Managers of the Gnat Creek Hatchery have found a way to raise more salmon with less water: Just add oxygen.

The Gnat Creek Hatchery, located 18 miles east of Astoria, was originally built to raise steelhead trout but now raises spring chinook. Unlike steelhead, spring chinook reach their maximum poundage during the summer, when low-water levels in Gnat Creek limit the number of chinook the rearing ponds can sustain.

Gnat Creek Hatchery typically produces around 850,000 spring chinook for the net-pen fish rearing program overseen by Clatsop County Fisheries Project Director Tod Jones.

But summer water constraints have left a full bank of the hatchery’s raceways – which could rear about 200,000 chinook – unused. Those ponds will be full of fish by next summer under a new system that stretches the available summer water supply by adding pressurized oxygen to the rearing ponds.

By raising oxygen levels in the water as it moves from one pond to another, the system allows more fish to survive in the ponds without drawing more water from the creek.

“When this water passes through a pond of fish, the oxygen is depleted,” said Warren. “We replenish the oxygen when the water goes into the second and third ponds … effectively reusing the water two more times after we initially pass it through the first pond.”

Over this past summer, Jones and Warren built a makeshift oxygen supplementation system with donated materials from Longview Medical Supply. As a result, the hatchery was able to raise an additional 80,000 chinook this year, Warren said.

With help from a $108,000 grant from the Oregon Department of Fish and Wildlife Restoration and Enhancement Program, the hatchery recently purchased a permanent oxygen supplementation system that should increase yearly production of spring chinook from the traditional 850,000 to 1.1 million.

The system involves six oxygen generators and 15 dispensers that “supersaturate” water with oxygen as it drops down into the raceways from Gnat Creek, Warren said.

As water passes through each pond, a new dose of oxygen is added to maintain the required levels for sustaining fish, raising oxygen levels by up to 175 percent during low-water periods July through October.

Oxygen supplementation will also boost the percentage of adult returns from released smolts, according to ODFW. By retaining fish at the hatchery for a longer period of time hatchery personnel are able to release higher-quality smolts into Youngs Bay and Blind Slough net-pens in November, when conditions are better suited for their survival.

In five of the past seven years, low summer water levels forced managers to move chinook to net-pens as early as Sept. 5, when water in the Columbia River is still about 70 degrees and contains a lot of pathogens, said Warren. Under those conditions, smolts are more susceptible to disease and need to be treated with antiobiotics.

The low head oxygen system is more efficient and easier to troubleshoot compared to other systems, according to Warren. Each pond at Gnat Creek runs on its own unit, so if one of the systems breaks down, the rest of the ponds still receive oxygen. Other systems use one central component that supplies oxygen for all the ponds simultaneously. If the central system fails, all of the ponds go without oxygen.