Experimental flow not expected to impact Lees’ Ferry fishery

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February 28th, 2008

Fishing news

Experimental flow not expected to impact Lees’ Ferry fishery
PHOENIX – An experimental release of up to 40,000 cfs for 60 hours from Glen Canyon Dam proposed for starting March 4 is not expected to negatively impact the world-renowned Lees Ferry trout fishery just downstream in picturesque Marble Canyon.

The U.S. Department of the Interior is proposing the experiment to study and improve Colorado River resources in Grand Canyon National Park.

The goal of the experiment is to better understand whether higher flows can be used to rebuild eroded beaches downstream of Glen Canyon Dam by moving sand accumulated in the riverbed onto sandbars. For more information, visit www.gcmrc.gov/research/high_flow/2008/ .

Larry Riley, an Arizona Game and Fish Department biologist, said the proposed peak flow of 41,000 cubic feet per second would mean an increase of about 4 feet at the Lees Ferry boat ramp.

”Wild trout are well adapted to stream conditions that include spring flushing flows. The proposed test flows could temporarily displace adult rainbow trout by changing water currents. However, the rainbow trout are unlikely to leave the immediate area and fish will return to desirable habitats rather quickly,” Riley said.

Riley explained that adult trout generally show a high degree of habitat fidelity, and long-term marking experiments at Lees Ferry have borne that out. Trout monitoring associated with the 1996 (spring) and 2004 (fall) high-flow experiments also noted no changes in density of adult trout in the Glen Canyon Reach.

The proposed 41,000 cfs flows would be about four times the average flow in recent years; however it’s perhaps a third of the historic pre-dam spring peak flows of around 120,000 cfs and less than half of the peak flow during spill operations in 1983. To put all that in perspective, in late January this year the Salt River was flowing at around 80,000 cfs into Roosevelt Lake.

Riley said the spawn is currently underway at the Ferry. “If the experiment is conducted, some trout redds (nests) could likely be scoured by the flow, as the spawn in recent years has peaked in late March. Also, some fry may be lost. Trout demonstrate compensatory survival to off-set these kinds of events. Increased survival and growth rates of fry not lost or displaced are attributed to reduced competition for food. Likely late spawns can make up for losses of fry as well.”

Riley said that some anglers have expressed concerns that the trout at Lees Ferry could experience a shortage in their food supply, especially freshwater shrimp, because of the experimental flows. “We don’t anticipate that happening. In fact, the flows will more likely have a positive affect on the algae habitats that support the shrimp populations.”

The timing of this high-flow event is at the beginning of the growing season when sunlight is returning to the canyon bottom, and the aquatic algae is poised to exploit the space, increasing sunlight, and available nutrients to grow and expand rapidly. “The experimental flows could trim the important alga beds of dead and aging fronds, just like trimming an aging tree, and could in fact create a bumper crop of high-quality food for trout.” Riley said.

Another concern by anglers is that the trout won’t be catchable after this high flow. “Angler catch rates really did not change significantly as a result of the 1996 experiment. Fish behavior may shift following this event, with trout adopting different feeding habits and seeking out food in locations different than the seasoned Lees Ferry angler may be used to, but it shouldn’t take long for anglers to adapt – it never does,” Riley said.

For example, he said, fish may use deeper water habitats following the high flow to capitalize upon food at depth making it more difficult for the wading angler to fish by sight. ”That’s a challenge to the skill of the angler, but not an adverse effect to the trout,” Riley said.

Guides and staff at local businesses learn quickly about alternative fishing tactics to catch these wild trout. “Consultation with these seasoned anglers and skilled listeners can provide tactics that will broaden an angler’s repertoire and improve their ability to catch fish,” Riley said.

Riley added that Game and Fish Department biologists plan to conduct fish surveys at Lees Ferry both before and after the proposed experimental high flow event. “Every time an experiment is conducted, we learn things that help us better manage these important resources. That is an important part of the adaptive management process.”

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