Zoellick examined the density and biomass of redband trout (Oncorhynchus mykiss gairdneri), relative to stream temperature, in the headwater reaches of Big Jacks and Little Jacks Creeks, in southwestern Idaho. Initially stream shading was greater and solar insolation was lower in Little Jacks Creek; otherwise the two streams were similar. Maximum water temperatures increased with distance from headwater springs in both streams, but increased more rapidly, and to higher levels, in Big Jacks Creek. Redband trout density and biomass were greater in Little Jacks Creek compared to Big Jacks. Trout density was negatively correlated with increases in water temperature and solar insolation in both streams. Trout biomass increased with stream shading and was negatively correlated with solar insolation. Warmer water temperatures in Big Jacks Creek were likely due to historical season-long livestock grazing, which drastically reduced riparian shading. Idaho water quality regulations prescribe that water temperatures not exceed 22 degrees C, with a maximum daily average of less than or equal to 19 degrees C. During 1996-1997, water temperatures in Little Jacks Creek met these criteria, while they were not met in the lower reaches of Big Jacks Creek. Differences in temperature and trout abundance between the 2 streams indicate Idaho's cold-water criteria were appropriate for protecting redband trout populations.
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