Kokanee Recruitment Response to Water Level Management in Lake Pend Oreille, Idaho

In the 1950s Lake Pend Oreille (LPO) produced an average annual kokanee Oncorhynchus nerka harvest of over a million fish, while providing a trophy rainbow trout O. mykiss fishery.  Kokanee abundance declined precipitously in the late 1960s following several biologic and hydrologic alterations, and has remained at a depressed level since.  The most significant alteration is thought to be a lower winter water level regime associated with power production at Albeni Falls Dam.  The majority of the natural reproduction takes place on the lakeshore of LPO.  Lower winter water levels are thought to decrease recruitment by reducing the availability of suitable spawning gravels.  Since 1996, the Idaho Department of Fish and Game has alternated between the preexisting water level and an experimentally raised lake elevation, to determine if raising winter lake levels would increase kokanee recruitment.  In this study, we evaluated the properties of the current natural recruitment metric and developed alternative strategies for testing the lake level hypothesis.  The existing egg-to-fry survival estimate was shown to be unsuitable for assessing the lake level hypothesis, so we used a stock recruitment modeling approach to evaluate the effect of water level management on shore-spawning kokanee recruitment over a 35 year timeseries.