Shocking Developments - Rainbow Suppression on the South Fork of the Snake


An Evinrude-powered jet sled slowly prowls a bankside eddy on the South Fork of the Snake River. The olive-jacketed passengers sport caps emblazoned with the Idaho Department of Fish and Game shield. Two of them man long handled nets to capture belly-up fish floating just below the surface. They are the stunned victims of electric currents that only seconds before had shocked the water where they were holding. It is harmless, actually. If not netted, they quickly regain their senses and flee to deeper, faster, safer portions of the stream. Those unlucky enough to be netted are placed in buckets of river water waiting onboard.

The prized fish they are after – cutthroats, browns, and rainbows – are measured for length (and some for age) before being released. These stats are recorded in a logbook before the boat moves on to other pieces of likely water where its crew repeats the process again and again.

This work is wet and cold and continues for days on end. It is a scene played out not just in Idaho, but on rivers across the country each autumn as a way to measure the health of fisheries. This day, however, is different. It is not autumn. It’s spring. And the goal is not to gauge fish abundance. Rather, the crew is specifically targeting spawning beds utilized by non-native rainbow trout. Those they capture are not released back to the river. Instead, they are placed in containers and trucked to other waters in the state where they are unlikely to compete and hybridize with native cutthroat trout. It is a dramatic shift in an almost two decades old suppression program that yielded satisfactory results some years, but has proved failing more recently.

The push for native trout preservation has been at the forefront of Rocky Mountain West fisheries management for almost two decades. Major efforts are underway in Colorado and Montana to safeguard greenback and west slope cutthroat populations respectively. The states of Montana and Idaho are also making a mad push to protect dwindling numbers of bull trout. In Yellowstone National Park, streams are being poisoned and mandatory kill regulations are in place to remove non-native brook and lake trout from many of its waters. In some of these locales, non-native trout have been long established. In others, their presence in substantial numbers is a rather recent phenomenon. Idaho’s South Fork of the Snake River falls into the latter.

Enter the Rainbow

For half a century, the South Fork of the Snake has been heralded as one of the strongest native trout streams in the continental U.S. A tailwater flowing from Palisades Reservoir near the border of Wyoming, it has traditionally hosted over 5,000 Yellowstone cutthroat per mile on its upper reaches. Rainbow trout were stocked by the hundreds of thousands on the South Fork up until the mid-1980s. Regardless, they never gained even a toehold. Their numbers were in the single digits of total trout percentages almost every year. This is despite the river sharing a confluence with the Henry’s Fork, a famed rainbow fishery for over a century.

“My earliest memories of the South Fork go back to the mid-1960s,” states longtime area fly fisher and writer Bruce Staples. “It was such a cutthroat river for the first several decades that I fished it.” Rainbows, according to Staples, were something anglers caught every once in a while.

In the late 1990s, however, rainbow trout numbers began to surge. Many identify the 1996 and 1997 flood years as part of the blame. Runoff was extreme with record to near record releases from Palisades Dam. Some anglers and fishery managers contend that these high flows allowed rainbow trout from the Snake main stem and the Henry’s Fork to navigate the higher gradient lower South Fork and access more hospitable habitat found on the upper reaches. Others believe the flooding of privately stocked ponds in the Swan Valley area permitted rainbows to slip down to the nearby river. Both presumptions are speculative.

2003 was a turning point. Electroshock data showed cutthroat numbers had plummeted to almost half of what they were a decade before. Rainbows and rainbow-cutthroat hybrids (cutbows) had risen to greater abundance than natives for the first time in the river’s history.

Idaho Department of Fish and Game (IDFG) faced a dilemma. Increasing rainbow abundance meant more than just a decline in Yellowstone cutthroat numbers. A crash in the population could trigger the fish being listing under the Endangered Species Act, with management taken out of the hands of IDFG and placed under the control of federal agencies. This, in turn, could potentially impact fishing opportunities through a limited season, as well as curtailment of municipal and agricultural water practices throughout the drainage. Furthermore, the department had to consider public sentiment. In angler surveys conducted every five years, the majority of respondents stated a desire for the South Fork to be managed as a native cutthroat fishery.

