Abstract

Spangled perch, Leiopotherapon unicolor, were exposed to concentrations of 5.0, 10 or 20 mg 1^-1 zinc, 0.1, 1.0 or 10 mg 1^-1 temephos or 0.1, 1.0 or 10 mg 1^-1 2,4-D at 25ºC for 2 h, during which time ventilation rate, heart rate and oxygen consumption were continuously monitored. Fish exposed to 10 and 20 mg 1^-1 zinc demonstrated a significant increase in ventilation rate; those exposed to 20 mg 1^-1 also developed a significant bradycardia. Exposure to 10 mg 1^-1 temephos caused an immediate reduction in ventilation rate and oxygen consumption, and also reduced heart rate during the second hour of exposure. Fish exposed to 10 mg 1^-1 temephos died within 12 h. Responses of fish to 2,4-D gave no indication of toxic effects on the cardio-respiratory system. Cardio-respiratory responses to zinc are attributed to damage caused to the gill epithelium, which results in impaired gas exchange and lowered oxygen tension in arterial blood. Effects of exposure to temephos correspond to cholinesterase inhibition in nerves supplying the respiratory musculature and the heart.