A radio telemetry study of the migration of Atlantic salmon (Salmo salar L.) and sea trout (Salmo trutta trutta L.) in the upper Rhine

Atlantic salmon disappeared from the Rhine River in the mid-fifties when pollution and canalization of the main course of the river blocked access to their spawning areas. After water quality improved in the 1980's, a reintroduction program was begun in the French upper Rhine basin in 1992. To better define the difficulties encountered by the returning salmon on their way between the Iffezheim dam, the first main obstacle in the Rhine, and the upriver spawning areas, we undertook a radio-tracking study. Among the fish captured by electro-fishing below the dam in 1996, we tagged 20 salmon and 10 sea trout with radio transmitters, released them upriver and followed them with portable receivers and automatic recording stations. While the salmon captured during the summer interrupted their migration or even migrated downstream after being released, those captured in the autumn migrated immediately upstream. We were able to study the behavior of salmon confronting 15 obstacles. Only two salmon were recorded in the Rhine above the confluence of the Ill River, the main tributary of the Rhine on the French side. Most of the salmon made their way in the Ill toward the Bruche, a tributary of the Ill where spawning was observed. Four sea trout regurgitated their transmitters within three months. Most of the trout migrated upstream to the second obstacle in the river, the Gambsheim dam, while two of them chose to migrate up the Ill. We observed that the fish blocked by the dams in the Rhine often explored and possibly Spawned in tributaries which confluence was located a short way downstream. The initial phase of our study provided useful information on the behavior of salmon and sea trout returning to the upper Rhine. It confirmed the need to improve migration passages at most of the dams and enabled us to better define migration equipment priorities. It also confirmed the good prospects of the Bruche as a spawning stream for salmon.