There is a toothy new predator in northern Idaho’s largest lake. Walleye, which were essentially non-existent in the lake ten years ago, have become increasingly abundant, to the point where they are now a significant player in the fishery. Lake Pend Oreille has long been known for its trophy rainbow trout and bull trout, having produced world records for both species. The trophy fishery, and to a very large extent, the sport fishery in general, is built on a foundation of kokanee.
In recent years, IDFG, with strong public support, has worked to manage and suppress lake trout, which threatened to collapse the kokanee population a decade ago. Walleye, like lake trout, are a top level predator, and even though they are a popular sport fish that benefits fisheries elsewhere, they pose a significant risk to the Lake Pend Oreille fishery.
Walleye were illegally introduced into Noxon reservoir in the early 1990’s. Noxon Reservoir is upstream of Lake Pend Oreille on the Clark Fork River. Walleye spawn below Thompson Falls dam (the upstream end of Noxon Reservoir) and their fry naturally drift downstream after hatching, which is likely how they got into Lake Pend Oreille. Walleye were first detected at low densities in Lake Pend Oreille in the mid-2000s. Since then their densities have been steadily increasing.
Walleyes are voracious predators, and in Lake Pend Oreille they commonly feed on kokanee in the deeper parts of the lake and yellow perch in the shallower areas. IDFG is concerned that walleye numbers may continue to increase to the point where they cause excessive predation on kokanee and compete with or reduce abundance of high-demand sport fish species, such as rainbow trout and bass, as well as native bull trout and cutthroat trout.
Walleye have dramatically changed fish communities in all of the places they have been introduced in the western U.S. We recently have started to learn more about walleye in Lake Pend Oreille since they are relatively new to the lake. However, the information we have thus far suggests that we should expect some of the same negative impacts that introduced walleye have had on other established fisheries in the western U.S. As a result, the presence of walleye in Lake Pend Oreille is cause for concern for anglers and fishery managers.
Late last winter, IDFG staff met with a panel of walleye experts from the Midwest and eastern Canada to solicit their input on how concerned we should be and what options might exist to manage the walleye threat. Based on the strong public desire to maintain and improve the kokanee and rainbow trout fisheries, and the public values placed on maintaining and improving bull trout, cutthroat trout, and bass populations, the expert panel concluded that IDFG should be very concerned about the continued expansion of the walleye population.
The group unanimously agreed that IDFG should approach this situation by developing an understanding of basic walleye biology in Lake Pend Oreille to guide future research and management efforts. In addition, the panel recommended evaluating harvest methods to curb walleye population growth. They concluded that angler harvest alone is unlikely to keep walleye in check. Therefore the panel recommended that IDFG immediately begin evaluating other approaches to increase harvest. While IDFG has not committed to a long-term suppression program, we are currently evaluating the feasibility of using commercial gill nets as a tool to reduce walleye abundance to a manageable level. At the same time, IDFG is promoting walleye angling and harvest using public outreach and by sharing walleye distribution data.
To understand basic walleye habitat selection and movements throughout Lake Pend Oreille, IDFG biologists initiated a telemetry study in March 2018. Twenty-one walleye were tagged with transmitters to allow biologists to identify their locations and movement patterns. Fish were tagged at four locations including: the Clark Fork River delta, near Dover on the Pend Oreille River, near Cabinet Gorge dam, and near Garfield bay. Tagged fish were periodically located using aircraft, boats, and stationary receivers.
Early results from our spring tracking indicate that some walleye move many miles. For example, we documented several walleye tagged at the Clark Fork delta moving downstream to the Sandpoint area. Other walleye tagged near Dover on the Pend Oreille River moved into Oden Bay and to the Clark Fork River. Also, during that time, some walleye stayed reasonably close to where they were tagged.
Earlier this spring and around the time of spawning, two large concentrations of walleye were identified near the Clark Fork River delta and near the mouth of Pack River. By mid-May, several walleye were documented moving upstream into the Clark Fork River, some as far upstream as the Cabinet Gorge Dam. More recently, several tagged walleye were located in Denton slough, while others were concentrated around Sandpoint near the railroad and Highway 95 bridges, Oden bay, Ponder Point, and Kootenai Point. Recent telemetry flights also showed several walleye moving downstream into the Pend Oreille River and two tagged walleye moved as far downstream as the Laclede area.
To evaluate the feasibility of gill netting as a tool to control walleye abundance, IDFG contracted a commercial netting company to net for three weeks between April 16 and May 4. Commonly, when biologists use gill nets to evaluate a fishery, nets are set during the day and checked the following day after soaking overnight. With overnight gill net sets, fish have a higher mortality rate. To minimize mortality on other species, we used short-set (4 - 5 hour) gill nets to target and remove walleye. Our nets were set at eight locations (Figure 1, Table 1) just before dawn each day and checked at about 9:00 am the same day.
During the three-week period, a total of 1,284 walleye were captured and 1,233 of those fish were removed (Table 2). We released 51 of the captured walleyes for a tagging study. We also removed 36 lake trout and 33 northern pike. There was very little incidental mortality for non-target species (Table 2), including 100% survival of bass. Table 1 provides the numbers of fish captured and released by netting location and Table 2 provides the number captured and removed. The removed fish were not wasted, and over 1,050 walleye, lake trout, and northern pike were brought to either the Bonner County Food Bank in Sandpoint or the ABC Food Bank (Athol Gleaners) in Athol for distribution to those in need.
Given what is known about walleye, it is currently not realistic to expect that walleye could be eliminated from Lake Pend Oreille. Walleye will be swimming in the lake into the foreseeable future. Our goal is to find effective ways to manage this new walleye population at a low enough density that they do not jeopardize existing fishery management goals. There are no seasons or bag limits on walleye in Lake Pend Oreille, and we encourage anglers to fish for and harvest walleye. Fortunately, walleye fishing is fun and they make excellent table fare.
The attached map (Figure 2) provides telemetry locations for walleye that we have located since early April. In general, most of the walleye have been located on the northern corridor of the lake between the Clark Fork River delta and Sandpoint area. The Sandpoint area is already a popular walleye fishing area and fishing should improve near the bridges and down into the Pend Oreille River as summer progresses. The information on walleye locations is currently at a broad scale, but as our monitoring continues to develop and improve, we will aim to provide more detailed information and get it out to the public. We will provide another update and a new map later this summer. Lastly, we will continue to use telemetry and other projects focused on walleye in the lake to better understand their impacts on the fishery and appropriate management actions.
Go to the image gallery to view the tables and figures referenced in the text.