Hope and Failure

IDFG’s 2004 iteration of the South Fork Snake River Management Plan called for a three pronged attack, including the construction of weirs on spawning tributaries to hinder the upstream movement of rainbow and cutbows and limit hybridization, timed spring releases from Palisades Dam to scour redds when rainbow spawning on the river itself is most intense, and a controversial change in regulations allowing anglers year round, no limit harvesting of rainbows and cutbow hybrids.

Over a 15 year run, the plan produced signs of both success and failure. Some years electroshock surveys showed cutthroat abundance grew greater than that of rainbows. Other years, cutthroat numbers declined slightly while rainbow numbers grew.

Weaknesses in each element of the three pronged strategy limited the plan’s overall success. Tributary weirs have proved effective at barring rainbow passage to upstream spawning beds. However, only a small percentage of rainbow trout attempt the arduous push up these streams each year. The timed freshet releases from Palisades Dam was considered the most important piece of the management plan. South Fork rainbow trout generally begin spawning a few weeks before cutthroat. Releases at the right time would not significantly impair cutthroat spawning. Henry’s Fork Foundation senior scientist Rob Van Kirk deemed 25,000cfs to be the mark at which freshets would have an impact. Palisades Reservoir, however, was established for irrigation storage and flood control, not for fish or fishing. As a result, effective releases are never guaranteed. During the first 15 years of the plan’s implementation, flows rarely exceeded 22,000cfs, let alone at the right time.

Open, no-limit harvest was and remains the most controversial element of the management strategy. Responses during public comment periods suggest robust support. Nonetheless, many of those backing the plan are personally hesitant to whack a reputable sport fish like rainbow trout. This is despite implementation of the Angler Incentive Program (AIP) in 2010. Under this program, invisible tags are placed in the snouts of a small number of rainbows during electroshocking. Each could equate to anywhere from $50 to $1,000. The heads of harvested rainbows can be placed in deep freezes at boat ramps along the river. Those heads are scanned by IDFG. If one contains a winning tag, the monetary reward goes to the submitting angler.

Veteran South Fork guide Ed Emory is amongst the most vocal in his opposition to harvesting. “Anglers have absolutely voted their opinion by not choosing to participate in Idaho’s program,” he states. “Less than .5% of the over 300,000 plus anglers have chosen to participate. That’s a deafening statement of a failed incentive program. And a strong vote to leave the fishery as it is.” Many guides and outfitters hold similar views.

Nevertheless, data suggests that a fair amount of harvesting is occurring. AIP has resulted in almost 29,000 heads being submitted since the program began, of which 425 contained winning tags. 4,466 were turned in 2019 alone, the highest number since the program began. This does not include creel surveys which are considered separate from AIP numbers. Results from the most recent creel surveys suggest only 13 percent of harvested rainbow trout are being submitted to IDFG.

Despite its shortcomings, the management strategy was declared at least somewhat successful in its early phase. Yellowstone cutthroats were holding their own. Rainbow and cutbow abundance exceeded 2,000/mile only once. IDFG credited angler harvest for staving off non-native dominance. Although socially unacceptable in some circles, in-the-field killing of rainbow trout appeared to be the one approach that had a semblance of effectiveness. This, the department felt, was a sign of consistent hope.

That hope was shattered in 2018. Electroshock data that year showed rainbow abundance exceeding 3,000/mile and cutthroat number dropping to just over 1,800/mile. Alarm bells began to ring once again.

A New Plan

Facing the continued deterioration of a native fishery, IDFG revisited an idea they originally proposed in 2003 – removing rainbows and cutbow hybrids during their annual electroshock survey. “We considered this when the department first designed the management strategy, and there was some backing amongst conservation organizations and members of the community,” says Upper Snake River Fisheries Manager Brett High, “But our office faced a lot of pushback. We were told in meetings throughout the region that if rainbows were to be removed, they (the fishing public) wanted to be the ones to do it.” 15 years later, circumstances were different. Now removal via shocking seemed like a much needed part of the plan.

