BILLINGS — More brown trout than usual, less water than in some past years and other factors apparently are leading to a fatal outbreak of fungus on some fish in the Bighorn River below Yellowtail Dam.
Fishermen on the blue-ribbon trout stream in recent weeks have reported seeing hundreds of dead or infected brown trout.
Montana Fish, Wildlife and Parks biologists said this week that the fungus is part of a natural cycle. But this year reports seem to indicate that it is worse than usual.
The fungus appears as a white or cloudy growth on the skin and scales of infected fish. It generally takes a few days to a week to spread over the fish and kill it.
FWP regional fisheries biologist Mike Ruggles said the fungus spores are ubiquitous in most water in Montana. Healthy fish are able to block the fungus spores from attacking their skin with the slime that covers their exterior. Fish that are stressed or at less-than-peak condition often cannot ward off the spores, which burrow through their slime and into their skin.
This year, a number of conditions combined to stress brown trout in the Bighorn River, leaving them susceptible to the fungus, Ruggles said.
First, high water during the past few years created excellent reproduction conditions for the fall-spawning brown trout. FWP surveys last September showed more than 6,500 brown trout per mile in the 13 miles of the Bighorn River immediately below the Yellowtail Afterbay Dam, Ruggles said. That is the largest number of brown trout measured since 1999 and in the top half dozen years since FWP has kept records. That means that fish have to work harder to compete for a limited amount of food in the water.
Second, relatively low flows in the river this fall and winter concentrated the fish in spawning areas, FWP regional fisheries manager Ken Frazer said. Fish do not eat during spawning, which taxes their health anyway. Fish – particularly the big males – that spend more energy competing for spawing places and fighting with other fish are stressed and can lose part of their body weight. That decreases their ability to ward off fungus spores in the water, he said.
Third, the fungus spreads more quickly when fish are packed in close proximity to each other, Frazer said.
Finally, fish were subjected to nitrogen supersaturation in the water this past summer, Ruggles said. Gates in the Afterbay Dam designed to prevent the phenomenon were under construction. The resulting gas-bubble disease causes blisters and lesions on the fish, which stresses them and exposes them to fungus.
The fungus apparently is attacking larger, older brown trout. Ruggles said. Rainbow trout, which do not spawn until spring, are not affected by the current outbreak.
The fungus should run its course in the next few weeks, leaving slightly fewer – but much healthier – fish in the river, he said.
The fungus poses no danger to people.
Fishermen also can help by limiting the amount of time they handle fish before releasing them, Ruggles said.
While the fungus is present in most Montana waters, anglers and boaters still need to be careful not to inadvertently carry microscopic spores between streams and lakes. FWP recommends that all anglers inspect, clean and dry their boats and equipment every time they leave a lake or stream to prevent the spread of invasive species and diseases.