Identification of a sensitive index during fish bioassay of an azo dye methyl red (untreated and treated).

Acute and chronic toxicity of methyl red (untreated) was examined on a freshwater fish Poecilia reticulata, using indices viz; mortality, reduction in RBC counts and their morphological abnormality (poikilocytosis and anisocytosis). Similar studies (acute toxicity) were also made in physicochemically and biologically treated methyl red. Data comparison of these four indices revealed poikilocytosis as the most sensitive index, since it measured higher toxicity of methyl red when fish mortality was either minimum at its low concentration (5 ppm) during both acute and chronic toxicity or even nil in the biologically treated 100 ppm methyl red, during acute toxicity. Mortality was next to poikilocytosis though it ranked 1st at higher concentration of methyl red during acute toxicity. The reduction in RBC counts however, was found to be the most sensitive parameter only in case of prolonged exposure (4 weeks) to 5 ppm methyl red. Amongst the four indices used for quantifying toxicity; anisocytosis was found to be the least expressive. Based on these findings we recommend quantification of data on fish mortality and poikilocytosis during acute toxicity whereas reduction in RBC counts and poikilocytosis during chronic exposure to methyl red.