Shocking seems easy. Running electric currents through the water column can and does bring up heavy numbers of fish in the affected area. Most who observe such operations are quickly impressed by how effective the method is. But there are limitations.

Late autumn electroshock surveys occur when rivers levels are low and current speeds are generally minimal. These conditions produce the best results. The downside is that these surveys are done to gauge trout abundance. Reliability of the data comes from shocking the same reach of river each and every year. In the case of the South Fork, data from the upper river, where the rainbow population is greatest, is collected from a 2.5 mile section. This leaves dozens of miles untouched by IDFG.

Spring, on the other hand, allows personnel to target highly concentrated spawning beds. Hundreds, if not thousands, of rainbows collect on very specific parts of the riverbed to take part in violently frenzied acts of procreation. This time of year, however, is when runoff and increased releases from Palisades Reservoir create higher flows and deeper water. These conditions limit the effect of electrical currents through the water column. Timing is also an issue. Rainbows are typically in the act for no more than a month.

In the end, IDFG determined an “all-hands-on-deck” approach was needed. With limited additional funding - $7,000 on top of their annual fall electroshock survey funds – the department would attempt to target both seasons.

In mid-April of 2019, a skeleton crew of Fish and Game officials manning a single Wooldridge jet sled cruised along the upper South Fork, targeting specific beds for a trail run of 19 days over a 30 day period. Their goal was 3,000 rainbow and cutbow hybrids. A total of 5,857 were removed. All were placed in holding ponds throughout southeast Idaho. These ponds are used by local communities for youth fishing programs and a place where adults new to fishing can gain experience before fishing more natural settings.

5,800 rainbows removed in a single month might seem like a lot of fish. Yet it is a drop in the bucket when considering the 2018 survey results of over 3,000 rainbows per mile on the upper reach of the river. Nonetheless, when considering the time and personnel limitations of this initial run, the results were encouraging. IDFG plans to double their efforts in 2020 pending increased funding.

Removal will continue in autumn, but spawning season will be the focus. “Sure, it’s a lot tougher to work in spring,” says fisheries biologist Patrick Kennedy, “but we found our method of shocking to produce in a big way. Targeting concentrated fish has its advantages. We also found that we captured a lot of rainbows before many of them actually spawned. This will impact their abundance over the coming years.”

The department is not abandoning their three original approaches. Harvesting is still being encouraged and tributary weirs will remain standing and manned for the conceivable future. And while it is unlikely that the timing and amount of water released from Palisades Dam will ever occur, IDFG is working with the Bureau of Reclamation (which manages most reservoirs throughout the Snake River System) to better guarantee flows deemed beneficial to Yellowstone cutthroat the rest of the year. Winter is a key focus, where higher flows can be conducive to the survival and development of juvenile cutthroat.

What the Future Holds

Data from the autumn 2019 survey indicate a positive impact. Results showed an increase in cutthroat abundance and a substantial decline in rainbow and cutbow numbers of over 800/mile. But this is one year, and a review of previous surveys display a wide level of inconsistency from year to year. Still, IDFG, conservation organizations like Trout Unlimited, Backcountry Hunters and Anglers, and the South Fork Initiative (a division of the Henry’s Fork Foundation), as well as a large portion of the fishing public are hopeful this new strategy might turn the tide in favor of native dominance.

The overall goal of the management plan for the South Fork of the Snake River is to 1) protect the genetic integrity of native Yellowstone cutthroat, and 2) reduce rainbow and cutbow numbers to pre-1990s levels, roughly 10% or less of total trout abundance. Success depends on a number of factors, not least of which is continued public support. IDFG feels this support remains strong. The only question is what strategy the public is willing to back and, perhaps, take part in.

The department is also looking into additional approaches. A mandatory kill regulation – similar to what exists for lake trout on Yellowstone Lake – is one option. This measure, however, is considered extreme by many and is unlikely garner public backing to the same degree as the other approaches currently implemented. Mechanisms that target juvenile rainbows and eggs are another option. “New technologies are always coming about in fisheries management,” Kennedy says. “We just don’t know what kind of tools we will have just a few years from now.”

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