Henry's Fork Science


Almost anyone with an interest in fly fishing for trout is at least familiar with Idaho's Henry's Fork. Those who know a little about the river know about the quality fishing it historically has provided. Those who know it a little better, however, are aware that the system has not been free from problems and that the fishing in recent years has not been consistent.

Prior to 1978, the Henry's Fork fishery was maintained by stocking of fingerling and catchable-size trout. The river has been under special regulations ever since, and from 1988 to the present it has been managed as a catch-and-release, wild fishery. Surprisingly, however, the population has continued to exhibit a general decline in numbers since that time.


One notable exception to this pattern occurred in 1993. As an unintended consequence of the accidental drawdown of Island Park Reservoir that had occurred the previous year, there was a short-lived increase in both the abundance of trout and in the angler catch rate. The mishap dumped large volumes of sediment and large numbers of stocked fish (from the reservoir above the Island Park Dam) into the system. The latter made many people happy, at least temporarily, and prompted renewed calls by some to reestablish a program of stocking on the river.


Recognizing the value of the Henry's Fork as a wild-trout fishery, the Idaho Department of Fish and Game (IDF&G) instead decided to make a detailed study of the river. To the biologists the answers almost certainly would be found with juvenile fish and, on a broader scale, behavioral patterns that influence their survival. The reasoning behind their focus was simple: Without adequate recruitment (i.e., the annual addition of young), no population can maintain itself for very long. This obviously held true for the Henry's Fork. But the biologists only had good information on adult fish, which showed a pattern of declining numbers, indicating a problem.

Beginning in the spring 1995, Dr. Matt Mitro, then a graduate student in a doctoral program at Montana State University, began extensive field research that specifically focused on fish in their first and second year. "I set out to quantify production and recruitment, to identify factors limiting the population, and to propose management options to enhance recruitment," explains Mitro. Then, over the next four years, working in all seasons, often at night, he did just that. What he ultimately found suggests that there is reason to be encouraged about the river's fate.

His study was concentrated on the section from Island Park Reservoir/Dam downstream to Riverside Campground. While a lot of time and effort was spent examining aspects of spawning success, it became clear early on that fish were in fact being produced in good numbers — they just weren't surviving for very long. Understanding why required a detailed look at where and when juvenile fish moved.

Using a modified drift boat equipped for electrofishing, Mitro and his field technicians captured, tagged, and released more than 10,000 juvenile rainbow trout during the course of the study. The fish were marked with elastomer, a dyed latex gel that is packed in a syringe and injected between the rays in a fish's fin or into the clear tissue just behind its eyes. Using a variety of fluorescent colors and placing the tag at different locations (i.e., on different fins, behind different eyes), each individual fish received a unique tag combination that specified both the year-class to which it belonged and the location of its initial capture.

This large number of tagged fish was needed to obtain an accurate measure of movement throughout the entire system. An even larger number of fish, however, was required to obtain the estimates of production and of overall juvenile abundance in each section of the river. In the end, approximately 30,000 age-0 and age-1 fish were marked by clipping small sections from the margins of various fins (the size of the clips were kept small to allow for regeneration).

Information from subsequent recaptures from within each section was then used to piece together a picture of mortality rates, seasonal habitat utilization, and the timing of large-scale movements. These patterns, in turn, highlighted the real problem.

"I found that spawning was not a limiting factor," says Mitro. "Yearly production ranged from about 150,000 to 250,000 age-0 trout at the end of the summer growing season. However, few of those fish survived through their first winter."

Such a finding is not surprising. The winter period is considered to be critical by many biologists. Rapidly declining temperatures slow a fish's metabolic rate, which slows energy uptake. As a result, habitat features such as deep pools, substrate interstitial cover (the spaces between boulders, cobble, and gravel), and woody debris jams that provide refuge from higher current velocities are essential for the fish to minimizing the depletion of its energy reserves. If such features are lacking, the fish is in trouble.

This is especially true for the youngest and therefore smallest fish. These fish have higher metabolic rates but less stored fat (lipids) per unit weight than larger fish. Viewed in this light, quality winter habitat takes on added value for trout in their first year; when it is locally unavailable or proves inadequate, the fish seek out areas where their needs can be met.

The movement of Mitro's tagged fish showed that this applied in the Henry's Fork as well. Throughout the summer, autumn, and even into early winter, juvenile fish were found throughout the entire study area, a great many of them using mid-channel vegetation beds as cover. At some later point, however, the young fish disappeared from most parts of the river, either from emigration from habitat that was no longer suitable, or from outright mortality.

