Mortality causes of Atlantic salmon and sea trout smolts in a restored river and the adjacent estuary
A. Koed & H. Baktoft
Danish Institute for Fisheries Research, Department of Inland Fisheries, Vejlsøvej 39, 8600 Silkeborg, Denmark, e-mail: firstname.lastname@example.org, Phone: +4589213100, Fax: +4589213150
The mortality causes of wild salmon Salmo salarand trout Salmo trutta smolts in the recently restored River Skjern Å and the adjacent Ringkøbing Fjord was investigated using radio telemetry. Their downstream migration was monitored by manual tracking and by seven automatic listening stations distributed along the river and one within an estuarine nesting colony of herring gulls Larus argentatusand cormorants Phalacrocorax carbo sinensis, 13 km outside the river mouth. Due to drainage and subsequent subsidence of the river valley a lake of 250 ha developed when the dikes were removed in connection with the river restoration. Until mid-May the river water entered the upper end of the lake on a stretch of c. 500 m. The water re-entered the river though a channel situated in the lower end of the lake. During April and May 54 wild salmon and 53 trout smolts were tagged and released. Pike Esox lucius in the river predated four. Twenty-eight transmitters were retrieved in the estuarine nesting colony (22 salmon and six trout), the majority predated by cormorants. Of these five (four salmon and one trout) were predated in the river and 23 in the estuary. The survival of salmon smolts was positively correlated to length weight-relationship (K-factor). There was no correlation between salmon smolt survival and length of fish or gill Na+, K+-ATPase activity. The survival of trout smolts was neither correlated to fish length, K-factor nor gill Na+, K+-ATPase activity. A comparable study was performed in 2000 prior to the river restoration. The present study indicates that the river restoration had an indirect negative effect on the smolt run, mediating bird predation within the river system. As also demonstrated in 2000, bird predation in the estuary had a major adverse effect on the smolt run. Jointly the smolt mortality in the river and in the estuary may threaten a self-sustaining salmon population in River Skjern Å.