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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