An urgent investigation is taking place on a Tyneside lake after nearly 10,000 fish were found dead over a period of two days.
About 6,000 dead roach have been recovered from the lake
Environment Agency scientists are taking samples from Killingworth lake as the numbers of dead fish rise.
Fisheries officers went to the lake on Monday after a call from an angler who spotted some dead fish.
Initial tests on the water appear to show nothing out of the ordinary and investigations are continuing.
Killingworth lake is a popular urban fishery often used as a venue for tutoring budding anglers and has seen a lot of improvement work recently, including a habitat creation project using recycled Christmas trees.
The work was done in January and was designed to provide a good environment in which young fish can mature.
But in the past 48 hours, 6,000 dead roach and another 1,500 perch, bream and carp have been collected from the lake.
Officers say there are about 2,500 more dead fish still to be recovered.
Graham Siddle, from the Environment Agency, said: "At the moment all I can say is that we've never seen anything that has had such a devastating but localised effect like this, so we're asking for every test we can to get as much information as possible."
Initial tests show that the pH level of the water is average. The amount of oxygen in the water is low, but it is not thought to be low enough to have caused such a major reaction.
The results of more detailed tests are due back soon and officers hope these will give a better indication of what might have caused the deaths.
An incident hotline number - 0800 807060 - has been set up and Mr Siddle has appealed for information.