The major exception to this pattern was in the area immediately below the dam, and this proved to be the key. "The interstitial spaces necessary for overwinter habitat are largely confined to Box Canyon, especially along the banks," says Mitro. "Consequently, most overwinter survival has been in this area, while trout observed in other sections in autumn have not been found in those same sections in the following spring."

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More interesting still, the survival rates in the canyon were strongly related to the flow volume coming out of the dam after midwinter. As discharge increases, the water level in Box Canyon rises and floods more rocks and woody debris, creating additional bank habitat. The higher the flow at that time, the more fish survive through the season.

"A high level of discharge during the latter half of winter will improve the recruitment of age-0 trout," says Mitro. "The water stored in Island Park Reservoir is currently managed for multiple uses, including power production and irrigation. In past years, excess water has been released from the reservoir in early winter in anticipation of a large snowpack. This study showed that if the water is released later in winter, more habitat along the banks in Box Canyon can be created at a critical time for age-0 trout survival. Managing discharge has the greatest potential for benefitting the fishery."

For the time being, those charged with managing the reservoir and the river seem to agree. In the winter of 1998-1999, higher discharges were maintained after the second half of winter, and plans are to continue the practice in the future.

The sampling effort started in the study is also something that will be carried into the future. The IDF&G continues to use tag recaptures to develop more information on the fishery.

As for Mitro's personal view on the fate of the river, he remains optimistic. "The Henry's Fork is an excellent wild-trout fishery. It's great right now, and with the right flow management it can possibly be even better in the future."

To be sure, those who know even a little about the river, hope he is right.

Matthew Handy lives in Burke, Virginia.

Research Spurs Improvements

In 1984, the Henry's Fork Foundation was organized in an effort to protect this magnificent resource and fund scientific research. It's difficult to summarize the results of the research in a few paragraphs, or pinpoint one cause of the fishery's decline. It's easier to discuss the important factors that influence the Henry's Fork fishery.

Island Park Dam is both a blessing and a curse to the Henry's Fork fishery. Almost all of the water that enters Island Park Reservoir comes from springs. In the reservoir, the water is enriched by sunlight before it spills through the dam, and this helps produce the prolific insect hatches and rapid growth rates in the river's trout.

Probably the most significant influence the dam has had on the fishery below has been the repeated flushing of trout from the reservoir during drawdowns in 1977, 1979, 1981, and 1984. These flushes kept the river's trout numbers up and increased average fish size. After 1984, the trout population in Box Canyon declined steadily until September 1992, when the reservoir was drawn down to its lowest level in history. This drawdown flushed fish out of the reservoir, and the following year the trout population in the Box Canyon shot back up.

In 1992, fish passage from the reservoir to the river was almost completely cut off when Fall River Electric Cooperative retrofitted the dam with a hydroelectric facility. The intake penstock is now screened to prevent fish passage, although a few fish bypass the turbines during high flows in May and June.

The Henry's Fork Foundation has worked closely with Fall River Electric to ensure that the new facility does not have a negative impact on the fishery. The current licensing agreement requires Fall River Electric to meet federal water-quality standards for dissolved oxygen, total gas pressure, water temperature, and turbidity. This was not always the case before the project went on line. The facility also provides a second outlet from the dam, and this alleviates the need for periodic shutdowns for maintenance. I believe the facility helps the Henry's Fork, due to its state-of-the-art and continuous computer monitoring system.

The Henry's Fork Foundation has also worked closely with the Fremont-Madison Irrigation District to negotiate higher stream flows during the winter months. The results have been optimum flows from Island Park Dam, which have significantly increased the young-of-the-year survival and growth rates in adult fish. With higher stream flows and better management of wintering trumpeter swans, aquatic vegetation fares better in winter.

Island Park Dam blocks the trout's access to traditional spawning beds in the river's headwaters. The spawning and wintering habitat below Island Park Dam is limited, especially during low-water years. On the Buffalo River, a spring-fed Henry's Fork tributary in the upper Box Canyon area, access to spawning areas has been blocked by a small hydro facility for nearly 60 years. The recent installation of a fish ladder at the Buffalo River facility now provides passage from the Henry's Fork into the upper Buffalo River, opening a substantial amount of spawning and young-of-the-year rearing habitat.

The efforts of the Henry's Fork Foundation (Box 550, Aston, ID 83420, [208] 652-3567, www.henrysfork.com) and others have really started to pay off, and Mother Nature has also chipped in with consecutive good water years, helping the entire river system to rebound.

Mike Lawson is former owner of the Henry's Fork Angler. He lives in St. Anthony, Idaho.

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