the running waters of Southern Belgium (Wallonia). Barbel used as stocking material were produced by artificial spawning of wild and hatchery-reared brood-stocks. Fry, fingerlings and sub-adult fish were cultivated at optimum temperature (18-24°C) in ponds and tanks supplied with a heated effluent (Tihange nuclear power plants on the R. Meuse). For the period 1983-1989, 85,495 barbels (3,553 kg) of all sizes (4-50 cm) were stocked in 14 streams, in order either to rebuild extincted populations (as in the R. Méhaigne) or to sustain declining ones (as in the R. Meuse). Stocking performed in the R. Méhaigne resulted in the rebuilding of high density (50-100 kg/ha) populations contributing to over 50% of total ichthyomass in several stations. Barbel stocked in early June at a size of 11-12 cm (22 g) attain a size of 10.0-16.8 cm, 19.3 -21.4 cm and 23.8-24.9 cm at the end of their first, second and third summer season, respectively. The average annual survival rate of the stocked barbel is 56%. Angler's catches (legal size = 27 cm fork length) amount to a minimum value of 14.6% of the number of catchable fish available. Most (62%) tagged > 20 cm barbel stay within a +- 0.2 km river stretch but some fish (3.2%) move over long distance (2 km and over). Hatchery-reared barbel acclimatizised in the R. Méhaigne were proven to spawn naturaly in May-June, 1989 and to produce wild fry and fingerlings. Growth studies performed on tagged > 20 cm barbel stocked in the R. Meuse indicate that a 30 cm long fish at release attains a size of 35 cm after 1 year and 40 cm after two years. Recapture rates by anglers are relatively low (maximum 6.2% in one station) and 15.8% of the catch is composed of barbel having spent over 400 days in the river. Analysis of angler's recapture reveals a high proportion (62%) of tagged barbels moving over distances > 1km (maximum recorded = 14.5 km) from their point of release. Most movements are in an upstream direction (towards the fast flowing and turbulent water zones just below navigation weirs) or towards the lower course of tributaries offering gravel spawning grounds. As a conclusion, the experimental stocking of Southern Belgian running waters with hatchery-reared barbels has produced very positive results both in term of ecology (rebuilding of extincted populations, as in the R. Mehaigne) and sport angling (increased catch, as in the R. Meuse). In the future, this type of stocking may be continued and even extended provided that the rearing technologies used allow high quality fish (geneticaly and behaviouraly) to be produced. Nevertheless, a long term preservation of the barbel ecological and fishery resource can only be achieved by taking measures to conserve, improve or restore natural habitats, to increase the success of natural reproduction and recuitment and to reduce the angling pressure (mortality) on large late-maturing (> 35 cm and 8 years) females.