Assessment of upstream and downstream passability for eel at dams

The American eel (Anguilla rostrata) population has experienced a marked population decline. Habitat loss resulting from dam construction to improve the control and use of freshwater discharge is one of the factors involved. There are some 5600 dams in rivers draining to the St. Lawrence River in Quebec (Canada). Their passability to eels migrating upstream and downstream has been assessed using the Quebec Dam Database. Eighteen percent of the dams are used for supplying water and 13% for hydroelectricity, but >50% are used for recreational purposes. Although the majority of the dams are <3 min height and are made of concrete or earthfill, dams present a great variety of physical characteristics. Passability ranks were assigned to each category of dam based on three assessment criteria: the height of the dam, the materials used in its construction, and its use. Passability to upstream migrants was also assessed from photographs for a subset of dams. The two methods (statistical analysis and the use of photographs) may yield different results, but the two methods were consistent to identify the impassable dams. This analysis shows overall that the problem of passability is more significant for upstream passage than it is for downstream passage.