Exploring Impacts of Lake Level Decline on a Critical Forage Species in a Large Terminal Desert Lake

Authors: Sarah Barnes, Phaedra Budy, Robert Al-Chokhachy

Year: 2022

Abstract

In endorheic saline lakes in arid regions increasing drought conditions, water diversions for agriculture, and development cause significant lake level decline, potentially resulting in fish littoral habitat loss. Pyramid Lake, a large, deep, endorheic lake on Pyramid Lake Paiute Tribe land in Nevada, is home to Lahontan Cutthroat Trout, Oncorhynchus clarkii henshawi, a unique sub-species of cutthroat trout listed as Threatened under the Endangered Species Act. In Pyramid Lake O. c. henshawi success relies heavily on their primary forage species, the Tui Chub, Siphateles bicolor, a prolific, large-bodied, native minnow, whose abundance, according to long term gill net surveys, may be in decline. Here, we evaluated potential impacts of littoral habitat loss on S. bicolor spawning success. We combined S. bicolor abundance and habitat data, collected monthly at 9 sites from May to August, to investigate S. bicolor spawning adult and larval fish habitat selection. We sampled spawning fish with gill nets (5-30 m of depth), and larval fish with larval tows (5-60 m depth). We deployed temperature loggers at depths of 3, 15, and 30 m at each site for the duration of the sampling effort. We assessed substrate and vegetation from 5-30 m of depth using hydro acoustic transects conducted at every 500 m of shoreline, which we verified by SCUBA survey. Spawning adult catch-per-unit-effort (CPUE) was significantly higher at depths <10 m (p <.001) supporting our hypothesis that littoral habitat is important for S. bicolor spawning success. We observed greater than 5% composition of boulder and cobble substrates at 3 of the 9 sites surveyed using SCUBA, with 2 of the 3 sites coinciding with our highest observed spawning adult abundance, suggesting spawning adults may be selecting for the limited amount of boulder and cobble substrates within the littoral zone. In contrast to spawning adult abundance, we observed the highest larval fish density at the two most southern sites, a trend which may be driven by prevailing northerly winds. Integrating our results with lake bathymetry and elevation data would allow managers to better understand the relationship between lake elevation, littoral habitat, and S. bicolor and O. c. henshawi populations in Pyramid Lake. Mitigation strategies could include securing more water for Pyramid Lake and altering O. c. henshawi stocking regimes to not exceed S. bicolor carrying capacity.