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Dead rainbow trout floated on the South Platte. Could fish detectives crack the case?

When an angler discovered the kill, he quickly put in motion an investigation to determine if the culprit posed a threat to the river’s health -- or was just a quirk of nature

John Davenport fly fishes in the South Platte River
John Davenport with Denver Trout Unlimited fly fishes along the South Platte River near Elitch Gardens on July 17, 2018 in Denver. (Kathryn Scott, Special to The Colorado Sun)
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Two weeks ago, Rick Mikesell started his morning the way he often does before heading in to his job as operations manager at Trouts Fly Fishing in Denver — he wet a line in the urban stretch of the South Platte River near the Interstate 25 overpass.

An isolated downpour the day before had bumped up the river’s flow, and perhaps cooled it off a bit amid a stretch of blistering heat, and just as he suspected, his beloved carp were in fine form. Then he noticed a couple of dead rainbow trout floating nearby.

He didn’t think much of it — rainbows are cold-water fish, and though some have been stocked by private interests and have adapted to this relatively warm stretch of the South Platte, it’s not uncommon to find one that has succumbed to high temperatures. But then he saw another. And another. And another. 

When the count reached a half-dozen, he grew concerned. 

“The thing that didn’t make sense was that the carp were healthy and happy,” Mikesell said. “So were the smallmouth bass, and I saw one adult (rainbow) at the Mile High bridge. If there was a chemical spill, all species would be hurting.”

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But why all the dead rainbows? The circumstances were troubling enough that he pulled out his cell phone, took photos of the deceased — all mature fish ranging from 18 to 22 inches — and sent them off to John Davenport, the conservation chair for Denver Trout Unlimited. That set in motion a prompt investigation to try to determine the cause of death.

Ultimately, data from water temperature sensors would be collected, flows would be analyzed, oxygen levels would be tracked and theories would be formulated. Deductive reasoning eliminated some possible culprits. And while the aquatic detective work didn’t exactly culminate in a Hercule Poirot moment, it compiled enough evidence to finger a most likely suspect.

“Naively, I thought when I got a sample for Colorado Parks and Wildlife, I assumed in my mind that they’d take the fish and put it into a mass spectrometer,” Davenport said, “then read out all the components in there, and get right to the reason the fish died. But it doesn’t work that way. 

“There’s no CSI for fish.”

But this came close, at least in citizen-science terms. And the exercise confirmed how effectively anglers can serve as an early warning system for river health. Ultimately, Davenport described this incident as a “minor fish kill” — ultimately about 15 were discovered along this segment of the river, among thousands that can populate any given mile. 

But in 2011, fly fisherman Trevor Tanner sounded the alarm on what turned out to be a dangerous chemical spill seeping from the Suncor oil refinery. But his first clue that something was amiss came when he noticed dozens of carp clustered in the water beneath a footbridge near the confluence of Sand Creek and the South Platte in Commerce City.

As he waded through the river seeking a spot to cast, he noticed the oily sheen on the water and smelled the foul odor. The problem appeared to be pouring into the river from Sand Creek and he found himself standing in the middle of it.

Although workers in the area had noticed the smell for days, it wasn’t until Tanner made his discovery that authorities were alerted. The spill was found to contain benzene, a known carcinogen, and though it didn’t cause a fish kill, it created an environmental hazard that took months to address. 

We all use the river every day. If something really crummy like that happens, a chemical spill or some major toxic event, if we don’t take care of it quick, we’re going to lose a huge resource.

Rick Mikesell, angler

The difficulty Tanner encountered finding the right reporting channels led to Denver Trout Unlimited and the U.S. Environmental Protection Agency creating a process. “Reporting cards” were printed and distributed to fishing enthusiasts with an 800 number and instructions to call if they spotted anything that might indicate a waterway could be in distress. 

“Something like that is not only a concern to the fish population but to human health,” said Paul Winkle, aquatic biologist for CPW, of the Suncor incident. “Anglers should keep their eyes open. We can’t be everywhere all the time to see what’s going on.”

So when Mikesell saw the floaters in the river, he wasted no time triggering the process.

“We all use the river every day,” Mikesell said. “If something really crummy like that happens, a chemical spill or some major toxic event, if we don’t take care of it quick, we’re going to lose a huge resource.”

Starting with Mikesell’s email that morning, Davenport logged everything. He notified the National Response Center for potentially toxic spills at 11:01 a.m. that day, reporting Mikesell’s find of the dead fish. That triggered more than a dozen notifications, ranging from the Colorado attorney general’s office to city and state health departments.

Colorado Parks and Wildlife called him back, as did the EPA and Colorado Water Conservation Board. 

This “reporting card” was created after the 2011 incident in which seepage from the Suncor oil refinery found its way into Sand Creek and the South Platte River. The problem was first reported by a man fishing for carp near the intersection of those waterways. (Provided by John Davenport)

“Had there been an obvious cause like an oil sheen or a chemical odor the response teams would have been dispatched, but in this case that could not happen because the cause was and actually still is unknown,” Davenport said. “When I reported it, we could not tell whether it was a minor incident or a major incident that was unfolding and would warrant immediate action. Luckily, it turned out to be minor, but it was a good response and the agencies were ready.” 

Davenport emailed the Denver Department of Health and Environment and alerted Ashley Rust, a research associate in civil and environmental engineering at the Colorado School of Mines who also advises Revesco, developer of The River Mile, on trout habitat. Then he headed down to the river.

