Report Regarding a Research Proposal for Inclusion in the Columbia River Basin Fish and Wildlife Program:
Lake Pend Oreille Fishery Recovery Project

March 7, 1997  |  document ISAB 97-4

read full document > (90k PDF)

Background

Lake Pend Oreille is Idaho’s largest (86,000 ac) and deepest (1,200 ft) natural lake and is economically important for the tourism and sport fisheries it supports. The lake was formed after recession of continental glaciers but the Pend Oreille River was dammed in 1952 at Albeni Falls, 25 mi downstream from the natural lake outlet, resulting in a reservoir that extends upstream to Lake Pend Oreille. The dam allows the lake level to be regulated for power production and flood control. The major inflowing tributary to the lake, Clark Fork River, was dammed at Cabinet Gorge, also in 1952.

Fishery resources in Lake Pend Oreille are estimated to be potentially worth millions of dollars (Maiolie and Bennett 1996). Native salmonids in the lake include westslope cutthroat trout and bull trout. In 1942, the Gerrard strain of rainbow trout was introduced and world record-class rainbow trout were caught in the ‘40s, ‘50s, and ‘60s. Lake trout have become established in Pend Oreille, and another non-native species, kokanee, entered the lake in the 1930s by migrating downstream from Flathead Lake. After invading Pend Oreille, the newly established kokanee prospered. By the early 1950s, sport fishing pressure on kokanee exceeded 100,000 angler days per year and there was a commercial kokanee fishery, resulting in a combined sport and commercial harvest often in excess of 1,000,000 fish. Kokanee also served as an important forage species for the lake’s large predator populations.

Beginning about 1964, the Pend Oreille kokanee population began a steady decline (Maiolie and Elam 1993). By 1976, annual harvest had dropped from one million to approximately 200,000 fish. The population has remained at depressed levels since that time, although interannual variation in harvest is high. Over the last decade, harvest levels have ranged between 82,000 in 1985 and 227,000 in 1991.

Idaho Department of Fish and Game (IDFG) believes that regulation of winter lake levels by the operation of Albeni Falls Dam has been the primary factor contributing to the decline of kokanee (Maiolie 1993). Prior to dam construction the lake was low in winter and spring (average elevation ~ 2,048 ft), rising to ~ 2,063 ft in late spring and early summer with snowmelt runoff, and then falling back rapidly to low levels in July. After the dam was built the lake was maintained at high, stable levels (~2,062 ft) throughout the summer and then drawn down to ~ 2,056 ft in winter from 1952 to 1965. Beginning in late 1966, Pend Oreille was drawn down to a lower level of 2,051 ft throughout the winter.

IDFG notes that the winter drawdown to 2,051 ft corresponded with the onset of the kokanee decline. They believe the cause of the kokanee population reduction has been reduced reproductive success caused by loss of spawning gravel along the lake shoreline (Maiolie and Bennett 1996). According to this hypothesis, kokanee spawning was highly successful when winter lake levels were maintained at ~ 2,056 ft, which resulted in ample amounts of clean, wave-washed gravel being available at key spawning locations. However, when winter drawdown lowered the lake to 2,051 ft, IDFG believes many of the preferred spawning areas became unavailable and kokanee were forced to spawn in gravels of inferior quality, i.e., gravels that contained more fine sediment. They found that year class strength was greater than average in 1968, 1982, 1983, and 1985 – years when the winter pool was held at higher than average levels. Overall, the relationship between winter drawdown and kokanee harvest five years later has been significantly negative (r = -0.71, P < 0.005). IDFG is concerned that the kokanee population in Lake Pend Oreille is at considerable risk. They believe it is possible that the stock may be driven so low that a “predator trap” might develop, in which the kokanee population level becomes limited by predation on fry to the extent that it cannot be rebuilt.

^ top

Use Adobe Reader to view PDF documents