Wyoming outdoors: Fungus outbreak killing old Bighorn trout

Wyoming outdoors: Fungus outbreak killing old Bighorn trout.

Saturday evening I received an email and photo from a friend. Kent Andersen, facilities director at Sheridan College, had fished the Bighorn River and was disturbed to see a number of dead brown trout littering the bottom of the river. The photo he sent me was of a brown trout covered with fungus and looking like parts of the fish had been encased in an opaque plastic wrap.

Andersen wanted to know if I had any idea of what caused the problem. He noted that he had seen dead fish in the river over the years, but nothing like what he saw Saturday.

I surmised that the dead and infected brown trout were casualties from spawning, but I forwarded Andersen’s email to Ken Frazer and Mike Ruggles. Frazer is the regional fisheries supervisor for Region 5 in Billings and Ruggles is a fisheries biologist for the Bighorn River.

Frazer responded that he had received a high number of reports of infected and dead fish. He went on to say: “We get some reports of fungus and dead fish each winter, but it seems to be worse than normal this year… There are high numbers of brown trout in the river after a couple of good water years and now they are concentrated by lower flows. The low flows limited available spawning areas this fall so the fish were even more concentrated during spawn which probably increased competition and fighting resulting in more wounds to start the fungus.”

Frazer had pointed out to me a long time ago that during the spawn, trout rub off most of their protective slime when they are building redds. Of course, the trout are jostling one another when they spawn so the rubbing and the fighting causes even more disruption to the skin.

Another thing that Frazer has preached for as long as I have known him is that the Bighorn River is only as productive as the flows — the higher the flows, the more fish. We had high flows in the Bighorn River from 2008 through 2011. The flow was dropped to 1,750 cubic feet per second in 2012 and that crowded a bunch of fish. (An aquatic ecosystem’s carrying capacity is expressed in pounds of fish per surface acre, unlike terrestrial ecosystems where the carrying capacity is expressed in numbers of animals per acre.)

Just like the flu infects more humans in the winter when people are crowded inside, when trout are jammed into less water and spawning, they are more apt to be exposed to diseases and catch them. The trout population in 2011 was probably the highest it has been in more than a decade because of good water years. In 2012 those higher numbers of fish were concentrated in a lot less water. Couple the fungal outbreak with the fact that brown trout only live three to four years in the Bighorn River and a lot of brown trout either die of old age or from the fungus.

There were a fairly high number of rainbow trout that died this summer after the spawn, too. Most of the fish had the fungal infection and appeared to be bigger, older fish. (Again, remember that trout have relatively short life spans, especially where they experience fast growth. So a lot of old die each year after the spawn).

On the bright side of this situation is the fact that with fewer trout in the Bighorn River the remaining trout will have more food available to them so they will have better growth rates, i.e., they’ll have larger girths. So anglers can look forward to fewer catchable brown trout in the Bighorn this year, but they will be huskier.

In time the trout population will balance out with the flows and the large die-offs will subside, but for now I wonder if the trout realize that spawning is hazardous to their health.


~ by John McGranaghan on January 31, 2013.

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