Redband Trout (Oncorhynchus Mykiss) Diet and Foraging Behavior in an Intermittent Stream

AUTHORS: Makenna Doramus, Collin Glass, Hayden Krause, Aleah Mendiola, and Andrew Wymore
FACULTY: Dr. Chris Walser
DEPARTMENT: Environmental Studies

ABSTRACT

Redband trout (Oncorhynchus mykiss) is native to the interior Columbia River drainage including Dry Creek in southwestern Idaho. Dry Creek has perennial headwaters but middle and lower reaches experience summer intermittency. Climate models for the Pacific Northwest predict a decrease in summer flows due to climate change. Declines in stream flow can have major impacts on stream biota. The objective of this study was to examine the impact of a changing flow regime on redband trout foraging behavior and diet. We chose three mid-reach study sites. Hydrologic conditions in Dry Creek were categorized as flowing (lotic) or nonflowing (lentic). We used GoPro cameras to collect videography of fish foraging behavior. We gathered approximately 10 hours of fish behavior footage. Terrestrial, benthic, and pelagic macroinvertebrates were collected to measure availability of trout prey. We used gastric lavage to determine trout diet. Over the study period, the density of benthic macroinvertebrate prey remained stable, while terrestrial and pelagic macroinvertebrate densities decreased. Trout fed four times less often during lentic conditions than during lotic conditions. During lotic conditions, trout fed primarily in the drift (pelagic + surface) with terrestrial macroinvertebrates composing 40% of their diet. During lentic conditions trout fed significantly less at the surface and equally as often in the benthic and pelagic zones. Terrestrial macroinvertebrates comprised just 15% of trout diet during lentic conditions. We contribute the change in trout foraging behavior and diet to decreased flows, which reduced the availability of drift macroinvertebrates and caused stressful water quality conditions.