Bad patch

Shooting Times & Country Magazine - - FRONT PAGE -

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

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