Environment Agency officials are waiting on the results of a series of tests after the death of 1,000 fish at Drayton Reservoir in Northamptonshire.
National fisheries manager, John Ellis, has confirmed that a huge quantity of roach had been affected and, although the deaths peaked last week, fish are still dying.
An investigation has been carried out but the cause of death remains a mystery.
Mr Ellis, of the Canal and River Trust (formerly British Waterways), said: “The investigation has so far ruled out any contamination from parasites, pollution and poisonous algae.”
He added that the possibility of ‘low dissolve’ oxygen has also been tested for and dismissed.
Experts believe an internal bacterial infection or a virus spread between the fish are the most likely causes. Both are still under investigation.
Mr Ellis added: “It could have been a spawn related issue - the water may not have been warm enough for the fish to lay eggs.”
Low water temperatures can cause the fish to die because they simply can’t give birth. Mr Ellis explained inconsistent weather this summer could have been contributory factor.
The full results of the tests carried out on the reservoir by the environment agency will come within the next fortnight.
Drayton Reservoir is known to UK anglers as one of England’s premier match fishing venues. It was stocked with 50,000 small carp with some having grown up to 20lb.
Anglers have previously been advised to ensure they have angling equipment that can handle the large fish and that they watch their rods and poles; hundreds of rods and poles have been pulled into the reservoir by fish.