Acetylcholinesterase inhibition in federally endangered colorado squawfish exposed to carbaryl and malathion

Toxicity of technical carbaryl and malathion to endangered Colorado squawfish (Ptychocheilus lucius) was estimated by 24-h in vivo brain acetylcholinesterase-inhibition tests. Effect concentrations were estimated by ANOVA and an alternative procedure utilizing a linear-plateau regression model. NOECs were 29.3 μg/L carbaryl and 371 μg/L malathion. Threshold concentrations estimated by linear-plateau regression were 7.40 μg/L carbaryl and 150 μg/L malathion. NOECs showed that carbaryl was approximately 13 times more toxic than malathion. Relative potency of the two toxicants was confirmed by linear-plateau regression analysis: Threshold estimates showed that carbaryl was approximately 20 times more toxic than malathion. NOECs for acetylcholinesterase inhibition were 15 times lower for carbaryl, and four times lower for malathion, than NOECs estimated from growth and survival in 32-d early-life-stage tests. The difference between effect concentrations estimated by the two methods was attributed to development of tolerance in 32-d early-life-stage tests and greater sensitivity of biochemical processes compared to whole-organism responses.