Outdoors

Added nutrients boost fish in Dworshak Reservoir

About 15 yeas ago, Dennis Harper became an advocate of fertilizing Dworshak Reservoir waters to grow more fish. Then the Orofino chiropractor put his money where his mouth was.

“I took a week off work and spent a couple thousand bucks to bring in experts for a conference to see if what was working in Canada could be done at Dworshak.

“I looked into this for totally selfish reasons,” he admitted. “I love to fish.”

The 12-year effort required cooperation of jurisdictions, including the Corps of Engineers, tribes, state and federal agencies and local governments.

“But everyone signed on after seeing what was going on in Canada as close as Arrow and Kootenay lakes,” Harper said.

“Basically, (fertilization) saved their fisheries,” said Sean Wilson, Idaho Fish and Game fisheries researcher.

“Kokanee stocks in Arrow and Kootenay lakes had all but collapsed from the effect of upstream dams trapping all the nutrients. Fertilization increased the fecundity of the fish and they got better reproduction. They’ve been able to maintain the size of the fish with two or three times the densities.”

This is the fourth year of a similar project in Dworshak. Research indicates the fertilization is beginning to result in bigger fish, but it’s too early to say much more than that, Wilson said.

The effect of the increased nutrients starts with the tiniest aquatic organisms and trickles up the food chain to the fish.

It’s taken four years for the fish to get the benefits, Wilson said.

The Corps of Engineers has been spending more that $100,000 a year to apply nitrates from a barge that motors down the middle of the reservoir once a week from spring to fall.

Idaho Fish and Game has committed that much and more annually to the nearly completed five-year study as it tries to balance the amount of phosphorus and nitrogen found in the reservoir’s water.

The goal is to cut down on toxic blue-green algae while boosting phytoplankton, a tiny algae eaten by zooplankton, the main food source for kokanee.

Size of the kokanee can vary from year to year depending on fish densities. A big hatch of fish results in smaller sizes as they compete for food.

“Fertilization should enable us to have bigger fish at higher densities,” Wilson said. “The fish might not be longer in this year’s abundant year-class, but it looks as though we’re already seeing heavier fish, better body condition.”

This is a big numbers year for Dworshak kokanee, which had a million 1-year-old kokanee last year.

“Figuring in the losses, we could have a half a million or more 2-year-olds out there now,” he said.

The fish were running about 9 inches long last week. They should be 10-11 inches long in July.


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Rich Landers

Rich Landers

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