Date of Award:

5-2018

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Ecology

Department name when degree awarded

Watershed Sciences and the Ecology Center

Committee Chair(s)

Jereme Gaeta

Committee

Jereme Gaeta

Committee

Nicolaas Bouwes

Committee

Mary Conner

Abstract

Multiyear drought is expected to increase in occurrence and severity in dry regions across the world, such as the American Southwest, endangering native species and ecosystem health. The effects of multiyear drought are often worsened by human water use for consumption, energy production, and agriculture, which, in lakes and reservoirs, cause reduced lake elevation. Reductions in lake elevation may leave previously submerged habitat stranded along shorelines and make tributaries inaccessible to fishes. Here, we show that decreasing lake elevation, associated with drought and water withdrawal, reduces the availability of shoreline cobble habitat to fishes and increases tributary distance by creating an elevation-specific shoreline habitat map and an elevation-specific tributary channel distance map, respectively. We combined 17-years of fish survey data and lake elevation history with our elevation-specific maps to determine whether the number of fishes observed and the number of fish hatching are related to drought-driven changes in shoreline habitat and tributary channel distance. We surveyed the shoreline zone of Bear Lake, UT-ID, from full pool to a depth of >18m, totaling 94.86 surveyed km2. As lake elevation decreased >6 m from full pool to the lowest historical elevation, shoreline cobble decreased by >97%, and tributary channel distance increases up to 400% that of full pool. Bear Lake sculpin, a cold-water fish species of concern that relies on cobble for spawning, catch per unit effort decreased by >75% at the minimum cobble area available, and the number of sculpin successfully becoming adults declined by as much as 86%. The number of Bear Lake Bonneville cutthroat trout, a sport fish that relies on connections between tributaries and the lake to complete their life cycle, successfully becoming adults declined by as much as 62%, but annual growth was not related to drought. Our research is an example of the severe consequences of multiyear drought conditions and water withdrawal on lakes and reservoirs, as well as the fishes they support. Indeed, understand the effects of extended drought on aquatic habitats is critical to maintain healthy lakes and reservoirs, to sustain native species, and to maintain the health of popular sport fisheries.

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