Fish and their environment in large European river ecosystems. The River Meuse.

The River Meuse flows through three countries : France (506 km), Belgium (182 km) and The Netherlands (183 km). Its flow regime (mean annual discharge of 250 m3/s at the border between Belgium and The Netherlands) has a maximum in February and a minimum in July-September. Its thermal regime is disturbed by the input of heated effluents which cause a rise in temperature of 2 to 10°C. The water quality is degraded, mainly downstream from Liege, where dissolved oxygen drops, and pollution by cyanides and heavy metals may cause fish kills. Physical habitats have been drastically modified by the canalization of the river between Namur and the Dutch border. The biocenoses are relatively wellknown. Phytoplankton is dominated by diatoms and Chlorophyceae. Zooplankton mainly consists of Protozoa ands Rotifers. Sixty-six families of macro-invertebrates were registered in the Belgian Meuse and the Insect group is dominant with 41 families. Forty-eight species of fish (40 native, 7 introduced or acclimatized and 1 with an undetermined status) are living or have been living in the Belgian part of the Meuse. One of the most abundant species, the roach, has a slow growth (198 mm fork length when 8 years old) in relation to the deterioration of the environment. In Belgium, there is only a sport fishery but in the Dutch section of the Meuse, a small commercial fishery (eel, pike, perch, sea trout, roach) still exists. Fisheries for salmon and others migratory fishes disappeared at the beginning of the XX th century, when new weirs were built in The Netherlands. At present, many fish species are being restocked. The restocking of some species, such as roach especially, and perch is demanded by anglers. For other species such as chub, pike and barbel, it aims at restoring the populations. The fate of the fishes released in the river is not known. However, in some unfavorable conditions, there are indications that these restockings sustain the populations. Some studies on the fate of the barbel in the Meuse basin have recently begun. The impact of various populations on the Meuse fish were studied in relation to the thermal and radioactive outflow from the nuclear power-station of Tihange. The thermal effluents directly or indirectly cause earlier reproduction, a change in the feeding habits and an increase in growth rate. These studies have now been discontinued. The radio contamination of fish by Cs 137 was important during the first years of the study, but it was lower in 1983. Finally, studies on the contamination of fish by heavy metals have started recently. Among possible parasites, only Bucephalus polymorphous, which is responsible for larval and intestinal bucephaloses, was systematically studied in the Meuse. The rate of infestation in roach is high (65 %) especially in the reaches where pike perch occurs.