Some statistics.
We have looked at the catch statistics of Surna as far back as 1970. It is generally accepted that the drift netting took about half of Norway’s salmon catch and we assume for Surna the double harvest before 1989. The catch including Catch and Release (CR) is the green line. CR does not give any more salmon or bio mass. Surna was regulated the year 1968 and the permission and conditions for water power generation in Trollheim Power Plant were given 1962.
You might argue the fit between 1988 and 1889 in the figure below but the mean tangent or slope of the rose salmon harvest line can’t be ignored. You might detect a deeper slope after 1996 when Statkraftverkene changed from a public authority to the company Statkraft A/S and increased the daily varying power run of the river. It is obvious that Statkraft is responsible for Surnas lack of biological diversity and the revision of Folla-Vindölareguleringen is most important to stop the decline of salmon. Surna is appointed a National Salmon River by the Norwegian Parliament february.25 year 2003.
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Figure. Catch as killed salmon is rose line with double catch before 1989, sea trout is blue line and catch of salmon including CR is green line.