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Drawdown, habitat, and kokanee populations in a western U.S. reservoir

March 25, 2023

Greater drought frequency and severity due to climate change will result in greater drawdown of water storage reservoirs. However, changes to oxythermal regimes due to drawdown are reservoir specific and interface with fish species-specific habitat requirements, producing varying effects on coldwater fish populations. We examined the effect of drawdown on the oxythermal habitat and relative abundance of kokanee Oncorhynchus nerka, a coldwater salmonid, in Island Park Reservoir on the Henrys Fork of the Snake River, Idaho. A measure of relative kokanee abundance was negatively, exponentially related to drawdown. Oxythermal patterns measured in the reservoir during 2021, a severe drought year, revealed that drawdown reduced kokanee habitat by increasing water temperatures and decreasing dissolved oxygen concentrations. Oxythermal refugia for kokanee appeared to relate to inflow from the spring-fed Henrys Fork and other groundwater inflows. However, we did not quantify groundwater flow or connections, and we did not study kokanee population demographics or mortality. Reducing these sources of uncertainty is a priority for future study. Still, our study highlights a potential mechanism connecting reservoir drawdown to fish populations and the unique yet predictable mechanisms by which reservoir drawdown interacts with reservoir morphometry to affect fish habitat availability.

Publication Year 2023
Title Drawdown, habitat, and kokanee populations in a western U.S. reservoir
DOI 10.1002/nafm.10879
Authors John S. McLaren, Robert W. Van Kirk, Arthur J. Mabaka, Soren Brothers, Phaedra E. Budy
Publication Type Article
Publication Subtype Journal Article
Series Title North American Journal of Fisheries Management
Index ID 70255214
Record Source USGS Publications Warehouse
USGS Organization Coop Res Unit Seattle
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