Mortality Curves of Bluegills (Lepomis macrochirus Rafinesque) Simultaneously Exposed to Temperature and Zinc Stress

Bluegills were exposed to a temperature-zinc interaction consisting of five temperature regimes and four concentrations of zinc which were 1.0, 5.6, 10.0 and 32.0 mg Zn++/1. All of the above concentrations of zinc and a control were tested on five groups of fish which had been acclimated to (1) 20 ± 1 C, (2) 30 ± 1 C and fish which had been acclimated to 20 ± 1 C and were exposed to temperature increases at the following rates: (3) 1 C/24 hours, (4) 1 C/1 hour and (5) 1.5 C/10 minutes until a maximum of 30 C was reached. This study showed that a rise in temperature could reduce the survival time of bluegills exposed to lethal and sublethal concentrations of zinc. Reductions in survival time were a function of the rates at which the temperature was increased. For example, bluegills acclimated to 20 and 30 and then exposed to a lethal concentration of zinc (32 mg Zn++/1) at each temperature died 2.6 times faster at 30 C than at 20 C. Fish acclimated to 20 C and exposed to a temperature increase of 1.5 C/10 minutes died 4.8 times faster than fish maintained at 20 C. A sublethal zinc concentration (5.6 mg Zn++/1) produced no mortalities in 20 C acclimated bluegills in 96 hours but was toxic to fish when the temperature was increased from 20 to 30 C at a rate of 1.5 C/10 minutes.