The individual and combined acute lethal toxicity of selected biocides on the juveniles of rohu (Labeo rohita) under tropical conditions

Abstract

In the natural ecosystems, fishes are exposed to more than one contaminant at a given time. The present study is an attempt to understand the individual and combined acute lethal toxicity of the common biocides - malathion, methyl parathion, endosulfan and 2, 4-D on early juveniles of rohu under laboratory conditions. The 48 h LC50 values of the biocides were: malathion-7.89 mg.l–1 (7.28–8.61); methyl parathion - 7.34 mg.1–1 (7.25–7.43); endosulfan - 0.0036 mg.1–1
(0.0025–0.0047) and 2, 4–D - 962.43 mg.1–1 (954.02–970.81). The average "additive index"
value and the average "magnification factor" for the different biocide pairs were: malathion –2, 4–D (0.218 and X 1.22); endosulfan –2, 4–D (–0.073 and X 0.93); malathion - methyl parathion (0.24 and X 1.24) and malathion-endosulfan (1.648 and X 2.65). It was found that 2, 4–D (chlorophenoxy herbicide) is "moderately toxic", malathion and methyl parathion (organophosphate insecticides) are "toxic" and endosulfan (organochlorine insecticide) is "very
toxic" to rohu juveniles under static (with renewal) 48 h LC50 bioassay. The combined toxicity tests indicated the "strictly additive" nature of the insecticide-weedicide combinations and the "more than additive" nature of the insecticide-insecticide combinations. The simultaneous or even asynchronous application of these chemicals in the paddy fields and plantations thus greatly increase their potential to be toxic in freshwater and coastal ecosystems of the tropics.