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Colorado Parks and Wildlife is investigating an outbreak of sores on rainbow trout in Eagle County waterways, with bacteria and stress the primary suspects.
The rainbow trout have been reported by anglers and guiding companies in recent days, and CPW has engaged with those parties to collect samples of the affected fish, which were sent to the state’s Aquatic Animal Health Lab in Brush.
While the results are not yet in, CPW aquatic biologist Kendall Bakich said she has seen similar lesions on fish in the Eagle River in the past, as well as other nearby water bodies. A case in Steamboat Lake showed a similar pathology in rainbow trout, Bakich said, occurring directly after the spawning season during warmer water temperatures.
“But it was really this stress-related disease that is caused by bacteria in the water,” Bakich said. “That’s what we suspect we are going to come back with, we’re not exactly sure what bacteria, that’s what the fish lab will figure out.”
Tough times after spawning
Rainbow trout spawn in the spring, unlike brown trout which spawn in the fall. The annual reproduction ritual can be very taxing on the fish, and in the months that follow, trout are at their most lethargic as they recover from the process. In its weakened condition, a trout’s immune system can be compromised after spawning, making bacteria-born diseases better able to take hold.
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The urbanized settings that surround the waterways of Eagle County offer plenty of opportunity for bacteria to glide off impervious surfaces into a creek or river.
“We’ve had many recent events like the right rain storm hitting a denuded hillside, or a particular parking lot,” Bakich said.
Those events deposit bacteria directly into the waterway, clouding the once-clear water and causing stressful conditions for fish. With rainbow trout at their weakest currently, they’re least able to fight off disease, while brown trout don’t appear to be having an issue, Bakich said.
“The browns look great,” Bakich said. They spawned last fall, they got to eat afterward, they’ve had all winter to recover and in spring the food productivity goes up to where they just get to stuff themselves. They don’t have much they’re worried about, they’re starting to develop for their fall spawn, but their lives are not as stressful as the rainbows at this time of year.”
Only the strong
But not all rainbows are being affected, Bakich said, so the “survival of the fittest” law of nature may be at play in local waterways, as well. Rainbows with open sores have been reported from Gore Creek to the Eagle River and even the Colorado River in Eagle County, but as those reports are coming in, so are plenty of reports of rainbow trout being caught which look perfectly healthy.
“Most of the affected fish appear to be bigger fish, which would be older fish,” Bakich said. “This is also at the end of spawning season, a time when you would see old fish having their last spawn, and they’ve been lucky enough to make it to an old age — they haven’t been predicated upon or harvested or anything like that — so often this time of year you’ll see fish that just don’t look very healthy, they’re at the end of their lives.”
Meanwhile, some of the fish that have been seen with lesions “still seem somewhat healthy,” Bakich said. “The rest of their bodies are healthy, they are behaving normally, so you would anticipate those individuals will be able to fight the infection as waters calm down, the environment stabilizes their food production starts to increase.”
As the water slows down in local rivers and creeks, fish may not always be as comfortable with the warmer water, but often fish food — those little bugs known as “macroinvertebrates” — become easier to find.
“Snow melt is really cold, so everybody is not producing as much, including the macroinvertebrates,” Bakich said. “And as the water warms up a little bit, those macroinvertebrates will start to reproduce and emerge, and that provides a lot of food for fish.”
While Bakich anticipates this could create conditions where some of the affected fish recover, “we’ll also have some individuals that will just be old and just won’t make it,” Bakich said.
Closures becoming more welcome
When the water temperatures get too warm, into the high 60s and low 70s (Fahrenheit), fishing closures will go into effect on local waterways, something that will also reduce pressure on rainbows trying to recover from spawning season.
The closures often frustrate anglers, but this season, for some, the closures will be welcome.
Local angler John Moore, who has been fishing the Eagle County waterways for 25 years, says he’s concerned about the future of the local fish population overall, and incidents like the current lesion outbreak aren’t making him more optimistic about the future.
“My concern is ultimately the sustainability of the river, and the pressure that it undergoes each year,” he said. “It seems like it ramps up every year.”
Moore said he’s often shocked by the number of boats he sees at local put-in and take-out areas along the Eagle River.
“The window is just so tight, so everybody wants to get out there, but I think we’re getting to the point where there could be better management on how many boats go down that river and when,” he said.
“In three weeks or so it will be too warm, and it will be shut off to fishermen,” he said. “Which is good, it’s becoming apparent with what we’re seeing with the rainbow trout here that these shutdowns are really needed.”
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July 07, 2024 at 12:05AM
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Colorado investigates strange sores on fish in Eagle County waterways - Vail Daily
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