Effect of the Operation of Kerr and Hungry Horse Dams on the Reproduction Success of Kokanee in the Flathead River System, 1986 Annual Progress Report.

Abstract

The 1985 kokanee spawning run in the Flathead system was the strongest in five years. Escapement to the Flathead River system was 147,000 fish, including 123,000 in McDonald Creek and an estimated 20,000 in the main stem. Enumeration of spawners and redds in the Flathead River was hindered by high fall flows and early freezing in November. The upstream spawning migration from Flathead Lake began in late August. Schools of kokanee were seen six miles above the lake on September 4. We counted 1,156 redds in Flathead Lake, distributed primarily along the southeastern shore. An unusually high proportion (90 percent) of lakeshore spawning occurred in the zone above minimum pool, where egg mortality is very high because of exposure from drawdown. Escapement to the Swan River was 1,350 fish. Four year old (III+) fish comprised 95 percent of the spawning run in the Flathead system. This continues a five-year trend toward dominance of the III+ year class. The age composition of spawners has varied considerably for the past 15 years. The average size of spawning fish was 365 mm, which is identical to the average size of the parent year class in 1981. One of the goals of managing Flathead kokanee is to produce mature fish 300-330 mm in length. In the main stem Flathead River, pre-emergent survival was 80 percent. Survival in McDonald Creek, unaffected by hydroelectric operations, was 83 percent. Sampling showed few hatched alevins, probably due to unusually cold winter temperatures. Egg survival at Blue Bay, a spawning area on Flathead Lake where redds are concentrated below minimum pool, varied in relation to depth and dissolved oxygen concentration in the substrate. Eggs survived 78 days at 2,880 feet where dissolved oxygen was 5.7 mg/l. Eggs survived 35 days at 2,870 feet where dissolved oxygen concentration averaged 2.9 mg/l. Low dissolved oxygen contributed to poor survival to emergence at all elevations in Blue Ray. Experiments in Skidoo Bay confirmed that survival of eggs above minimum pool depends on redds being wetted by groundwater seeps. After 40 days exposure by drawdown, eggs in groundwater seeps showed 86 percent survival, whereas outside of the groundwater seeps eggs survived less than six days. These results confirm that exposure by drawdown is the primary factor that limits kokanee reproductive success in redds above minimum pool. We surveyed the west and south shoreline of Flathead Lake to locate potential kokanee spawning habitat. We found conditions which could support incubating eggs at two sites in South Ray and two sites on the west shore of the lake. Seven other sites on the west shore were not suitable due to low groundwater discharge or low dissolved oxygen. In all these areas suitable substrate existed only within the drawdown zone. The lake should be drafted earlier in the fall, and filled earlier in the spring to improve recruitment from lakeshore spawning. We conducted creel surveys during 1985, and estimated that anglers caught 192,000 kokanee. Anglers harvested 49,200 fish during the ice fishery in Skidoo Bay, 129,000 fish during the summer fishery on the lake, and 13,800 during the fall river fishery. Estimated fishing pressure for the year exceeded 188,000 angler hours. The abundance of mysid shrimp in Flathead Lake, measured at six index stations, increased to 130/mIf in 1986. My&Is increased tenfold from 1984 to 1985, and about threefold from 1985 to 1986. Monitoring of mysid shrimp and zooplankton populations in Flathead Lake is supplementing an investigation of the growth and survival of juvenile kokanee. Kokanee and mysid shrimp feed primarily on planktonic crustaceans. This work was designed to detect a potential decline in kokanee recruitment or growth brought about by competitive interaction with mysid shrimp. Fluctuation in adult kokanee year class strength is in part attributable to the negative effects of hydroelectric dam operation on reproductive success in the main stem Flathead River and in Flathead Lake. Our results show that egg survival in the river has improved in response to stabilized discharge from Hungry Horse Dam. Drawdown exposure continues to limit egg survival in lakeshore redds. Study of the variability in growth and survival of young-of-the-year fish in Flathead Lake will further the understanding of factors which cause fluctuation in the kokanee population.