The influence of depth and location on abundance of fishes in gill net surveys in Bear Lake, Idaho-Utah

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Aldwin G. Keo, Natural Resources, University of Idaho, Moscow, ID, Aldwin G. Keo, DDCSP Collaborative, Aldwin G. Keo, SEEDS Program, Megan R. Heller, Kerri T. Vierling and Anne Yen, Department of Fish and Wildlife Sciences, University of Idaho, Moscow, ID, Courtney J. Conway, U.S. Geological Survey, Idaho Cooperative Fish and Wildlife Research Unit, Moscow, ID

Background/Question/Methods
Bear Lake is home to many species of fish including Bonneville Cutthroat (Oncorhynchus clarkia), Lake Trout (Salvenlinus namaycush), Utah Chub (Gila atraria), and Common Carp (Cyprinus carpio) and is large natural lake that has four major tributaries that feed the lake. Bear Lake is in southeastern Idaho and northeastern Utah and is bisected by the state line. Bear Lake has 4 unique species of fish that are endemic to the area: Bear Lake Sculpin (Cottus extensus), Bear Lake Whitefish (Prosopium abyssicola), Bonneville Whitefish (Orospium spilotnotus) and Bonneville Cisco (Prosopium gemmifer). Bear Lake is a natural and oligotrophic lake, characterized by low productivity of nutrients and abundant oxygen levels in the deeper parts of the lake. The objective of the study was to investigate the species assemblage structure and fish abundance of Bear Lake at different depths. We estimated species assemblage structure of Bear Lake by sampling species richness and abundance at different depths of the lake. Gillnets were set on both the east and west sides of Bear Lake, split between the Idaho and Utah state lines at 5 different depths (9 m, 15 m, 20 m, 25 m, and 35 m). Monofilament experimental gill nets were used in the study and were 53.3 m long, 6.1 m deep, and had seven panels with 12.7, 19.1, 25.4, 38.1, 50.8, 63.5, and 76.2 mm bar-measure mesh. The nets were set at the thermocline or at the bottom of the lake for a 24hr period and were set either parallel or perpendicular to the lake shore. In 2019, the gillnets were set on September 8th to September 12th, and in 2020, the gillnets were set from July 13th to July 16th.
Results/Conclusions
In general, the total number of fish species did not vary when nets were sampled on the eastside or westside of the lake. However, we did find that species richness increased with depth and the abundance of fish generally increased at shallower depths. Historically, Bear Lake waters were drawn down for irrigation and power and this would negatively affect the fish that did not do well in shallow waters, for example Bear Lake Sculpin. Having a range of water depths from shallow to deep can increase the species richness of a lake because the different depths provide habitat for different species of fish.