Over the last few weeks a few diseased fish have been reported, all carrying the distinctive white fungal blotches. Once badly infected fish often move into the margins or slack water where they can hold station more easily and where they are much more visible. It is important to state that the white patches seen on all the dying fish reported are not UDN, rather they are caused by fungus, a secondary infection. The symptoms of UDN are quite different – lesions on the head, initially as grey patches on the skin on the top and sides of the head often developing into lesions or open wounds. Fish with UDN lesions can become infected with fungus but often the lesions heal up once the fish moves upriver. I have been trying to obtain samples from a fresh run fish with UDN head lesions all year with no success, the incidence appears to be much lower than observed last year and more akin to the normal background level.
Despite this over the last few weeks there has been an increase in reports of fish either dying or already dead, all of which have carried extensive white patches of fungus.
Fungal spores are everywhere, hence food left out in the kitchen soon develops mould from airborne fungal spores. It is the same in the river although the fungus spores are carried in the water, and any damaged fish is susceptible to fungal infection. This time of year always seems to be the worst for fungus but it usually clears up. It did last year following an increase in river levels and cooler water. We may well see an increase in affected fish in the next week or two but a change in conditions usually helps.
Similar fish have been reported from many rivers this year including the Dee, Tweed and Annan.