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Trout infected with fungus found dead in Big Sky

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Earlier this month, FWP Fisheries Biologist Mike Duncan discovered several dead rainbow trout specimens in Big Sky. UNSPLASH PHOTO

Despite fish kill, FWP fisheries biologist ‘not concerned’

By Gabrielle Gasser EBS STAFF

BIG SKY — Last week, Mike Duncan, a fisheries biologist for Region 3 of Montana Fish, Wildlife and Parks, found several dead fish infected with Saprolegnia parasitica in Big Sky.

The fish were located just upstream of the pond near the Big Sky Community Park. The fungus they suffered from is common, according to Duncan, and is found in most waters throughout the state of Montana. 

The fungus exists year round but tends to mainly affect fish during the fall when the fish are most susceptible. Factors causing fish to succumb to the fungus include high water temperatures, stress from secondary infections or chemicals, according to Duncan. 

“It’s not uncommon to see a handful of fish this time of year with it,” he said. “Usually fish end up becoming infected when they are stressed from something else, whether it is high water temperatures, spawning, or something else going on.”

The fish that Duncan found in Big Sky were rainbow trout, but the fungus can affect any species of fish. Duncan said he sees the fungus occur often in brown trout in the fall when they’re spawning.

It’s possible for fish to pass on the infection to each other and Duncan noted that, “usually it’s just a single fish, but it’s not unusual when you find one fish, you find a couple.”

Currently, it’s unclear exactly what caused the fish in Big Sky to become infected with the fungus.

“I can’t really speak to why the fish died or why they picked up the infection because they had died at least four days before I found them,” Duncan said. “At that point, it’s a little too late to figure out what is going on.”

Duncan noted that he did not see any fish that had recently died while collecting the specimens.

“I’m not concerned unless I see more fish,” he said. “It’s something that pops up on occasion and if it’s an isolated event to a couple of fish then it’s not something I’m going to be concerned about. If it persists or spreads, then that’s something we’ll take a closer look at.”

For now, Duncan says the fungus is nothing to be alarmed about, but says he will “keep an eye on it.”

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