The latest electroshock data for the South Fork of the Snake River shows a surprising increase in the number of rainbow and cuttbow hybrids on the upper portion of the stream (in the vicinity of Conant Boat Ramp). The 2009 estimates show 2270 Age 1 and older rainbows and cuttbow hybrids per mile at Conant, a significant increase from the 925 recorded in 2008. The 2009 estimates for Conant also show 5363 yearling rainbows and hybrids per mile for Conant. This is the first year that we have seen rainbows and hybrids significantly outnumber native cutthroats at Conant since 2003 and the highest number of rainbow/hybrid trout recorded since monitoring began in 1982. Disturbing news for those of us concerned about native cutthroat preservation, and a bit of a downer when considering the good news with the lower number of rainbow/hybrid trout we have been experiencing over the past few years.
So why the sudden increase in rainbow/hybrid trout, and why such a large increase? I talked with Dan Garren at the Idaho Department of Fish and Wildlife and he suggests that the artificially low winter flows we have been experiencing over the past few seasons have been to the detriment of cutthroat while benefiting exotics like rainbows. This is something that many area anglers have complained about for a couple decades. Detrimental flows are generally considered to be anything below 1000 cfs. In most years, we are lucky to see flows anywhere close to that. What is more, freshet releases (spikes in flows from reservoirs that are intended to mimic natural runoff) from Palisades have been occurring too late in the season to be of benefit to cutthroat (and to the detriment of rainbows). Ideally, these release should be occurring in mid to late May, but it is more common to see them occur a month or more later.
Bureau of Reclamation, the agency that regulates releases on the Snake River, have their hands tied because releases are dictated by the irrigation needs of the agricultural industry in Idaho. Until that changes we are going to see a lot more of this. We either need several big winters in a row, or some kind of change in policy that will give native trout better chances at survival. If the latter does occur, it will be at a glacial pace. There are no guarantees either way, but I would almost want to put my money on the weather.
There is some good news, however. Because of the two strong winters we had in 2007-2008 and 2008-2009, which resulted in greater storage in the system, flows from Palisades Reservoir this year were at 1800 cfs through January and then gradually ramped down by 200 cfs every couples of weeks until the 925 cfs that we stand at right now. This is good news for the South Fork's native cutthroat. Idaho Fish and Wildlife believes that this will result in much better numbers for cutthoat this fall when they conduct their next survey. But with the sub-par winter we had this year, it is doubtful we will see this trend continue over a two year period.
In addition, the Bureau of Reclamation is planning on this years freshet to occur in May, which will hopefully help the matter.
The IFG Summary for the 2009 Survey is below -
South Fork Snake River 2009 fall population estimates
The Idaho Department of Fish and Game continued our annual fish population monitoring at the Conant and Lorenzo monitoring reaches of the South Fork Snake River in the fall of 2009. This is the initial summary of our 2009 data and may be subject to change with further review. In 2009, we observed high abundances of trout in the South Fork Snake River at Conant with an estimated 4,096 trout per mile. This estimate approaches our all-time high abundance of 4,857 trout per mile that was estimated in 1999. Unfortunately, over half of this abundance (55%) is rainbow trout (including rainbow x cutthroat trout hybrids. Results from our current survey show an estimated 2,270 age I and older rainbow trout per mile at the Conant monitoring reach, which is a significant increase over last year’s estimate of 925 rainbow trout per mile. Many of these rainbow trout are yearling fish which were affected by reservoir releases in 2008. Spring flows have been linked to trout abundances in the South Fork through several studies, all of which have indicated that the timing and duration of spring “freshets” that mimic a natural hydrograph can significantly affect both rainbow and cutthroat trout abundances. The 2009 estimate of yearling rainbow trout is 5,363 fish in the Conant reach, which is by far the greatest abundance of rainbow trout yearlings that we have observed to date. This is the first year ever where rainbow trout significantly outnumber native Yellowstone cutthroat trout since monitoring began in 1982.
Since 2004, flows out of Palisades Dam have been modified to enhance Yellowstone cutthroat trout spawning while concomitantly limiting rainbow trout recruitment. Spring freshets have been implemented to some degree every year since 2004, but the timing and magnitude of the freshet are important for limiting rainbow trout recruitment. Modeling combined with field observations suggests when the spring freshet occurs in mid to late May, rainbow trout recruitment is low, but this impact is lessened quickly the later in the year the freshet is released. The freshet released in 2008, which had the greatest chance of shaping the population of yearling fish observed in the current study was later in the year than desirable, and lower in magnitude than what was necessary to curtail this year class of fish. Of additional concern is the 2009 freshet, which cannot be evaluated until next fall. Although the freshet in 2009 was of sufficient magnitude, the release occurred well into June and even July, which we suspect will have a minimal impact to the rainbow trout fishery. Therefore, it is likely that there are two strong year classes of rainbow trout entering the South Fork Snake River population.
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