The Lake of Menteith reopens to angling tomorrow with “natural causes” now seen as the “most likely” reason for thousands of mystery fish deaths.

The fishery, one of Scotland’s prime game and coarse angling venues and set to be a venue during the World Fly-fishing Championships next month, called in a world veterinary authority on fish welfare to report on the poisoning.

He rated the “fish kill” event as level 3 on a five-point scale but said that natural causes were most likely to blame, concluding that a rare combination of organic “matter, gases and associated bacterial toxins” rising up from the coldest depths of the lake had been involved.

Prof. Ronald Roberts: rare natural causes to blame
Prof. Ronald Roberts: world authority

Professor Ronald Roberts, president of the World Aquatic Veterinary Medicine Association, chairman of fish health and welfare at the European Food Safety Authority and the EU’s Food Safety Risk Assessment organisation, visited the Stirlingshire fishery yesterday.

He discounted toxic algae as a cause and ruled out any infection, but said he believed hydrogen sulphide poisoning had been a “significant factor”, adding:

“It is unlikely however to be the only factor, as anerobic breakdown of organic matter releases a range of similar products.  This will of course have been a very temporary situation and there is now no evidence of any such contaminants remaining.”

Such events were rare, he said, and he had no personal experience of them recurring.

Thousands of prime rainbow trout and specimen coarse fish such as pike, died suddenly more than two weeks ago and the 700-acre lake was quickly closed while scientists studied the possible causes.

Fishery manager Quintin Glen called in experts from around the world for guidance.  SEPA joined with leading biologists such as Professor Geoff Codd of Dundee University, an international authority on toxic algae, in examining water and fish samples.

Prof. Roberts ended his report noting that he had cooked and eaten fish from the main affected area of the lake yesterday.  “There was no muddy taste or algal taint and the fish has caused me no negative effect.”

Glen said he would be consulting water quality experts shortly “in relation to safeguarding the lake for the future – as an area for angling recreation.”

Full Lake of Menteith statement