North Idaho anglers have to be scratching their heads over the unofficial world-record 9.67-pound kokanee caught in northeastern Oregon’s Wallowa Lake on Sunday.

This fish, the latest in a series of state-record kokanee the lake has given up this year, usurped the 9-pound, 6-ounce world record land-locked sockeye caught in 1988 in Okanagan Lake, British Columbia.

Here’s the rub: Oregon fisheries biologists say Wallowa’s behemoth kokes are gaining their girth by feeding on mysis shrimp – the half-inch long introduced crustaceans that led to the crash of kokanee in Priest and Pend Oreille lakes.

Mysis shrimp were planted in numerous kokanee lakes in the Inland Northwest and Canada in the 1960s as fish managers tried to duplicate the big-kokanee bonanza luring anglers to B.C.’s Kootenay Lake.

A decade later, biologists who’d been under considerable pressure by anglers and business communities to duplicate Kootenay’s fishery, began getting the hint that a mistake had been made.

Kokanee fisheries in many of the lakes began a long decline headlined by the near collapse of Lake Pend Oreille’s kokanee 30 years later.

The mysis shrimp flourished in their new waters and devoured zooplankton, the primary diet of kokanee.

To make things worse, the mysis shrimp elude the mouths of most large kokanee by hanging out deep during day and swimming to the surface to feed at night – the exact opposite of the kokanee’s feeding pattern.

Researchers have learned that Kootenay Lake essentially has a wave action that sweeps up the shrimp and made them available to feeding kokanee. Lakes such as Pend Oreille and Flathead did not have the same phenomenon.

Initially, scientists thought the kokanee declines were the result of the competition with shrimp for zooplankton.

But in recent years they’ve learned that the shrimp became easy prey to the deep-holding young mackinaw.

Lake trout had struggled in these lakes because most of their young fish couldn’t find enough to eat. But once mysis shrimp were introduced, the young mackinaw had a feast and boomed into bigger macks that would devour kokanee, among other species.

The new predatory force tipped the balance: Lake trout boomed; kokanee declined.

The story is different at Wallowa Lake. By 1985, about 20 years after mysis shrimp were introduced, the lake’s kokanee were trending to bigger sizes, up to 17 inches.

State-record kokanee were caught from the lake in 1998, 2000 and 2001.

But a huge landslide in 2002 upstream from the lake affected kokanee spawning and the number of kokanee declined dramatically several years later.

In 2008, a sonar survey turned up only 70,000 kokanee in a 1,500-acre lake that’s about a mile wide and 4.5 miles long. (The annual harvest by fishermen was about 30,000 in the 1990s.)

The low densities would have been a strong signal the kokanee were crashing, but last year’s sonar surveys showed a nice rebound of 270,000 that are now more than 1 year old.

Bill Knox, Oregon’s fisheries biologist in Enterprise, says he’s holding his breath until he sees if this bumper crop survives to spawning age.

“If they do, great and we’ll have good fishing for smaller fish,” he said. “If they don’t survive well, then we have big problems”

Knox has enough funding for a little research coupled with some educated hunches to explain how these and other details are translating into a fishery that’s suddenly surged to international prominence while North Idaho kokanee have floundered:

•Mysis shrimp have reduced the number of zooplankton species at Wallowa Lake, but the shrimp densities are much lower than in Lake PDO.

•Wallowa apparently has much less spawning habitat for lake trout. Mackinaw have not boomed since mysis were introduced, although they still could.

•Low abundance of kokanee, possibly caused by the landslide, left the survivors with a bounty of food and room to grow.

•Wallowa Lake is in a glacier basin “bath tub” with depths to just under 300 feet compared to 500 foot depths at Lake PDO. Perhaps because the lake is shallower, some of Wallowa’s kokanee – not all of them – have learned to feed effectively on the mysis shrimp.

“The fish that have learned to eat the shrimp are the ones that are getting so big,” Knox said, adding yet another hunch:

“We had two recent years of thick, long-lasting ice and it’s possible that longer period of low light made the shrimp more available to the kokanee.”

North Idaho biologists are keenly interested in Wallowa Lake as well as northwestern Montana’s Little Bitterroot Lake, another relatively shallow lake where kokanee have learned to key in and feed on mysis shrimp.

“We already have mysis shrimp in Hayden Lake,” said Idaho fisheries researcher Andy Dux. “If it’s true that kokanee can figure out how to take advantage of the shrimp in shallower lakes, then maybe Hayden would fit that model.”

That could be a big deal.

Contact Rich Landers at (509) 459-5508 or richl@spokesman.com