Density and biomass of redband trout relative to stream shading and temperature in southwestern Idaho

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Density and biomass of redband trout (Oncorhynchus mykiss gairdneri) relative to stream temperature were examined in headwater reaches of Big Jacks and Little Jacks Creeks in southwestern Idaho. Stream shading was greater (mean of 80% versus 46%) and solar insolation was lower (mean of 7.9 versus 15.1 mJ · m-2 · day-1) in Little Jacks Creek (P < 0.04); otherwise the 2 streams were similar (e.g., width, depth, gradient, median substrate size). Maximum water temperatures increased with distance from headwater springs in both streams (P ≤ 0.07) but increased more rapidly and to higher levels (24–26 °C) in Big Jacks Creek. Daily maximum water temperatures (23 km downstream of headwater springs) during July 1996 were lower in Little Jacks Creek (ranged from 18–22 °C) than in Big Jacks Creek (20.2–26 °C, P < 0.001). Daily temperature fluctuations also differed between streams, averaging 3.6 °C for Little Jacks Creek and 7.8 °C for Big Jacks Creek (P < 0.001). Redband trout density and biomass were greater in Little Jacks Creek (means of 0.8 fish · m-2 and 25.0 g · m-2) compared to Big Jacks Creek (0.3 fish · m-2 and 8.9 g · m-2, P = 0.01). Trout density was negatively correlated with increases in water temperature (P = 0.03) and solar insolation (P = 0.09) in both streams. Trout biomass increased with stream shading and was negatively correlated with solar insolation (P < 0.1). Warmer water temperatures in Big Jacks Creek were likely due to historical summerlong livestock grazing, which drastically reduced riparian shading.

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ELECTRONIC FILE - Zoology

SWAP (2/19/2016) citation:
Zoellick BW. 2004. Density and biomass of redband trout relative to stream shading and temperature in southwestern Idaho. Western North American Naturalist. 64(1):18–26.