Upper Temperature Tolerance of Loach Minnow under Acute, Chronic, and Fluctuating Thermal Regimes

Abstract

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 32°C and 38°C, depending on the method and exposure time; however, temperatures as low as 28°C resulted in slowed growth compared with the control groups. In LTM trials, we increased temperatures 0.3°C/min and death occurred at 36.8 ± 0.2°C (mean ± SE) for fish (37–49 mm total length) acclimated to 30°C and at 36.4 ± 0.07°C for fish acclimated to 25°C. In CLM trials, temperatures were increased more slowly (1°C/d), allowing fish to acclimate. Mean temperature at death was 33.4 ± 0.1°C for fish 25–35 mm and 32.9 ± 0.4°C 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 32°C and the 30-d temperature lethal to 50% of the test animals was 30.6°C. Growth at static 28°C and 30°C was slower than growth at 25°C, suggesting that fish were stressed at sublethal temperatures. In ACE trials with diel temperature fluctuations of 4, 6, and 10°C and a 32°C peak temperature, over 80% of fish (20–40 mm) survived 30 d. Although brief exposures to 32°C were not lethal, the growth of fish in the three fluctuating-temperature treatments was significantly less than the growth at the ambient temperature (25–29°C). To minimize thermal stress and buffer against temperature spikes, we recommend that loach minnow habitat be managed to avoid water temperatures above 28°C.