Ongoing tests into the unexplained deaths of thousands of fish at the Lake of Menteith mean the fishery will stay closed until at least Saturday.

Quintin Glen, the fishery manager is expected to take a decision on reopening “on Thursday or Friday” this week, according to staff.

There have been no further fatalities at the 700-acre Stirlingshire fishery, billed as Scotland’s prime trout angling location, since the mysterious deaths of fish nearly two weeks ago.

Tests by the Scottish Environment Protection Agency (SEPA) are continuing but so far their results have ruled out pollution or infectious disease. They have also said there is no evidence of unusually high levels of algae which are sometimes related to oxygen depletion.

The agency said: “SEPA ecology staff are investigating the possibility that the deaths have been caused by a rare natural phenomenon involving toxins produced by naturally occurring bacteria.”

Examination of the fish revealed they died due to a toxins which severely damaged their gills and suffocated them by stopping oxygen uptake from the water.

The lake’s manager says scientists are now studying levels of ferrous iron concentrations in the water.  Glen said: “During the episode the water turned an unusual yellow browny colour consistent with high iron.

“We now need to look at the levels of ferrous iron in the sediment and gather further evidence from environmental fish experts.  Fish in the exit burn from the lake were unaffected which may add weight to this theory.”

Closure of the fishery has posed headaches for competitive angling organisers at both international and club level.  Two heats for the Scottish National fly-fishing championships had to be moved at short notice last week.

And the Scottish National Anglers’ Association (SANA), hosts of this year’s World Fly-fishing Championships due to open in Scotland in less than five weeks, face tricky logistical issues if the lake is not back to normal operations.

Menteith is one of four central Scotland stillwater venues for the blue riband angling event which sees competitors moving between locations as far apart as Loch Awe in Argyllshire and Loch Leven in Kinross for morning and afternoon sessions each day.