Once nearly gone, Lake Pend Oreille kokanee have rebounded in a big way--over one million fish

From the Columbia Basin Bulletin

July 19, 2014

The Lake Pend Oreille kokanee population has literally risen from its death bed over the past eight years, due in large part to an Idaho Department of Fish and Game strategy aimed at reducing predation on the smallish game fish. “We were right on the ragged edge of losing the kokanee population” in the large north Idaho lake, Jim Fredericks, the IDFG’s Panhandle Region fishery manager told the Northwest Power and Conservation Council’s Fish and Wildlife Committee during its July 8 meeting in Portland. A population that had into the 1990s commonly provided harvests of a million fish annually began to shrink. State fishery officials were forced to close the fishery beginning in 2000. The low point: an estimated abundance of only 9,994 adult fish in 2007. The declining trend was roughly mirrored by a rapid surge in the lake trout population in Lake Pend Oreille, a long-lived, large bodied fish that preyed heavily on the kokanee. The kokanee, a non-native species, were a prized angler target that also provided food for native bull trout, which are now protected under the Endangered Species Act, as well as introduced Gerrard rainbow trout and lake trout. The latter two species grow to trophy size. Kokanee became established in the lake in 1930s and became a keystone species -- historically supporting the most popular fishery in Idaho and serving as prey for other valued fish species. The Gerrard rainbow trout were introduced in 1941. Lake trout were introduced in 1925 but remained at extremely low abundance until late-1990s. Now the trends have reversed. A large-scale lake trout suppression program launched by the state in 2006 has resulted in the removal of 165,000 lake trout. More than 75,000 lake trout have been removed by anglers motivated by a $15 reward per fish. Another 89,000 have been taken from the lake via an extensive netting effort using contracted commercial fishermen from Lake Michigan. The kokanee recovery effort has largely been funded – at a rate of about $1 million per year -- by the Bonneville Power Administration through the NPCC’s fish and wildlife program. Avista Corporation has also provided funding. BPA markets power generated at the U.S. Army Corps of Engineers’ Albeni Falls Dam, which controls the elevation of Pend Oreille. Avista owns hydro projects upstream of the Pend Oreille on the Clark Fork River, the lake’s main tributary. Dam manipulations are factors that can affect kokanee and other fish and wildlife populations. The commercial adult lake trout catch rates have plummeted 82 percent from 2006 to 2013, an indication of a severe population decline. And juvenile catch rates have dropped by 83 percent. Meanwhile the estimate of kokanee adult abundance jumped to 1.3 million in 2013. And the estimated kokanee biomass in the lake more than doubled just from 2012 to 2013. The 2013 biomass total of 625 metric tons is up from the population’s low point of 74 tons in 2007. The rising kokanee population trend prompted IDFG officials last year to open the first kokanee fishery in the lake since 1999, albeit with a six-fish daily bag limit. With positive expectations, that bag limit was raised to 15 fish this year. After surveys of the lake were conducted in late summer of 2013, biologists estimated that the 2-year-old-going-on-3-year-old age group was “even larger” than the one that produced this year’s adult group. The surveys are conducted by trawling the lake to sample both older fish and fry, and through the use of hydrocoustics, which use SONAR technology to identify objects underwater. Adult fish are mature kokanee, meaning fish that are going to spawn that year. Hatchery fish mostly mature at 3 years old, while wild fish typically mature at age-4,” according to IDFG biologist Andy Dux. “Anglers are catching almost entirely age-3 and age-4 kokanee.” And they are doing well. “Last year people were easily catching their six-fish limit,” Dux said. This year they are reeling in the 15-fish limit within a couple hours. Fredericks told the Council that the lake trout suppression is working to achieve a variety of goals. The reduced predation has helped fuel the increase in the kokanee population in the lake. In addition the bull trout population appears to be stable or increasing, likely helped by the reduced predation threat and expanded kokanee prey. Rainbow trout growth rates have also been increasing. Pend Oreille, located in the northern panhandle, is Idaho’s largest and deepest lake. Frederickson said it is also one of the deepest lakes in United States with an average depth of 538 feet (164 meters) and a maximum depth of 1,152 feet (351 meters).