Whirling Disease Update: Is The South Platte A Dying River?

"One of the great trout fisheries of the West is gravely ill, and the prognosis is uncertain…." So concludes Denver Post outdoor editor, Bob Saile, in a recent feature story entitled "South Platte: A Crisis Runs Through It." Saile is the writer who first blew the whistle on Whirling Disease (WD) in Colorado two years ago, so his opinions carry weight.

"On the day we visited the river," Saile continued in his recent column, "the October sun coaxed the air temperature well into the 70’s, but there were entire runs and pools that had no anglers in them. Normally jammed parking areas were empty. Maybe that was the surest clue that something was wrong. On an outrageously gorgeous day at Deckers, where you sometimes have to take a number to find a rock to stand on, nobody was fishing."

Other disturbing signs he saw included:

The virtual absence of rising trout, despite an abundant trico mayfly hatch, on runs "…that would normally produce dozens of sipping, splashing rainbows and browns…"

Unprecedented algae growth on the river bottom, "…some of it trailing in the current like gobs of tinted toilet paper…"

A clear trend of fishermen reporting that "…the trout just aren’t here anymore, not in anything close to the numbers they were…"

Professional guides "…abandoning the Cheesman Canyon-Deckers stretch of the Platte for greener – no, let’s say, more productive pastures…"

The writer and his partner, both experienced fly fishers, hooking just four trout in six hours on the day described. "The river looked, and somehow felt, dead," he writes.

Most disturbing of all, Saile cites some new research by state WD expert Barry Nehring confirming that the rainbows have indeed "crashed" in this river section, leaving what is said to be "…the lowest rainbow trout biomass since special regulations were begun." At the monitoring site above Deckers, a local hamlet considered the center of the fishery, rainbow density has plunged 89 percent since 1989. Even more alarming, he says brown trout, which supposedly have a natural immunity to WD, have declined 65 percent in just four years at this location.

"Just what the hell is wrong at Deckers other than WD?" Saile asks in his article. "Is it [the South Platte] simply experiencing a dramatic down cycle in the sometimes roller-coaster life of a trout stream? Or is WD, normally associated with rainbows, a bigger threat to brown trout than anybody so far has thought…?"

For answers to these troubling questions – and to decide whether The Angling Report should issue a "Stay Away" warning about this stream – I recently sought out biologist Nehring, the state’s foremost stream researcher. Although based on information gleaned from this interview, the opinions about the South Platte situation which follow are my own.

A crash of naturally reproducing rainbow populations has indeed occurred in the Deckers area and most other portions of the river system, caused by WD…. a tragic consequence of poorly controlled commerce and trade in exotic fish species, and old-science dogma that it would cause no harm in the wild.

A "generation gap" of five rainbow year-classes exists on the South Platte in most locations, with young wild rainbows wiped out and their stocked successors still at juvenile stages. In coming years, this will mean an interval of fewer trophy-sized rainbows as older fish die out.

Although smaller rainbows are gone, there is still a good population of fish measuring between 14 and 20 inches around Deckers, particularly in the famous Cheesman Canyon section. Starting this year, a restocking program of rainbows beyond the age of WD-susceptibility has begun, under a more enlightened fish hatchery management philosophy at the Colorado Division of Wildlife.

At this writing, trophy-sized rainbows are still abundant (and will continue to be for some time). Although surviving trout biomass is concentrated in fewer, larger fish, this can’t be all bad, as evidenced by the fact that I personally took a 10-pounder in the canyon earlier this year.

The worst losses are not rainbows per se, which can always be replaced by stocking, but from devastation of the naturally reproducing rainbow gene pool, built up from stockers in the first half of this century, producing the river’s stunningly beautiful wild strains.

Brown trout have largely, though not entirely, replaced the South Platte’s missing rainbows. Fishermen have to understand that browns have different habits and habitat requirements than rainbows, and are harder to catch.

Brown trout can also be severely impacted by heavy infestations of the WD organism, as proven on the Colorado River, despite acquired immunities from co-evolution with the disease in Europe. The Deckers monitoring site may be one of those elusive "hotspots" of infectivity identified elsewhere, but the brown trout population system-wide is fat, healthy and asymptomatic.

Evidence from other locations, like the Frying Pan, indicates that brown trout populations may overproduce, and then decline themselves from natural causes related to habitat limitations. The Deckers study site (only 600 feet of stream) is an area where only a few large fish will likely survive over time.

Brown trout numbers have declined in the Deckers study area by 78 percent since 1991. But they’ve also declined 59 percent since 1990 in the higher-quality Cheesman Canyon study area upstream – which now has the highest density of browns over 14 inches since special regs began in the 1970’s. This paradox is explained once again by the concentration of post-WD trout biomass in fewer, larger fish, particularly browns.

