Resistance of three freshwater fishes to fluctuating thermal environments

by DeHart, Douglas Alan

The resistance to elevated temperatures of three species of freshwater fishes, Oncorhynchus kisutch, Rhinichthys osculus nubilus, and Cottus perplexus was studied. Rock Creek and Greasy Creek, Benton County, Oregon, were surveyed to evaluate the nature of thermal stress encountered in the small cold water stream environment.

Resistance to constant temperature was determined experimentally by measuring time to death at fixed levels. R. osculus and C. perplexus were found to be more resistant to elevated temperatures than O. kisutch. For fish acclimated to 20° C the incipient lethal level was found to be 25° C for C. perplexus, and 29.5° C for R. osculus.

An apparatus was constructed to simulate the diel temperature regime observed in the study streams. The resistance of the three species to dielly fluctuating temperature was explored. The portion of the time temperature curve above the incipient lethal level was determined to be the critical region in relation to thermal resistance. Evidence was found for accumulation of thermal effects over periods of several days. Median resistance of O. kisutch exposed to 6 hours over their incipient lethal temperature per diel cycle with a diel maximum of 27° C was 1.5 cycles. A method of predicting resistance to cyclic exposures to elevated temperature was developed.

Implications of the nature of thermal resistance in the temperature regime in small streams are discussed.