The effect of summer intermittency on stream physicochemistry, macroinvertebrate abundance, and redband trout, Oncorhynchus mykiss gairdneri, condition in an Idaho stream

Regional climate change models for the Pacific Northwest, USA, predict increased summer streamflow intermittency. During low summer flows, fishes experience abiotic (e.g., low dissolved oxygen, high temperature) and biotic (e.g., competition, predation) stressors that can alter individual fish health, population persistence, and fish assemblage structure. In this observational study, we evaluated the impact of decreasing summer streamflow on water physicochemistry, macroinvertebrate abundance, and condition of resident (stream-dwelling) Columbia River redband trout (Oncorhynchus mykiss gairdneri) in Dry Creek, an intermittent stream in southwestern Idaho, USA. During the summer (June to September) of 2019, we used Onset®, HOBO® loggers to monitor stream temperature and dissolved oxygen at 30-min intervals in 10 study reaches within a 3.5 ​km stream section. Current velocity and habitat volume were measured weekly and drifting macroinvertebrates were sampled biweekly in each study reach. Redband trout were collected monthly and we used Fulton’s condition factor (K) as a surrogate for trout health. During the study period, current velocity and habitat volume decreased. Mean daily water temperatures (6.4–22.3 ​°C) were within the reported range for redband trout. However, in late summer, dissolved oxygen fell below the critical limit (5–6 ​mg/L) for salmonids. Macroinvertebrate drift density (number of macroinvertebrates/L) and macroinvertebrate drift biomass (mg/L) did not differ across summer months. However, macroinvertebrate drift rate (number of macroinvertebrates/15 ​min) was significantly lower in August than June or July (p ​< ​0.05). Median K for trout was lower in August (1.01) and September (0.91) than in July (1.25) (p ​< ​0.05). We propose that the decline in redband trout condition was because of intermittency-associated stressors (hypoxic conditions and decreased macroinvertebrate prey).