Thermal resistance of juvenile salmonids sublethally exposed to nickel, determined by the critical thermal maximum method

Abstract

The thermal resistance of juvenile coho salmon (Oncorhynchus kisutch) and rainbow trout (Salmo gairdneri) sublethally exposed to nickel was subsequently examined by the critical thermal maximum (CTM) method. Tests were conducted at a temperature increase rate of 6°C/h, usually 24 to 48 h after chemical exposure. Exposure and initial test temperatures were 15°C for coho salmon and 8°C for rainbow trout. Comparisons with unexposed controls were based on time and temperature of loss of equilibrium and death. Exposure to as low as 1·5 mg/litre nickel for 7 to 21 days significantly suppressed resistance of rainbow trout to elevated temperatures, but 28-day exposure to 0·9 mg/litre nickel did not. Suppressed thermal resistance corresponded to a twofold increase of nickel burden in gill and liver tissue of exposed rainbow trout. Resistance was also suppressed for coho salmon surviving a 14-day exposure to higher, lethal nickel concentrations, but this was not statistically validated. Since sublethal exposure to nickel, one of the least toxic of the metals, lowered fish resistance to elevated temperatures, the same response might be induced by the more toxic metals. One deviant test showed lowered thermal resistance in rainbow trout exposed to 0·06 mg/litre chlorine for 14 days. The CTM method is an efficient and sensitive way of quantifying the thermal resistance of fish sublethally exposed to chemical stressors.