Acute And Joint Toxicity Of Chlorpyrifos And Cypermethrin To Tilapia (Oreochromis Andersonii) And Zebrafish (Danio Rerio)

Pesticides use will continue into the foreseeable future because there are not yet good replacements for all of their good uses.However,they negatively impact on non-target aquatic organisms.It is therefore important to establish tolerance levels of different aquatic organisms to different pesticides to regulate use of pesticides.Therefore,the study aimed to determine lethal concentration(96-hour LC₅₀)effects of chlorpyrifos（organophosphorous pesticide）and cypermethrin（pyrethroid pesticide）,together with their mixture on exposure to Threespot tilapia（Oreochromis andersonii）and Zebrafish（Danio rerio）juveniles.Single acute toxicity was conducted in four separate experiments in this study under semi-static conditions.Experiment 1 studied acute toxicity effects of chlorpyrifos on juvenile Threespot tilapia（Oreochromis andersonii）whereas Experiment 2 studied acute toxicity effects of chlorpyrifos on juvenile Zebrafish（Danio rerio）.Experiment 3 and 4 studied acute toxicity effects of cypermethrin to Oreochromis andersonii and Danio rerio respectively.All these experiments used the same research design.Seven different logarithmic concentrations were selected for each experiment with Oreochromis andersonii to chlorpyrifos using concentrations of 0,19.1,25.1,33.1,43.7,57.5,75.9 and 100μg/L while Danio rerio to chlorpyrifos used concentrations of 0,0.2,0.3,0.4,0.6,0.7,1.0,and 1.3 mg/L.For cypermethrin exposure,Oreochromis andersonii used concentrations of 0,4.5,6.3,8.9,12.6,17.8,25.1 and 35.5μg/L,whereas Danio rerio used concentrations of 0,3.2,4.2,5.4,7.1,9.2,12.0,and 15.7μg/L.A control（de-chlorinated tap water）accompanied every set of experiment where all experiments were run in triplicates.Fish（n=10）were randomly exposed to these concentrations to determine the lethal concentration(LC₅₀)of chlorpyrifos and cypermethrin on exposure to Oreochromis andersonii and Danio rerio.Experiment 1（chlorpyrifos to O.andersonii）gave a 96-hour LC₅₀ of 55.187μg/L（confidence limits;44.207-68.895μg/L）with a significant positive correlation（P<0.05）,whereas Experiment 2（chlorpyrifos to D.rerio）gave a 96-hour LC₅₀ of 0.213mg/L（confidence limits;0.166–0.274mg/L）with a significant positive correlation（P<0.05）as well.Experiment 3（cypermethrin to O.andersonii）and 4（cypermetrin to D.rerio）recorded96-hour LC₅₀ of 11.692μg/L（confidence limits;9.780-13.978μg/L）and 5.723μg/L（confidence limits;5.138–6.374μg/L）respectively,all with significant positive correlation（P<0.05）.There was generally observed dose and time-course behavioral alterations in relation to the levels of chlorpyrifos and cypermethrin concentrations for either of O.andersonii and D.rerio.Clinically,abnormal behavior signs including loss of equilibrium,abnormal swimming and abnormal ventilatory functions were observed with abnormal behavior presenting early in exposure of fish to cypermethrin（8.9μg/L in 24 hours for O.andersonii and 3.2μg/L in 72hours for D.rerio）.On the other hand,chlorpyrifos presented abnormal behavior in O.andersonii at concentration of 43.7μg/L within 24 hours while behavior of D.rerio was altered at 0.2 mg/L within 5.5 hours of the experiment.Cypermethrin therefore proved super toxic to both O.andersonii and D.rerio whereas chlorpyrifos proved super toxic to O.andersonii but highly toxic to D.rerio.Furthermore,acute joint toxicity effects of chlorpyrifos and cypermethrin to O.andersonii and D.rerio were determined in two separate experiments（Experiment 5 and 6respectively）under semi-static conditions.Both of these experiments used the same experimental design.Joint toxicity test was conducted using 96-hour LC₅₀’s of chlorpyrifos and cypermethrin obtained in the single acute toxicity tests with ratio of 1:1.In triplicates,either of the fish species（n=10）were randomly exposed to the different concentrations after acclamation.One bucket holding pesticide free medium（de-chlorinated tap water only）was kept to serve as a control for either species.Using the Additive Index（AI）,joint toxicity effect of chlorpyrifos and cypermethrin to O.andersonii was found to be antagonistic whereas synergistic to D.rerio.Moreover,the results showed abnormal behavioral patterns including loss of equilibrium,abnormal swimming and abnormal ventilatory function on exposure of either fish species to the mixture with dose and time-course behavioral alterations.To O.andersonii,abnormal behavior started showing after 48 hours of exposure at the least cypermethrin+chlorpyrifos mixture concentration of 4.45μg/L+21μg/L whereas to D.rerio,abnormalities manifested within 48 hours of the experiment at the least set cypermethrin+chlorpyrifos mixture concentration of 0.0016mg/L+0.0595mg/L.Overall,the results of the study indicated that chlorpyrifos and cypermethrin are toxic to O.andersonii and D.rerio with their toxicity varying amongst the fish species,dose and time of exposure.The study reflected a neutralized mixture of cypermethrin and chlorpyrifos on exposure to O.andersonii with a less than additive effect（antagonistic）,indicating a lower toxicity of the mixture than that of single pesticide applied individually.Interaction of cypermethrin and chlorpyrifos mixture on O.andersonii may have occurred at the same receptor site or the compounds detoxified by carboxylesterase activity through hydrolytic enzyme Carboxylesterases（b-esterase or non-specific esterases）.Furthermore,the study reflected a synergistic effect of cypermethrin and chlorpyrifos mixture on exposure to D.reio,indicating a higher toxicity of mixture than that of either pesticide applied individually.As an organophosphate,chlorpyrifos may have inhibited detoxification of cypermethrin（pyrethroid）and hence enhanced its toxicity to D.rerio.However,more research is needed to not only quantify acute toxicity thresholds of chlorpyrifos and cypermethrin for different fish species but also to establish their joint effects to different fish species.Studies on effects of chlorpyrifos and cypermethrin as individuals and in mixture on different biomarkers in O.andersonii and D.rerio are needed to confirm their effects to different target organs.