Densité et biomasse des unionidés (Mollusca) de l'étang de Virelles (Hainaut, Belgique)

In the course of the autumn 1986, the pond of Virelles (Hainaut, Belgium) has been dried out. Unionid mussels (Anodonta cygnea and A. piscinalis) were then sampled on a total area of 1,408 m² (about 0.2% of the pond area). We estimated their density was nearly 875 ind./ha and their biomass 204 kg fresh weight/ha. The length frequency distribution of the mussels is quite surprising since the larger individuals are the most numerous. We think this situation is derived from a higher predation rate on small individuals. Musk rats (Ondatra zibethicus) and large carps (Cyprinus carpio, mean weigth 5.5 kg; mean biomass 86 kg/ha) probably enconnter less difficulties in handling the smaller mussels than the larger ones.