Study: Priest Lake has fewer lake trout than thought

PRIEST RIVER - Among the most surprising findings of the Idaho Department of Fish and Game's study of the Priest Lake fishery: There are far fewer lake trout in Priest Lake than previously thought.

Last week, Idaho Department of Fish and Game officials outlined the findings from a two-year comprehensive study designed to learn more about the Priest Lake fishery. Biologists from IDFG partnered with the University of Idaho and Kalispel Tribe, and have been specifically looking into the abundance of Lake Trout along with growth rates, food habits and population age and size restructure. The cooperative study included a large-scale effort to mark, release and recapture lake trout, identify angling effort through aerial surveys and learn more about catch rates and the harvest of various species on the lake through a creel survey.

Biologists felt understanding lake trout will help to understand how they affect other fish in the system and what types of management options are available.

"We learned a lot of good information through the study and we're here to bring the public up to speed," said Jim Fredericks, IDFG regional fisheries manager for the Panhandle District. "It's a long process and we're in the middle of it."

The most surprising finding is that there are far fewer lake trout in Priest Lake than previously thought. The total population of lake trout is estimated at 45,000 fish. Biologists previously believed the fishery to hold somewhere between 100,000 and 300,000 fish.

"We didn't have a good estimate prior (to the study)," Fredericks said.

Lake trout were introduced in 1925 to the Priest Lake fishery, which held abundant populations of bull trout and kokanee and was world-renowned for its cutthroat trout. The lake trout population at Priest Lake exploded after mysis shrimp were introduced in 1965. The IDFG introduced the new food source with the thought the shrimp would help the kokanee, but it actually led to a subsequent increase in the lake trout population and fewer cutthroat and bull trout for anglers. "The recruitment of lake trout was very limited in Priest Lake until the introduction of the mysis shrimp," Fredericks said. "In places where you have mysis shrimp and a good fish spawning habitat the population increases where it overwhelms the existing fish community. The Priest Lake fishery changed from predominantly kokanee and bull trout to predominantly lake trout, which is not an uncommon story."

Idaho's state fish, cutthroat trout, showed harvests around 4,000 fish per year in the 1950s but declined to hundreds of fish by the 1980s. Bull trout were federally listed as a threatened species under the Endangered Species Act in 1998 by the U.S. Fish and Wildlife Service.

During the spring and fall of 2013, the cooperative research team conducted intensive netting with gill and trap nets with the intention of tagging as many lake trout as possible. Of the approximately 4,000 fish caught 3,000 were released and 104 were recaptured with a mortality of 812. Just under 400 fish were sacrificed for scientific research. The study showed a survival rate of 85 percent each year with an exploitation of 13.6 percent, which reveals a very low natural mortality rate.

Lake trout grow rapidly in their first few years of life, up to 10 inches, before tapering off around 10 years of age showing an evidence of density dependence. Biologists explained that normally an adult fish would transition to a fish diet rather than mysis shrimp but the fish are simply not available. The lake trout lives its entire life in the lake unlike native salmonids and have a much longer lifespan than native freshwater fish. The lake trout can live anywhere from 50-75 years while cutthroat trout traditionally live six to nine years, bull trout eight to 15 years and kokanee three to four years. Another interesting finding from the study showed that about 30 percent of the female lake trout were characterized as "non-spawning" meaning that although they were sexually mature, their gonads were poorly developed. The phenomenon is believed to be a function of a lack of nutrition and is not something seen in lake trout populations where food is abundant, such as Lake Pend Oreille.

A creel survey from 2013-2014 revealed a total angler effort of 49,941 with about 75 percent of anglers fishing for lake trout, 20 percent fishing for kokanee and five percent fishing for small bass, perch and even pike minnow. The survey showed that of the 25,000 lake trout caught 15,000 are harvested each year. Of 12,000 kokanee caught, 4,000 were harvested.

"It shows a bump in angling effort from 2003 to present but not much difference," IDFG regional fishery biologist Rob Ryan said.

Ryan also said from 2001-2010 only a few thousand kokanee were present until the season was re-opened and the habitat rebounded. In 2011, the kokanee population hit approximately 20,000 and climbed to 35,000 by 2013. The 2014 estimate was 15,000. Looking at management options for Priest Lake and Upper Priest Lake IDFG officials said they felt they could either discontinue operations and allow the lake trout to dominate the habitat or develop a plan seeking funding to suppress the fish. Under the 2013-2018 management plan, information is currently being gathered and the IDFG is working with stakeholders to make information based decisions before coming back to the public in 2018 to discuss the next six-year management period.