Cooperative Fish and Wildlife Research Units Program: Utah
Education, Research and Technical Assistance for Managing Our Natural Resources

McLaren, J. S., R. W. Van Kirk, A. J. Mabaka, S. Brothers, and P. E. Budy. 2023. Drawdown, habitat, and kokanee Oncorhynchus nerka populations in a western U.S. reservoir. North American Journal of Fisheries Management. DOI: 10.1002/nafm.10879. USGS FSP IP- 136834.


Increased drought frequency and severity due to climate change can compound development pressures on water storage reservoirs throughout the western U.S. with uncertain effects on economically and ecologically valuable fisheries. Island Park Reservoir is a mid-sized hypolimnetic-release water storage reservoir on the Henrys Fork River in Idaho that supports a popular sport fishery for kokanee Oncorhynchus nerka. A region-wide drought in 2021 provided an opportunity to use Island Park Reservoir as a case study to examine how drought-induced drawdown affects kokanee habitat. We hypothesized that drawdown reduces the volume of water in the reservoir featuring “optimal” water quality for kokanee. We monitored water quality at five locations in Island Park Reservoir from spring turnover to fall turnover, from which we calculated the approximate volume of “optimal” (<17 C, >5 mg/L O2), “survivable” (<20C, >3 mg/L O2), and “lethal” (>20 C, <3 mg/L O2) conditions for kokanee. We found severe drawdown had a disproportionately negative impact on kokanee habitat availability. Although there was spatial and temporal heterogeneity in the availability of optimal and survivable conditions, drawdown for downstream water supply was the central mechanistic driver of declines in optimal and survivable conditions. Our findings explain interannual patterns in long-term kokanee spawning run return numbers, which were higher with higher reservoir carryover. Water conservation and precision management in the Henrys Fork watershed saved ~39 million m3 (32,000 acre-feet) in Island Park Reservoir in 2021, resulting in a ~150% increase in future spawning kokanee numbers from ~250 to ~650 in the 4,500 m study reach. Our study highlights the effects of drawdown on fisheries in water storage reservoirs and illustrates the degree to which hydrologic management, including projects to reduce water consumption, can benefit reservoir fish populations.