REPUBLIC -- Washington Department of Fish and Wildlife (WDFW) biologists are studying the fish population and management of Ferry County's Curlew Lake, where traditional rainbow trout catches have been changing.
WDFW district fish biologist Curt Vail explains that this summer's catch rates at the popular lake just north of Republic were lower than in the past, although those trout caught were bigger, 14 to 16 inches or more.
In the summer of 2000, catch rates were good, with many limits of five 8-9-inch trout reported. By the summer of 2001, catch rates were declining, with more 14-16-inch fish and fewer 8-9-inchers.
WDFW has maintained Curlew's rainbow trout fishery through fall stocking of hatchery fry that average 20 fish to the pound, or about 4-5 inches in length. WDFW also provides fry for an in-lake net pen rearing operation from which trout are released at 8-9-inch size in the spring.
Despite an increase in fall fry stocking (from 70,292 in 1997 to 168,600 in 2001), Vail notes, trout survival and catch rates were not increasing. A growing number of northern pike minnow (formerly known as squawfish) was suspected of preying on trout fry.
"That's when we decided to try tiger muskies," said Vail.
Tiger muskellunge are a sterile cross between northern pike and muskellunge that are used as a predatory biological control and trophy fish option. Four hundred tiger muskies were first stocked in Curlew Lake in 1998 to reduce the northern pike minnow population. Another 100 were stocked in 1999, 200 in 2000, and 300 in June of 2002.
The numbers aren't high because tiger muskies are stocked at a size of 12-16 inches to make sure they don't get eaten first, Vail explains, which limits the number affordable. But some have grown to top predator status, the biggest caught so far at almost 20 pounds.
"I know some people think the tiger muskies are eating the trout fry and that's why the trout catches have changed," said Vail. "But our sampling of tiger muskies so far doesn't show that."
The sampling is mostly done by electrofishing - using a boat equipped with an electrical system that puts a charge into the water to temporarily stun fish, allowing biologists to collect them in nets from the surface so that can be weighed, measured, and stomach-pumped to determine what they're eating. The electrofishing is done with lights at night when more fish are in shallow water and easier to collect.
Tiger muskie stomach sampling so far has not shown a significant number of identifiable trout fry, Vail says. It also, however, has not shown a significant number of identifiable northern pike minnow. The overall number of tiger muskies in the lake is so low that collecting a large enough sample is a challenge, he explains.
"We're continuing this and other kinds of sampling work to try to learn more," Vail said, "but we're also looking into the timing of the trout fry plants as the potential problem."
Trout fry need to be stocked in Curlew Lake after the annual fall "turn-over" or de-stratification of the water layers, Vail explains, to reduce northern pike minnow predation rates. As water temperatures decline, the lake water column of various oxygen concentration layers is mixed. Before this de-stratification, oxygen is available only in the top 25 feet of water, leaving newly-stocked trout fry more vulnerable to predation.
"Unfortunately," Vail explained, "we may have stocked trout fry at the wrong time in recent years. Now we're planning to watch the lake's turn-over more closely and stock fry later."
WDFW is also working with local private net-pen operation volunteers to optimize rearing conditions for better growth and survival of those trout. "We need help from more volunteers to assist with the net pens like feeding, regular net cleaning, and general maintenance," Vail said.
"We can also use help from fishers," he said. "When you see angler survey boxes at the boat ramps or are asked to fill out questionnaires, please do so. The information you provide will help us restore a top quality trout fishery at Curlew Lake."