The Environment Agency (EA) and MAFF have been investigating for the last 10 years or so and have come to the conclusion that this is a pathegon naturally caused/brought on by the stress of the fish trying to cope with the lack of disolved oxygen in the estuary.
The Tyne has a very long tidal estuary, is heavily industrialised and has nasties in the silts from the shipbuilding days. The new sewage interceptor network has rejuvenated the quality of the estuary but in hot dry summers we always have a problem, especially with Salmo salar
We have a large reservoir at the head of the system and it took us 5 years to persuade the EA to release water, to encourage fish out of the estuary, into the system and hopefully safety. The EA had always maintained it was not natural conditions and that the fish would not move on this water, we urged them to try it, what had they got to lose, we had fish to lose!! The result was, of coarse, that the fish did run on the release and most survived because of it.
The EA still appear reluctant to sanction a release until the fish are dying on mass, its true they have other obligations to other water users other than we anglers, but dead fish is guaranteed to get our blood up!!
Hence my original question, have fish been dying elsewhere during this hot dry summer?