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Photo: Ove Skilbrei
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Tough times for wild salmon

In recent years, there has been a sharp fall in the number, as well as the size, of salmon returning to western Norway from the Norwegian Sea. Possible causes include a lack of food, high levels of sea lice and smolt predation in the fjords. Since 2001 The Institute of Marine Research has been releasing tagged smolt into the River Daleelv in Hordaland, and the results of that work may now help to explain what is happening.

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.

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.

Facts on Atlantic salmon

Latin name: Salmo salar
Other english names: Fry, parr, smolt, jacks, grilse
Family: Salmonidae
Maximum size: Up to 150 cm and 40 kg (males)
Life span: 2-8 years
Distribution: Lives in rivers on both sides of the Atlantic ocean, from Spain to Northwest Russia and from Maine to Northern Canada. A separate population lives in the Baltic. In the marine part of its lifecycle the salmon is spread over large parts of the Northern Atlantic ocean.
Spawning area: Rivers
Spawning time: October-January
Food: Juveniles in freshwater, mainly insects. As smolts and postsmolts in seawater, plancton and fish fry and as larger salmon, shrimp, pelagic fish as herring and lanternfish
Predators: Birds (e.g. Mergansers), coalfish, pollack and cod. In some areas sea mammals
Special features: The salmon is andromous, i.e. it is born and lives in freshwater for one to five years before it smoltifies and migrates into the ocean. It stays in the ocean for one to four years before it returns to the river to spawn.

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