Towards the end of the season I caught a brace of trout, each with a white furry patch about an inch square on their nose. What is it?
It sounds like saprolegnia, a common fungal disease of fish. Trout are particularly susceptible in both autumn and spring when the water temperature is on the move. Post-spawning fish are also susceptible.
Cuts and lesions can occur to female fish when kicking up a redd and these can become infected with saprolegnia, while male fish can pick up scars from fighting with other males over female fish. It can cover the whole body of the fish and in such cases the fish may die, but many fish with a small patch on their head or body during the autumn or spring will shrug off the fungal infection in time.
Saprolegnia can be a particular problem in trout-rearing units and for years it was treated with malachite green. Many rivers would run bright blue during the autumn until tests in the US found that malachite made the ears fall off mice and it was subsequently banned.
For many years, I treated infected fish in our rearing unit with a salt bath, which seemed to work, though we were operating at very low stocking densities. There are other treatments available, but if you only have a few fish with small spots of saprolegnia on their nose it doesn’t sound like too much of a problem. CDC