The impacts of dredging and weed cutting on a population of freshwater mussels (Bivalvia: Unionidae) Original Research Article

Regulated lowland rivers generally require management to control macrophytic vegetation and sediment build-up. Such management can have deleterious effects on much of the biota. It has long been a concern that indiscriminant river management has played a part in the world-wide decline in freshwater mussel populations. This study investigated the impact of dredging and weed-cutting on the population size, structure and distribution of four species of unionid mussel: Anodonta anatina, A. cygnea, Unio pictorum and U. tumidus. Dredging removed between 3% (A. anatina) and 23% (A. cygnea) of the mussel population. The weed bucket removed a maximum of 3% of any species, but its higher frequency of use results in removing a similar number of mussels to dredging in the long-term. Marked stones placed in the river during dredging suggest that the excavator drags mussels across the river bed. This is supported by the relatively high density of mussels in the channel closest to the bank from which the excavator habitually operates. Tagged mussels moved only small distances following dredging (generally <15 cm after 55 days) and showed no tendency to disperse. The impact of river management on unionid populations can be reduced, while retaining its channel maintenance function, by dredging and weed cutting only within the centre of the channel. Marginal vegetation should be cut to 5 cm above the river bed using weed boats with an annual alternation between banks to preserve refugia of invertebrates and fish.