Peter Rivinoja

Post-release and pre-spawning behaviour of simulated escaped adult rainbow trout Oncorhynhus mykiss in Lake Övre Fryken, Sweden.

Using radio telemetry, the present study simulated the escape of 48 adult rainbow trout Oncorhynchus mykiss from a net-cage fish farm in the Lake Ovre Fryken, Sweden. The post-release dispersal of O. mykiss was fast, showed long-range dispersal behaviour, low winter survival and lacked the ability to find suitable spawning habitats. Thus, the present study suggested that reproducing for the first time may be an obstacle to the establishment of escaped farmed O. mykiss.

Monitoring Juvenile Atlantic Salmon and Sea Trout in the River Sävarån, Northern Sweden

Downstream migrations of Atlantic salmon (Salmo salar) and sea trout (S. trutta) smolts were monitored in the in northern Sweden to evaluate the river’s potential as a national index river for salmonid populations in the Baltic region, and to acquire information for stock conservation plans. A rotary screw trap was installed and operated from 2005 to 2008. Recapture rates of tagged smolts were high (from 8% to 31%), which allowed smolt abundances to be estimated. Between 2,600 and 3,900 salmon and 500–1,500 trout emigrated from the river each year. Genetic analyses made it possible to identify the origin of salmon and trout smolts and to determine whether they originated from the Savaran stock or from stocks in neighbouring rivers. In 2005 and 2006, most of the salmon were from the Savaran stock, which previously had been thought to be extinct. By contrast, 58% and 52% of the salmon analysed in 2007 and 2008, respectively, originated from stocks other than Savaran. The density of salmon and trout 0+ parr (i.e. recruitment success) based on electro-fishing was a poor predictor of smolt abundance 2 to 3 years later. The smolt trap data combined with genetic analyses provided valuable information to develop stock conservation plans.