Small Eels, Big Barriers Employing Open Data to Assess the Effects of Dam Removals and Nature-like Fishways on Juvenile Eel Migration

Sammanfattning

The critically endangered and diadromous European eel, Anguilla anguilla, has had its freshwater habitat greatly reduced as a consequence of dam development in European waters. Fishways of different kinds and removal of dams are management measures used to restore connectivity in freshwater systems. Recent work have shown that fishways are not equal in their ability to restore connectivity for many fish species in general and for the European eel in particular, and that they can be highly ineffective. This study investigated how the upstream migration of juvenile eel is affected by the implementation of nature-like fishways and dam removals. Open GIS data on dams, dam removals, fishway solutions, and monitoring data from electrofishing were combined and used to assess whether upstream migration of juvenile eel in freshwater is affected by dam removal and the construction of nature-like fishways. Confounding effects, such as water temperature, electrofishing effort, distance travelled from the sea, and eel restocking were controlled for. I found that the data available could neither confirm nor deny that dam removal or the construction of nature-like fishways restore connectivity for juvenile eel, based on assessing eel occurrence upstream and downstream of a dam before and after dam removal or construction of a nature-like fishway. The completeness and quality of the open data used in this study was scrutinized and discussed.