Fertile plan would fatten Dworshak silvers

DENT, Idaho — Bill Knapp of Pierce runs his knife up the belly of a small kokanee.

He quickly guts it, rinses it and tosses it in a pile of about two dozen of the silvery fish. None are bigger than 8 inches long and most are closer to 6.

The landlocked salmon are plentiful in the 55-mile-long Dworshak Reservoir this year. So plentiful that few of them are growing to sizes greater than 10 inches long.

“I’ve been fishing here a long time and this is the smallest I’ve ever seen them,” Knapp said. “I guess if a guy had a sardine can he’d be all right.”

The problem is simple: There is too many fish and too little food in the reservoir. The solution may be simple too. Officials from the Idaho Department of Fish and Game and the U.S. Army Corps of Engineers want to add nutrients to the lake to boost growth at the bottom of the Dworshak food chain.

The extra growth, they say, will travel up the food chain, grow bigger kokanee for anglers and lead to a healthier ecosystem in the reservoir as well as its headwaters, which offer some of Idaho’s best trout fishing.

“The concept of this project is to try to increase the productivity of the reservoir so there is more food for more fish,” said Paul Pence, natural resource manager for the corps at Dworshak. “The idea is to increase the amount of nutrients in the reservoir so there is more and bigger kokanee to take the nutrients upstream and eventually wash downstream. We think this is going to be a win-win and not just for the fishermen and the reservoir but for all the fish and animals way up in the headwaters.”

All reservoirs become less productive as they age, said Ed Schriever, regional fisheries manager for the Idaho Department of Fish and Game at Lewiston. Before Dworshak Dam blocked off the North Fork of the Clearwater River more than 30 years ago salmon and steelhead used to spawn in the headwaters. When they died their bodies, rich with nutrients from the ocean, fertilized the system.

Kokanee were introduced in the reservoir, in part, to help transfer nutrients upstream. When reservoirs are new they tend to be very productive because of all the nutrients in recently submerged soil and vegetation. But that only lasts so long.

Schriever said water quality tests at Dworshak reveal that phosphorous and nitrogen are out of balance. There is too little nitrogen and too much phosphorous. The lack of balance encourages the growth of a blue-green algae in late summer. Zooplankton that kokanee like to eat doesn’t like the algae and their numbers crash.

The idea is to add nitrogen to the water to bring the water back in balance and encourage the growth of a type of algae that is eaten by zooplankton.

Schriever said most people won’t notice a difference and the added nitrogen will not lead to large mats of algae that plague some small lakes and reservoirs that are nutrient rich. The goal is not to grow trophy-sized kokanee. Instead, Schriever said the idea is to strike a balance between numbers of fish and size.

The corps and fish and game hired a Canadian expert on nutrient enrichment. The practice is common in British Columbia. Pence said the corps is planning public meetings in Orofino and Lewiston in August. Before it happens the agency will have to consult with the U.S. Fish and Wildlife Service and the National Marine Fisheries Service to make sure it won’t harm bull trout in the reservoir or salmon and steelhead down stream.

Care also will be taken to make sure the nutrients won’t interfere with Dworshak National Fish Hatchery or Clearwater Hatchery, both of which get water supply from the North Fork.