Rule curves for irrigation drawdown and kokanee salmon ( Oncorhynchus nerka) egg to fry survival

Abstract

The survival of kokanee salmon ( Oncorhynchus nerka) eggs and pre-emergent fry in the shoreline spawning areas of Banks Lake, Washington State, U.S.A., is reduced by annual drawdown of the reservoir. Kokanee salmon spawn exclusively along steep talus shorelines during the months of October and November. No suitable spawning streams enter the lake and nearly all water is pumped into this irrigation reservoir from lake Roosevelt on the Columbia River. Preferred spawning depths range from 1.5–4.6 m; however, when large populations were present, spawning was observed to 13.7 m. Fry emergence was estimated at depth intervals from conical trap catches deployed in an array over a major spawning area in the reservoir. Emergence occurred from late March to early June; however, the timing of maximum emergence shifted from 28 April to 10 May during three consecutive years due to changes in the thermal regimes during incubation. The relationships between the depth range of spawning area, timing of spawning, egg incubation and fry emergence, and the spring reservoir drawdown for 1977, 1978 and 1979 were analyzed graphically. Rule curves defining rate and magnitude of reservoir drawdown to achieve 75% fry emergence were developed. These rule curves can be used to determine a water management strategy for natural kokanee salmon production or to estimate the loss when exceeded.