February 23rd, 2004

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The Big Horn River Just Goes With The Flow
By Bob Krumm

As many anglers know, the Big Horn River has experienced lower than average flows for the past three years. For instance, over the past two years the average flow has been 1,500 cubic feet per second (cfs) while the normal average low flow would be 2,500 cfs. Along with the low flows have come some substantial changes in the trout fishery and the aquatic ecosystem.

Perhaps the most notable change has been the dramatic decrease in the number of trout per mile. Back in the good old days of the mid-90s the total number of brown and rainbow trout per mile in the upper river was estimated to be 7,200. Nowadays the figure is pinned at 1,600.

What has happened? Well, one obvious difference is the decrease in the amount of surface acres per mile of river. Many of the side channels and shallow riffles are high and dry. Consequently, there are fewer places for trout to live.

Another aspect of the loss of the side channels has been that the young of the year trout have to inhabit the main channel where the big browns can feed on them. For the first couple of years of the diminished flows, the browns decimated the young fish and all but eliminated the year classes.

What isn't reflected in the population per mile estimates is the average size of the trout. Unlike terrestrial animal populations, population ecologists don't express the carry capacity of a stream in numbers per surface acre but in pounds per surface acre. While the number of fish per surface acre is down markedly, the average size of the trout has increased dramatically. Back in the good old days, the average brown was about 14 inches while average rainbow was about 16 inches. Nowadays the browns and rainbows are between 18 and 20 inches.

The brown trout have had the opportunity to become what I call, "Real brown trout." What I mean is that in free-stone streams, brown trout normally switch from an aquatic invertebrate to a predominantly fish diet when the browns reach about 16 inches in length. Subsequently, the free-stone browns would become lunker-sized and hard to catch. The Big Horn has few baitfish species, particularly in the upper stretches. Previously the browns would have to continue to feed on the invertebrates. They would also have to feed continuously since they couldn't obtain enough food to be full. The habit of continuously feeding made the browns more susceptible to being caught by anglers.

When there were large numbers of smaller 12 to 16-inch brown trout in the river, many of the larger browns would become gaunt and starve to death because they couldn't eat enough of the small invertebrates to maintain condition.

During the past couple of seasons the browns have become harder to catch and also have sported girths that we had up until then only seen on rainbows. In short, the small trout diet has enabled the browns to be more like their free-stone stream cousins.

During the heydays of the Big Horn, the brown trout outnumbered the rainbows about six to one; presently the ratio is close to even with maybe the rainbows holding a slight edge. According to Montana Fish, Wildlife & Parks fisheries biologist, Ken Frazer, "It is typical for rainbows to be the predominant species in a tailwater fishery; the change that is occurring is not exceptional."

A slight digression is necessary at this point. Last year there were erroneous reports that there were only 800 trout per mile. The persons who posted this information had not read the population estimate correctly. The statistical model used for estimating the population is called the Lincoln Method or Mark and Recapture. The method entails capturing the trout by electro-fishing and marking them. In the case of the Big Horn trout, a hole is punched in the tail of each trout.

A few days later, the same stretch of river is electro-fished again. The biologists note how many of the fish are marked in the total of all the fish captured. A simple equation then can be used to make the population estimate: the number of marked fish in the second sample is to the total number of fish in the sample as to the total number of marked fish is to the total population. For instance, if there were 100 marked fish in the second sample of 500 and there were 500 total marked fish then the total population would be 2,500 fish.

What happened was that the biologists marked very adequate numbers of browns and rainbows on the first pass. On the second pass, the biologists recovered a statistically valid number of browns but not rainbows. They were able to estimate that there were 800 browns per mile in the upper river but could not make a statistically valid estimate for the rainbows. Ken Frazer told me that if he had to make an estimate, it would be at least as many rainbows as browns. Hence, 1600 trout per mile is a more realistic estimate not the 800 that so many have posted on the Internet.

Big Horn Rainbow

As I stated earlier, there is a lot less trout habitat in the Big Horn River due to the decreased flows. The decreased flows also mean that the water temperatures will average colder than when there is a high flow year. In 2003, I never recorded a water temperature in the upper river that exceeded 58 degrees. It wasn't until August that the temperature warmed into the mid-50s.

