Influence of Maximum Water Temperature on Occurrence of Lahontan Cutthroat Trout within Streams

Abstract We measured water temperature at 87 sites in six streams in two different years (1998 and 1999) to test for association with the occurrence of Lahontan cutthroat trout Oncorhynchus clarki henshawi. Because laboratory studies suggest that Lahontan cutthroat trout begin to show signs of acute stress at warm (>22°C) temperatures, we focused on the maximum daily temperature. The maximum daily temperature associated with the occurrence of Lahontan cutthroat trout ranged from 18.9°C to 28.5°C. Occurrence was more likely at colder (<26°C) sites. In two streams, the maximum daily temperature associated with the occurrence of these trout was relatively cool (≤20°C). In these streams, the response to temperature may have been modified by extreme declines in streamflow in late summer. The air temperatures measured during the study were not consistently related to maximum water temperatures, but air temperature is useful for predicting the distribution of Lahontan cutthroat trout at larger scales (e.g., the geographic pattern of fish distribution among streams as distinct from occurrence within streams). The results of this work suggest a link between the thermal tolerance of individuals in the laboratory and the occurrence of fish within streams and a provide a mechanism for the observed geographic patterns in the distribution of local populations of Lahontan cutthroat trout.