The Impact of the Channeled Part of the Aulne River (France) on the Upstream Migration of Returning Adult Atlantic Salmon as Determined by Radio-Tracking
Abstract.—A 2-year radio-tracking study of Atlantic salmon Salmo salar upstream migrants was conducted on the Aulne River in France. The objective was to survey upstream migration in the river, of which 70 km are channeled. The behavior of 126 salmon was studied at 26 weirs (approximately 2 m high) from the beginning of July 1999 to the middle of December 2000. The mean percentage of fish able to pass upstream of weirs fitted with older fish passage facilities was less than for more recent fishways installed since 1994. An evaluation of the cumulative effect of the weirs indicates that only 2–3% of returning adults are likely to pass through the channeled part of the river and reach upstream areas with suitable habitat for spawning. The most obvious blockages occurred at weirs in the lowermost portion of the channeled river. Environmental conditions such as low flows and poor water quality may also make it difficult for salmon entering the river to progress upstream. Moreover, salmon appear to exhibit nontypical migration behaviors in this environment, which is heavily influenced by human activities. More than 26% of returning adults migrated downstream during the study, leaving the Aulne before the spawning period began and entering and migrating upstream through other neighboring rivers. We also estimated the impact of fishing on upstream migrants, which appeared far greater for spring salmon than for grilse. This study provides fisheries managers with a tool to assess restoration plans for Atlantic salmon and better define the actions needed to reach restoration goals.