Colder temperatures mean slower growth rates, not only for fish, but for aquatic invertebrates as well. Hatches that normally occur in early July don't begin until late July or early August. Consequently, anglers who are accustomed to fishing specific July hatches as yellow Sallies or pale morning duns are out of luck if they plan on fishing early in the month.

Another aspect of the low flows is that the habitat for specific aquatic invertebrates has been diminished. Species that require heavy riffles and large gravel or cobblestone have declined in numbers. Over the past two years, the yellow Sally and pale morning dun hatches have been minimal to say the least.

Both black caddis and tricos have had sparse hatches through the low flow years.

Mahogany duns, who were rare prior to the low flow years, have burgeoned in numbers. The mahogany duns are silt dwellers. Without heavy flows, silt deposits have increased, particularly in the lower stretches of the river where irrigation return waters are laden with silt. Also, the plains streams flowing into the river carry heavy loads of silt.

During the high flow years of the mid to late 90s, scuds were relatively rare and sowbugs were numerous. As the flows decreased the sowbugs followed suit while scud numbers climbed.

Anglers had become accustomed to landing lots of trout each time they fished the Big Horn. With fewer, but bigger trout, anglers have had to settle for considerably fewer trout landed per day's fishing. However, anglers hook a fairly high number of trout but the landing percentage is abysmal—the large trout are too big and powerful to allow a high landing percentage. An expert fly fisher can expect to land only about one half the trout he or she hooks; novices probably will land less than one quarter.

The tried and true method of consistently catching trout on the Big Horn is nymph fishing. While rainbows continue to feed heavily on invertebrates throughout their lives, browns switch to a fish diet. Anglers wishing to catch more browns will have to change tactics in order land the slab-sided browns. Of course, I am told "Streamers don't work on the Big Horn except during the brown trout spawning time." Maybe it is time to rethink this credo.

In the previous decade any pattern imitating sowbugs would get the job done, but now scuds seem to have become the go to fly. While the tan ostrich herl nymph has done a great job as a sowbug imitation, it is also doing a passable job of imitating scuds—the tan herl with the fire orange thread under wrapping gives a hint of orange just as the natural does.

There are higher numbers of midges in the river nowadays thanks to the silt deposits so midge patterns are effective from early on, say March 1, to the end of June. Some days the midge clusters can be awesome to behold and to fish. The ideal midge day is a cloudy, calm one—a phenomenon that occurs four to six times a month. When such a day occurs, it is not unusual to see in the late afternoon midge clusters the size of dimes floating down the river.

Another aspect of the Big Horn River that I would like to address is the crowds or lack thereof. In 2003, the Big Horn had large numbers of anglers, particularly on the weekends in April and May. Many of the anglers came from points south, in other words, Colorado. As the season wore on there were weeks where the crowds were sparse and a person could fish the river without seeing but a handful of boats. June and July were low numbers months. August and September had a few more anglers but at no time were the conditions crowded. In October and November the river was close to deserted.

While the snow pack conditions aren't encouraging, it is safe to say that the flow won't go lower than 1,500 cfs. If we have a wet late winter and early spring, it might be such that the flow could be bumped up a bit but I wouldn't hold my breath!

The Big Horn has settled down and should provide good fishing for fairly large trout for several seasons to come. This year there will be an influx of 12 to 14 inch browns and rainbows to compliment the large trout in the river. However, the big fish will probably most all die off this fall. Next year they will be in the 16 to 20 inch range and a good number of trout in the 10 to 14 inch range. Basically, the population will adjust to the flows and will come into equilibrium with it.

When we finally have high flows, the first year will be tough fishing because the lower number of trout will be spread out over a large area. (Then, too, the fish will have so much more feed that they will grow at a geometric rate!)

Well, I hope I have helped to dispel some of the rumors about the Big Horn River and I hope, too, that you will try fishing it again. All you will have to do is to relax and go with the flow. ~ Bob

About Bob:

Bob Krumm is a first-class guide who specializes on fishing the Big Horn River in Montana, (and if there terrific fishing somewhere else he'll know about that too.) Bob has written several other fine articles for the Eye Of The Guides series. He is also a commericial fly tier who owns the Blue Quill Fly Company which will even do your custom tying! Bob is a Sponsor here! You can reach him at: 1-307-673-1505 or by email at: rkrumm@fiberpipe.net

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