(Semi-static bioassay flow, a simple equipment for measuring acult toxicity upon small tropical fishesfrom fresh Waters)

Abstract

A simple and practical technique for measuring acute toxicity in small tropical fishes is described. The equipment used, which has been developed to measure toxicity due to any toxic substance, was helpful in the evaluation of variation in physic-chemical parameters with precision and sensitivity in samples taken from treatments consisting of mercury nitrate in concentrations of 0; 100; 150; 175; 200; 225 and 250 µgHg.L^-1, every 24 hours during the entire bioassay. Due to the low values of D.M.S. (Minimal Significant Differences) both Fischer and Tukey tests showed differences between blocks and also between treatments. However, these differences were not biologically important, falling within an acceptable variability. After sampling, 5/6 of the initial volume were replaced by an identical solution previously prepared. The fishes remained in good health in the aquaria during water replacement. No death was observed in the control and it is therefore possible to state that lethality was due only to toxic substances.