Effect of the Operation of Kerr and Hungry Horse Dams on the Reproductive Success of Kokanee in the Flathead System, 1987 Final Report.
Studies of kokanee reproductive success in the Flathead system from 1981 to 1987 have assessed the losses in fish production attributable to hydroelectric operations. We estimated that the Flathead Lake shoreline spawning stock has lost at least 50,000 fish annually, since Kerr Dam was completed in 1938. The Flathead River spawning stock has lost 95,000 spawners annually because of the operations of Hungry Horse Dam. Lakeshore spawning has been adversely affected because Flathead Lake has been drafted to minimum pool during the winter when kokanee eggs are incubating in shallow shoreline redds. Egg mortality from exposure and desiccation of kokanee redds has increased since the mid 1970's. When the lake was drafted more quickly and held longer at minimum pool. Escapement surveys in the early 1950's, and a creel survey in the early 1960's have provided a baseline to which the present escapement levels can be compared, and loss estimated. Main stem Flathead River spawning has also declined since the mid 1970's when fluctuating discharge from Hungry Horse Dam during the spawning and incubation season exposed redds at the river margin and increased mortality. This decline followed an increase in main stem spawning in the late 1950's through the mid 1960's attributable to higher winter water temperature and relatively stable discharge from Hungry Horse Dam. Spawning escapement in the main stem exceeded 300,000 kokanee in the early 1970's as a result. Spawning in spring-influenced sites has comprised 35 percent of the main stem escapement from 1979 to 1986. We took that proportion of the early 1970's escapement (105,000) as the baseline against which to measure historic loss. Agricultural and suburban development has contributed less significantly to degradation of kokanee spawning habitat in the river system and on the Flathead Lake shoreline. Their influence on groundwater quality and substrate composition has limited reproductive success in few sites. Studies of the effects of hydroelectric operations on the reproductive success of kokanee in the Flathead system have been ongoing since 1980. Results of these studies have been published in a series of annual progress reports which are detailed in Appendix G. The reports summarize spawning site inventories and spawning escapement, egg and alevin mortality rates and the mechanisms by which water level fluctuations influence mortality, creel surveys, and investigation of the population dynamics of Flathead kokanee. The Region 1 offices of the Montana Department of Fish, Wildlife and Parks distribute this material to the scientific community and the general public. Until recently, it was considered feasible to recover losses to the Flathead kokanee fishery by enhancing and diversifying natural reproduction. But the establishment of opossum shrimp (M. relicta) in Flathead Lake has reduced the availability of zooplankton forage in the spring and summer, and may reduce the viability of juvenile kokanee. In 1986, research was redirected to quantify this competitive interaction and to investigate artificial means of enhancing the kokanee fishery. The average density of mysid shrimp in Flathead Lake has increased to 108/m{sup 2} in 1987, and at some locations density exceeds 500/m2. Mysid grazing pressure has delayed the pulse of zooplankton production in the spring and reduced zooplankton standing crop in the summer. Cladocerans such as Daphnia thorata, the preferred food of kokanee of all ages, are the most markedly affected species. The peak density of D. thorata in the summer has declined from 4.8/liter in 1983 to O.9/liter in 1987. Growth rates of underyearling and yearling kokanee have declined, apparently as a result of the reduction in their food supply. Spawning escapement has also declined, falling from 150,000 in 1985. to 25,000 in 1986, to 600 in 1987. Fry-to-adult survival has declined from 2.5 percent to near zero. The causes of high mortality, and which age-classes are most susceptible, are not completely understood, but the observed decline in juvenile growth rate implicates mysid-induced change in the trophic ecology of Flathead Lake. These biological changes in the Flathead system wi11 delay the formulation of a recovery plan for kokanee. Three scenarios represent the possible alternatives for such a plan. If mysid-induced changes in kokanee survival do not persist, or are limited in scope, we can proceed with enhancement of natural kokanee reproduction. This alternative includes protection of main stem spawning with stable fall and winter discharge from Hungry Horse Dam, and enhancing spawning in other tributaries. If the increase in wild fry mortality is limited, main stem flows can be tailored to optimize production from wild escapement and the kokanee fishery can be supplemented with hatchery-produced fry.
- Research Organization:
- Montana Department of Fish, Wildlife and Parks
- Sponsoring Organization:
- United States. Bonneville Power Administration.
- DOE Contract Number:
- 1983BP39641
- OSTI ID:
- 901154
- Report Number(s):
- DOE/BP-39641-5; R&D Project: 198110500; TRN: US200713%%321
- Country of Publication:
- United States
- Language:
- English
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Impacts of Water Level Fluctuations on Kokanee Reproduction in Flathead Lake; Effects of Operation of Kerr and Hungry Horse Dam on Reproductive Success, 1983 Annual Report.
Impacts of Water Level Fluctuations on Kokanee Reproduction in Flathead Lake, 1984 Annual Report.