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Biologists use flushing flows to aid North Platte trout population and improve fishing

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CASPER, Wyoming — Fishing on the North Platte River is sometimes a thing of legend. Anglers and guides tell stories about 50-fish days, the water teeming with fat and healthy brown and rainbow trout.

That wasn't always the case. Trout numbers in the Gray Reef section of the river fluctuated, peaking in 1987 with more than 14,000 fish per mile before plummeting to less than 1,000 fish per mile from 1991 to 1996.

Biologists realized low fish numbers weren't because of over fishing, predation, polluted water or drought, but because fish eggs weren't receiving enough oxygen to survive. They died before any hatched, said Al Conder, Casper area fisheries supervisor for the Wyoming Game and Fish Department.

Fisheries biologists found a solution in flushing flows — the deliberate release of more water from upstream dams. Several years after the Bureau of Reclamation started flushing flows in 1995, fish numbers have steadily increased, Conder said. This year's flows started Monday and will continue for 10 days in the spring, a change from historic five-day periods in the spring and fall. Biologists say the change could improve the fishery even more. Despite the muddy water, local guides say within days the fishing will be surprisingly good.

Most water in the North Platte River below Gray Reef Reservoir flows clear and sediment-free from the dam. But each creek and inlet into the river brings a steady flow of fine dirt that spreads like a blanket over the bottom of the river.

When trout spawn, both the male and female dig a trench in the river's gravel bottom. The female lays eggs, the male fertilizes them and both work to cover the eggs with gravel. The gravel bed is an incubator of sorts, holding the eggs in place while allowing water to constantly flow around them, bringing much-needed oxygen, Conder said.

"A lot of times right after the spawn you will see fish with tails that are really eroded from working the gravel with their tails," Conder said.

When silt and fine dirt covers a riverbed, water does not flow around the eggs as it would through gravel, causing them to suffocate and die.

Flushing flows mimic a dam-free river where runoff rushes down a stream, carrying away any fine dirt. The Bureau of Reclamation opens the Gray Reef dam for about six hours each morning, increasing river flows from 500 cubic feet per second to 4,000 and back down again.

PHOTO: John Ripley watches as water rushes from the Gray Reef Dam early Wednesday, March 19, 2014 near Alcova, Wyo. The flow of water on the North Platte River is increased this time of year by the Bureau of Reclamation. (AP Photo/Casper Star-Tribune, Dan Cepeda)
John Ripley watches as water rushes from the Gray Reef Dam early Wednesday, March 19, 2014 near Alcova, Wyo. The flow of water on the North Platte River is increased this time of year by the Bureau of Reclamation. (AP Photo/Casper Star-Tribune, Dan Cepeda)

After flushing flows began, trout began naturally reproducing so successfully that biologists stopped stocking fish in 2001 in the Gray Reef section, Conder said.

The increased flows began as five days in the spring when rainbow trout spawn and five days in the fall for brown trout. This year, the Bureau of Reclamation will increase flows for 10 days in the spring and not at all in the fall. The bureau made the changes to address issues with aquatic plants and algae in the fall rushing down the river and into the Dave Johnston Power Plant, Conder said.

Not all Wyoming river systems with dams can use flushing flows. It works on the Platte because extra water flowing out of Gray Reef is held in Glendo Reservoir, said Lyle Myler, deputy area manager of the Bureau of Reclamation.

Blake Jackson, fishing manager for the Ugly Bug Fly Shop, would like to see a fall flush to help the brown trout population, but appreciates the spring flush for rainbows.

He touted the long-term benefits of flushing flows and also the short-term boon. The first few days of increased flows are muddy and the fishing slow. Even though the flush will continue another week, fishing should improve today, he said.

High flows also flush minnows, crayfish and aquatic bugs from their hiding places. Trout generally spend the first few days waiting for the water to clear, and then the river becomes a feeding frenzy. Fish are hungry and noticing food they don't normally find.

"I can't imagine the fishery without it," Jackson said. "This management technique has shown to work really well for the whole health of the fish population post flushing flow."

And if it improves fishing in the meantime, he said, even better.


Information from: Casper (Wyo.) Star-Tribune, http://www.trib.com

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