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Date of Award
Master of Science (MS)
Bodensteiner, Leo R.,1957-
Helfield, James M.
Rawhouser, Ashley K.
I investigated available prey items and the diet characteristics of juvenile fishes in three seasonally inundated tributaries to Ross Lake, Washington from March through June, 2013. Native fishes include Rainbow Trout (Oncorhynchus mykiss), Bull Trout (Salvelinus confluentus), and Dolly Varden (Salvelinus malma). Cutthroat Trout (Oncorhynchus clarkii), Brook Trout (Salvelinus fontinalis) and Redside Shiner (Richardsonious balteatus) comprise the introduced fishes in the lake. Both Cutthroat Trout and Redside Shiner are native to Washington, but not Ross Lake. Juvenile Bull Trout, Rainbow Trout, and Brook Trout are known to feed on items along the bottom of lakes or streams, such as larval and adult insects as well as items floating or drifting in the water column. Diet composition can be altered by the benthic macroinvertebrate community, season, and habitat type as well as anthropogenic interferences such as dams. During each sampling event the stream was electrofished, benthic macroinvertebrate samples were collected, all captured fish over 50 mm were lavaged, and during the initial visit to each site, a habitat assessment occurred. Three fifty-meter reaches were selected for each stream to have representative sites at low, medium, high, and full pool elevations. Rapid habitat assessment was completed following USFS Stream Inventory Handbook for Region 6 on each of the streams during the first site visit, benthic macroinvertebrate sampling followed a modified version of the Environmental Protection Agency’s Rapid Bioassessment Protocols for Use in Streams and Wadeable Rivers: Periphyton, Benthic Macroinvertebrates, and Fish, electrofishing followed American Fisheries Society and National Marine Fisheries Service Guidelines, and diet evaluation was completed using non-lethal gastric lavage of stomach contents mm following the modified protocols of Giles (1980), Strange and Kennedy (1981), Hartleb and Moring (1995). My study suggests adequate food, in the form of benthic macroinvertebrates, is present based on the presence of few fish with empty stomachs in the system. The benthic macroinvertebrates found in the tributaries to Ross Lake reflect those commonly found in Pacific Northwest Streams. A total of 3,645 individuals in 31 families were collected. Ephemeroptera was the most abundant and frequently occurring insect order across all samples, but not uniformly the most abundant at all sites, dates, or months. Abundance of families by date differed, but not site or reach. Diets varied by sites, months, and most pool elevations, but not species. Sixty-five of the seventy-three fish collected had at least one diet item in their stomach (89% off all fish). Including those taxa that were identifiable only to terrestrial origin or class Insecta and those unrecognizable even at the class level, there were sixteen categories for analysis, seven of which were considered major and included in all analyses. Using Index of Relative Importance, I determined Diptera was the most important prey item overall, followed by Ephemeroptera. Stomach fullness, calculated by Instantaneous Ration, was correlated to the number of prey found in individual fish stomachs. As expected, stomach fullness followed benthos abundance trends. Many studies are completed on adult feeding strategies, especially in comparisons between species or environments, but research on juvenile diets is less available, and to the best of my knowledge research on prey availability and selectivity on seasonally inundated streams is non-existent. Further research on Ross Lake juvenile trout diet, the most important prey taxa, and the benthic community they rely on will result in a better understanding of fish stock dynamics and Ross Lake ecology and perhaps influence management of the fish stocks and lake levels in the future.
Western Washington University
Ross Lake (Wash. and B.C.)
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Derenne, Emily, "Dietary characteristics of juvenile trout and char in seasonally inundated stream segments in Ross Lake, Washington" (2014). WWU Graduate School Collection. 382.