As he scouted the South Platte for more dead rainbows, Davenport heard a shout from across the water. It was Rust, who also had come on the scene to investigate. They joined forces and  came upon more rainbows, including one that looked strikingly familiar. 

In a catch-and-release fishing culture, some fish make multiple appearances at the end of an angler’s line. A rainbow Davenport had named Alice Meow Wolf — Alice “just because” and Meow Wolf to denote that she lived in the waters near the under-construction art company installation — fell victim to the kill. He has referred to her as his “favorite trout.”

He noted the discovery at 1:06 p.m.

Since the arrival of in-phone cameras and social media, images of the day’s catch often find their way to the public before they’re even released back into waterways. Regular anglers soon come to recognize certain fish by some combination of size, location, markings (in the case of rainbows, Davenport notes, the spot patterns are unique as fingerprints) and even wear and tear from repeatedly being caught.

Mikesell has gotten to know a giant carp dubbed Big Bertha who inhabits the waters in Globeville. A couple years ago, he landed her and held the 32-pound behemoth, but he hadn’t hooked the fish in the mouth, so by angling ethics the catch didn’t count. Still, she was the largest fish he ever touched. He knows her not only by size, but by one eye with a characteristic bulge.

Davenport landed a carp that ranks as his largest freshwater catch and named him John. They met again on two other occasions. A friend who also fishes the Denver stretch of the South Platte once caught the same fish four times within a month. He could tell it was the same one because a lip injury he noticed the first time gradually healed from catch to catch.

He also knows four anglers who caught the same carp, which they named Dorothy.

So seeing Alice Meow Wolf belly-up in the water reminded Davenport of the respect he and others felt for the trout.

“Fishing is a fair game,” he said. “We try to fool them, they try not to be fooled. We debarb hooks so the fish will heal quickly, and we try to make sure they live to fight another day.”

Just off the east bank of the South Platte River in Denver in July of 2018, John Davenport of Denver Trout Unlimited retrieves one of his previously placed sensors that has gathered temperature and oxygen readings. (Kathryn Scott, Special to The Colorado Sun)

So it was not so unusual for Davenport to recognize an old adversary. Catch-and-release is one of the reasons Colorado retains high fish counts per mile of waterway.

“We look at it as ‘catch and restock,’” Davenport said of the ethic that can be controversial among anglers. “Some other states are dependent on hatcheries to stock fish, so that’s a lot of recreation that disappears. In Colorado, most fly fishing anglers catch many more fish when they release them than if they were to keep them.”

Statewide, anglers have flocked to Colorado’s fisheries, possibly fueled by the coronavirus shutdown that pushed many Coloradans to seek recreation outdoors. With most of three months left in 2020, the number of fishing licenses distributed already has exceeded all of last year’s total — by more than 165,000, according to Colorado Parks and Wildlife.

Davenport and Rust continued their investigation. He noted that the heavy showers of the day before could have washed anything from air pollution — smoke from wildfires wafted over the city — to metals like zinc and copper shed by vehicles onto pavement into the South Platte.  

She gathered water samples. They checked temperature gauges positioned at various known fish habitats. Rust also checked for levels of dissolved oxygen, which various species require in different amounts.

Davenport initially wondered if the evening rain the day before had heated from the hot asphalt parking lots at Mile High Stadium before channeling into the river and spiking the temperature. The numbers didn’t show that to be the case. 

The temperature logs showed the water ranging between 65 and 83 degrees, not far out of the ordinary for a hot stretch of August. Rainbows struggle with temperatures over 60, so they often seek deep, shady pools or other cool nooks in the river. 

The dissolved oxygen level, which trout like at a minimum of 6 mg/L, had spiked to 9 mg/L with the runoff from the rainstorm. But then it sank below optimal levels. 

Evidence pointed to a cause of death “probably related to the natural environment, river flows, and high temperatures,” Davenport noted in his report, which also acknowledged Rust’s oxygen measurements.

“During summer is when they’re under the most stress,” he said of the rainbows. “In this case, there was no sheen on the water, no chemical smells, no indication that there was a toxin, because nothing else had died. We saw some that actually survived.”

John Davenport

Rust came to a similar conclusion.

“I believe it was just a combination of stressful circumstances for them, a warm environment and not much oxygen,” Rust said. “If (pollutants) were to run off the pavement, it might have been just enough to knock them off the edge. Small native fish seemed OK. That makes me think either the trout that survived found a place to hide from a pollution event, or the heat and oxygen levels alone were enough to kill a few fish.” 

In his report, Davenport noted some possible remedial measures to prevent a recurrence, including long-term improvements to low-flow channels, more shaded spots and deep habitat “holding areas” that provide the rainbows refuge from high temperatures, and more fish passageways that allow them to seek more hospitable waters. 

Aeration devices and “beneficial weed beds” could also help to increase dissolved oxygen levels, particularly when summer heat tends to lower them.

So while some mystery still surrounds the incident, the most dangerous perps were eliminated and the vision of maintaining the urban South Platte as an improbable home for rainbow trout remains intact. 

“We know what kind of fish we catch there,” Davenport said. “We’ve been catching trout all the way to Coors Field, sometimes as far north as 88th Avenue. Who knows why, but they figured out how to live there.”


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