Understanding thermal habitat use by migratory fish has been limited by difficulties in matching fish locations with water temperatures. To describe spatial and temporal patterns of thermal habitat use by migratory adult bull trout, Salvelinus confluentus, that spawn in the Lostine River, Oregon, we employed a combination of archival temperature tags, radio tags, and thermographs. We also compared temperatures of the tagged fish to ambient water temperatures to determine if the fish were using thermal refuges. The timing and temperatures at which fish moved upstream from overwintering areas to spawning locations varied considerably among individuals. The annual maximum 7-day average daily maximum (7DADM) temperatures of tagged fish were 16-18 degrees C and potentially as high as 21 degrees C. Maximum 7DADM ambient water temperatures within the range of tagged fish during summer were 18-25 degrees C. However, there was no evidence of the tagged fish using localized cold water refuges. Tagged fish appeared to spawn at 7DADM temperatures of 7-14 degrees C. Maximum 7DADM temperatures of tagged fish and ambient temperatures at the onset of the spawning period in late August were 11-18 degrees C. Water temperatures in most of the upper Lostine River used for spawning and rearing appear to be largely natural since there has been little development, whereas downstream reaches used by migratory bull trout are heavily diverted for irrigation. Although the population effects of these temperatures are unknown, summer temperatures and the higher temperatures observed for spawning fish appear to be at or above the upper range of suitability reported for the species.
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