By Beate Hoddevik Sunnset and Ove Skilbrei
In recent years there has been a significant decline in the recapture rate for smolt from the River Daleelv in relation to the results from 2001 and 2002 (see chart). Meanwhile, the average size of the grilse has fallen from just over 2 kg to 1-1.6 kg. The grilse’s condition factor, i.e. the length to weight ratio, has also been abnormally poor in recent years. We have found occasional grilse that have shown normal healthy growth (2 -3 kg). Several of them were released in the same group, and it is therefore possible that they by chance found an area of the Norwegian Sea where conditions were better. The exception from the deteriorating picture was 2004, when both recapture and growth rates improved.
Recapture rates in subsequent years for tagged smolt released in the River Daleelv from 2001 to 2008.
Our research in the River Daleelv clearly demonstrates that there are many factors that affect salmon populations. Unfortunately there are currently several negative influences that are working in parallel, and their cumulative effect over the years has left many salmon populations in western Norway vulnerable. In spite of the fact that fishing has been halted for a number of years, some populations have too few spawning fish in relation to their production potential, and may be beyond recovery. In rivers where the native salmon population is now low, even a small number of escaped farmed fish may constitute a high proportion of the spawning fish. There is a growing body of scientific literature warning that the ability of wild populations to adapt is reduced if they genetically mix with fish that have been reared at fish farms.
Salmon from the River Daleelv are staying longer at sea
The Daleelv population was previously dominated by grilse. In past experiments, 60-70 % of the salmon returned as grilse. However, since 2003 we have observed that when recapture rates fall, it is the number of grilse that declines most. This has caused the proportion of the catch made up of grilse to fall to only 15-50 % over the past five years. Although there have also been fewer than normal older fish, they have represented a much larger proportion of the catch than previously. A similar trend has been observed in many Norwegian rivers in recent years. In such cases, the catch measured in kilos has fallen less than the number of fish.
