Influences of a dam on Atlantic salmon (Salmo salar) upstream migration in the Couesnon River (Mont Saint Michel Bay) using hydroacoustics

A dam has been recently modified on the Couesnon River in the vicinity of Mont Saint Michel to control and remove the volume of sediment brought into the bay by tidal waters. This paper shows that this dam has an impact on Atlantic salmon (Salmo salar) migration and that adult fish experience adaptive responses. Upstream migration activities were recorded with a 70 kHz echosounder (SIMRAD EY500 model) during two monitoring programs in 2010 and 2011, combining a total of 172 days of data collection. The echosounder was fixed at an upstream location, 8 km from the dam, and its detection beam capacity was approximately one third of the river's cross sectional area. In addition to the time of year, the upstream direction, swimming speed limit and fish size range were the specific criteria used from our database to efficiently discriminate other fish species from the Atlantic salmon. Among a total of 116,000 fishes counted during the two periods of investigations, 189 salmon were identified using those criteria. This study enabled us to identify the driving factors that influence the salmon migration in this semi-natural environment. There was a significant difference in fish activity when the dam was operating and when the river flow was under "normal" hydraulic conditions. Furthermore, during the opening and closing phases of the dam, the migration activity was scattered during the day, with peak abundance occurring mainly during the beginning of the operating phases. However, during all valve-opening phases, the salmon activity occurred mainly during the night. Therefore, the results suggest that this dam has a strong influence on fish migration activity at this particular location. (C) 2013 Elsevier Ltd. All rights reserved.