We used a lethal thermal method to estimate the upper thermal tolerances of 11 native and 7 nonnative fish species found throughout southern Arizona. Fish were acclimated to 258C and 308C. For all species tested, an increase in acclimation temperature resulted in a higher thermal tolerance value. Among the species acclimated to 258C, desert pupfish Cyprinodon macularius, western mosquitofish Gambusia affinis, and Gila topminnow Poeciliopsis occidentalis were most tolerant to high temperature. Speckled dace Rhinichthys osculus, spikedace Meda fulgida, and loach minnow R. cobitis were least tolerant. Many native species demonstrated a limited ability to extend their upper temperature tolerances via acclimation. Our data suggest that several native species may be sensitive to increasing annual and large daily temperature fluctuations in Arizona's streams and rivers. Although southwestern native fishes were previously believed to be tolerant to high temperature due to their evolution in desert environments, this study suggests that many of these fishes are less tolerant than previously thought. In addition, many fishes introduced from the eastern United States had higher temperature tolerances than some of the native desert species tested. Increases in stream temperatures in Arizona could reduce the habitat available for native fishes and therefore may favor those nonnative species with higher thermal tolerances.

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We used four methods to estimate the upper lethal temperature of loach minnow Rhinichthys cobitis: the lethal thermal method (LTM), chronic lethal method (CLM), acclimated chronic exposure (ACE) method with static temperatures, and ACE method with diel temperature fluctuations. The upper lethal temperature of this species ranged between 328C and 388C, depending on the method and exposure time; however, temperatures as low as 288C resulted in slowed growth compared with the control groups. In LTM trials, we increased temperatures 0.38C/min and death occurred at 36.8 6 0.28C (mean 6 SE) for fish (37-49 mm total length) acclimated to 308C and at 36.4 6 0.078C for fish acclimated to 258C. In CLM trials, temperatures were increased more slowly (18C/d), allowing fish to acclimate. Mean temperature at death was 33.4 6 0.18C for fish 25-35 mm and 32.9 6 0.48C for fish 45-50 mm. In the ACE experiment with static temperatures, we exposed fish for 30 d to four constant temperatures. No fish (20-40 mm) survived beyond 30 d at 328C and the 30-d temperature lethal to 50% of the test animals was 30.68C. Growth at static 288C and 308C was slower than growth at 258C, suggesting that fish were stressed at sublethal temperatures. In ACE trials with diel temperature fluctuations of 4, 6, and 108C and a 328C peak temperature, over 80% of fish (20-40 mm) survived 30 d. Although brief exposures to 328C were not lethal, the growth of fish in the three fluctuating-temperature treatments was significantly less than the growth at the ambient temperature (25-298C). To minimize thermal stress and buffer against temperature spikes, we recommend that loach minnow habitat be managed to avoid water temperatures above 288C.

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