These fish all showed signs of fungal infections.
Reports have increased over recent weeks with numbers now in excess of 50.
Some of the fish appear to be already suffering from the infection when they enter the river from the sea, while others have developed it while in the lower river.
Environment Agency fisheries experts concluded it appears to be a fungal infection, which is a recurring issue in this area.
Low river flows combined with the high water temperature and other factors such as a number of weirs to pass are causing the fish to be stressed, which makes them vulnerable to infection and allowed it to take hold.
Richard Jenkins, Environment Agency fisheries team leader, said: “This is something we have seen happen before, quite frequently, in this area and we have previously carried out significant laboratory analysis to understand more about the issue. We will be doing further testing on the affected fish next week to confirm it.
“We believe it to be an environmentally driven natural infection, which affects fish which are stressed. We understand this is distressing for people to see and are grateful for the reports we have received.”
To report a new incident, such as pollution in waterways, contact the Environment Agency’s 24-hour incident number on 0800 807060.