Up to ten fish a day are being found dead in the Tyne Valley

The Environment Agency fear they may have stage a rescue if problems for fish in the Tees - caused by a low concentration of dissolved oxygen and high freshwater temperatures - worsen.

And there are also fears that fish in the Tyne Valley are dying because of a decrease in oxygen due to the low river levels.

Up to ten fish a day are being found dead with some showing signs of bacterial infection - a secondary condition which can happen to stressed salmon and trout.

David Carrick, of Trout and Salmon Magazine, said the fish were showing signs of blisters and areas of decomposing flesh.

We are monitoring the situation very closely to make sure there is no further deterioration Darren Starkey, Environment Agency

Roger Inverarity, of the Environment Agency, said: "It is down to opportunist bacteria that, when they find a fish that is already stressed, will take advantage of that.

"There is too much mud in the estuary. The best course of action is to let nature take its course and undo the dredging that has happened over the years, which is why we get the soft mud lying around in such large quantities.

"Over time, the oxygen demand in it will decay and conditions will get much better."

In the Tees, Darren Starkey, from the Environment Agency, said they had a few options to consider.

He said: "We have a monitor in the river which takes dissolved oxygen readings every ten minutes.

"We are monitoring the situation very closely to make sure there is no further deterioration.

"One option would be to try and net the fish and take them upstream to help them on their way, but at the moment the temperature upstream is actually higher than downstream.

"If the situation deteriorates any further what we can do is to provide extra oxygen in the water by means of pumping oxygen into the water which will raise levels."