A Fine Trout Steam

Things are quiet on the Big Thompson these days, or at least as quiet as the winter weather can make one of the most popular trout streams on the Front Range. With U.S. 34 snaking along just yards away and lots of public access, from Estes Park to Loveland, the Big T is about as accessible as a trout stream can get. Rarely is the river completely empty of anglers. Flows out of Olympus Dam keep at least a little water open year round, and if it's humanly possible to fish, there will be a truck in the parking lot of Wapiti Park, and at least one angler will be working the water that glides behind the go-cart tracks, batting cages and miniature golf courses that crowd along the north bank. It's as heavily fished as a trout stream can get. Names get attached to stretches of trout streams. Downstream, there is Cottonwood, Caddis Flats, Sleepy Hollow. The water right below the dam is a run of river some call The Petting Zoo.

Rainbow trout may or may not be the most caught of the trouts, but they are almost certainly the most killed of the trouts. Easy to rear in hatcheries, 'bows are the backbone of put-and-take trout fisheries in and on the fringes of trout country.

But rearing trout for the masses to catch and eat is an expensive proposition. It costs the Colorado Division of Wildlife (DOW) $1.10 to produce and deliver a 10-inch rainbow, the size of stocked fish. In 2008 alone, the DOW dumped 27,000 “catchables” into Lake Estes.

Meanwhile, wild rainbows, as has been well documented, have been decimated across the country in the past 20 years as whirling disease has taken hold. Wild populations have crashed, and brown trout — predatory, wilier and, in the opinion of many anglers, harder to catch — have moved into the void. Most of the trout streams on the Front Range — which were, between the time the natives (greenback cutthroats) were effectively wiped out and the time whirling disease moved in, primarily rainbow fisheries — are now loaded with brown trout.

Ben Swigle is a fish biologist keeping tabs on the Big Thompson for the DOW, putting him in position to quantify aspects of the river fishermen experience mainly subjectively.

“It's right near its top,” Swigle said when asked how close the river is to its potential as a trout fishery. “I don't think we can push it any further.”

From the dam to Waltonia, the river is managed as a catch-and-release fishery, and it has become well-known not just for the quality of the fishing, but the health of the wild rainbow trout population.

“The St. Vrain, the Poudre, even the South Platte down lower — those are all brown trout streams,” Swigle said. He reports that while the population of brown trout is healthy, especially as you move downriver and out of the catch-and-release water, the proportion of rainbows to browns is unprecedented in the region. “The Big Thompson is very unique in the fact that whirling disease hasn't taken its toll.”

For that, you can credit the government, even if a wild rainbow fishery is an accident of management of the river rather than a goal: the Bureau of Reclamation pushes a lot of water down the river, for the Colorado-Big Thompson (C-BT) water project and for other water users, and these big flows have the effect of flushing sediments out of the riverbed. As a result, the host of the parasite that causes whirling disease (WD) for part of its lifecycle, the tubifex worm, has less available habitat, and does not thrive in the upper river. The parasite that causes WD originated in the Old World; so did brown trout, which have some resistance to the deforming disease. But WD hammers rainbow trout in their first year. Blow out the tubifex worm and, other factors being favorable, rainbow trout thrive.

And they do appear to be thriving in the Big T. Swigle samples trout populations at five sites on the Big Thompson around the same time and same flow rate every year, when the river is running around 50 cubic feet per second (cfs) within a two-week timeframe annually, and the DOW has compiled 20 years of data on the river. (High flows in the fall of 2007 precluded sampling.)

Swigle said the quarter mile of water below the dam — the Petting Zoo — is not the best rearing habitat for rainbows but, with some in-stream improvements that have been done, it is very good holding habitat for bigger fish. His numbers bear that out. On the sample site below the dam, Swigle and volunteers counted 185 rainbow trout averaging 11.0 inches long in 2006. This year, the count rose to 188 and the average length to 11.7 inches.

“That is quite astounding, given the sample is only 593 feet of river and it receives enormous pressure,” he said.

You can guess where this is going: The effects of catch-and-release management are also clearly a major factor in the quality of the fishery. Swigle's numbers show that below Waltonia, where the general creel limit of four trout in any length take over, the average trout is smaller, and the total biomass drops.

“The differences are quite obvious,” he said.

Below Waltonia, the balance between rainbows and browns swings toward the latter, though the proportions have been stable for years, Swigle said.

“My guess is that the rainbows are often kept below Waltonia, allowing the browns to dominate,” he said.

Fishermen will find some other things Swigle can tell you about the river to be interesting as well. Sampling, he said, never turns up any brook trout or greenback cutts, but many of the rainbows are rainbow-cutthroat hybrids, known as cuttbows. The river holds native populations of long nose dace and suckers, which provide a forage base for brown trout.

He outlined growth rates on the river. For both rainbow trout and browns, in August to October, a six-inch fish with be two years old; a 10-inch fish will be three years old; a 14-inch fish will be four years old. Fish over 20 inches are likely six years old.

Those are pretty rare in the Big T, he noted. But they are out there.

The fall of 2006 and the summer of 2008 were big water years on the Big T below Olympus. Releases out of Estes Lake typically climb in the fall, and last fall maintenance in the C-BT system put a larger volume of water than usual down the Big T. This summer, the river ran well above the historical averages through September. In June, when the typical flow is 276 cfs, the average was 458. In July, it was 350, against an average of 149. In August, it was 191, compared to 138. The typical September flow is 75. This year it was 248.

Swigle calls those 12 months the “big flush.” What will the effects of the Big Flush be on trout in the river?

Swigle cautions that between the nature of the fish and the nature of the habitat, trout under six inches are hard to count. But his numbers do show that young of the year rainbows were depressed this fall as you move down the river, potentially indicating a reduction caused by a combination of the big water and an increase in predation pressure from brown trout.

“Big flows, late in the season, (are) not helpful for reproduction,” he said. “Obviously, the redds get pretty beat up. The adults get pretty beat up, as well, as they try to nest while battling higher flows. When browns and rainbows emerge from the redds (in May and June, respectively) they struggle to withstand enormous flows.”

“That is the cloudy bad news for this year's class of rainbow trout, but the fact remains we did find a new crop,” he said. “To better understand the strength of the class we will need to look at next year's crop of six-inchers, which are more susceptible to our sampling methods.”

But Swigle draws a picture of trout populations healthy enough to withstand the effects of the Big Flush. Sampling points to a solid year-class of fish that hatched in the spring of 2006, with no discernible difference in the number of two-year-olds counted in 2006, before the Big Flush, and in 2008, after it.

“This indicates that the two-year old rainbows survived the big flows this year and are poised to spawn next year,” he observed.

And, post-flush, the bigger fish were, on average about an inch larger in 2008 than they had been in 2006, a difference Swigle thinks is attributable to reduced angling pressure during the high flows, and the associated reduction in hooking mortality trout enjoyed during it.

“The numbers of adults in the river has not changed since 2006, and a solid population is poised to produce a 2009 year-class,” he said