Guides who’ve shifted to "more productive" sections of the South Platte have an understandable self-interest in their clients catching fish, which tends to mean where easier-to-catch rainbows are more abundant. They are going to have to work harder to teach their clients to catch browns.

The monitoring site in question, from Deckers upstream to Lone Rock campground, is the single most heavily fished section on the river. Catch-and-release regs notwithstanding, angler-caused hooking and handling mortality at least partly accounts for fish declines here.

Overall, electroshocking tests prove that trout production on the South Platte is still 100 to 200 pounds per surface acre, far above standards for Colorado Gold Medal Waters, meaning that this is still one of the top trout fisheries in the state or nation.

Opinions about the Deckers-Cheesman Canyon area have to be weighed against the standards of a decade or more ago, following establishment of no-kill regulations, when this river section became arguably the most productive trout stream in the world. Denver-area old-timers are understandably miffed, but perhaps also spoiled, by declines from what may have been, biologically, a temporary circumstance.

Green algae proliferation in the South Platte is likely due to the fact that upstream portions of the river flow through two of the three fastest-growing counties in the US (Douglas and Park). These headwater areas, now subject to an explosion of nutrient increases from human septic tanks, pass this fertility downstream to Deckers.

The new proliferation of algae in the river may be unnerving to fishermen, but it is not necessarily bad for fish, which probably benefit from the increase of crustacean food sources ("scuds" or amphipodae) now thriving in this mossy environment.

These expanding food sources may explain why fish don’t rise to an obvious hatch of trico mayflies, as reported. If the fish have a fatter and more abundant underwater organism to eat, why would they risk going to the surface for skinny little spinners?

Fishing the South Platte now requires adapting to the adaptations that trout have made themselves, keeping species (rainbows vs. browns), habitat requirements (riffles vs. edges), lighting conditions (mid-day vs. dawn and dusk) and food sources (insects vs. crustaceans) in mind. The hottest flies here now may not be the tiny emergers and midges so widely written about, but deep-drifted scuds and aquatic worms like those from other nutrient-rich tailwaters.

The devastations of WD may not necessarily be inevitable. New technologies are being developed to measure infectivity and identify the mysterious WD "hotspots" now known to be destructive to trout of most species. This could lead to new methods to fight back against this insidious organism – things like adjusting flow regimes to reduce the silty habitat of intermediate host tubifex worms (i.e. flushing); changing stocking timetables (to avoid infection peaks and thus reduce vulnerability); reducing the mucky habitat favored by the worms (by backhoes, if necessary); or possibly even introducing sub-species of tubifex like those from the Great Lakes area. This subspecies is not infected by, and doesn’t produce, WD spores, and could interbreed with or out-compete local strains, thereby reducing their impact.

In sum, the South Platte is not dead, but it is changing character. No "Stay Away" warning is warranted, except for the highly pressured area upstream from Deckers to the mouth of Cheesman Canyon. Experienced anglers should not fish this area anyway; it’s too easy, too vulnerable to depredation and abuse. Above and below, there are still many miles of the best trout fishing in America.

If you go, take the trouble to get a little off the beaten path to Deckers. Take the rugged trail below Cheesman Dam and you’ll have a shot at many healthy 14 to 20-inch rainbows – even a world-class trophy. Go to the Spinney Mountain section during the spring spawn, and you’ll almost be guaranteed a five to nine-pound rainbow or cutthroat. Go to the booming Eleven-Mile section, an upstream tailwater now under catch-and-release regs, and complain if you don’t take at least a 17-inch fish. Go in the fall, and try to catch one of the many five to eight-pound browns that move upstream from area lakes.

And don’t be shy about killing brown trout in Colorado, where regulations allow. The catch-and-release ideology, indiscriminate by species or location, can be a big obstacle to the preservation of species diversity. By releasing all browns religiously, anglers can thwart the efforts of biologists to prevent this dominant predator from taking over. It’s OK and politically correct to kill browns, where permitted, to keep them from overwhelming other fish, especially WD-weakened rainbow populations. (On native cutthroat tributaries, please take your limit of browns and brookies both.)

Whatever the problems of this river, it is still a great fishery. Fishing it just requires a higher level of skill and patience. The South Platte still delivers if you adjust to its changes. – Hugh Gardner.

(Don Causey Note: Contributing Editor Hugh Gardner has recently finished a documentary on Colorado’s native trout, Incredible Journey of the Greenback Cutthroats. For a VHS copy, send $16.95 to the Colorado Wildlife Heritage Foundation.)