Avoidance Response: A More Important Determinant of Population Immediate Decline in Guppy Fish (Poecilia reticulata Peters, 1859) Exposed to Pesticides

This study investigated the ability of Paraquat (herbicide) and Dichlorvos (insecticide) to elicit avoidance response in the population of Guppy fish, Poecilia reticulata and predict the population immediate decline (PID) of P. reticulata when exposed to both agrochemicals. A 96-h forced system (FS) bioassay was each conducted in five duplicate systems, each with a control experiment. The avoidance response was examined using a non-forced multi-compartmented static system (NFS). The guppies (n = 3 guppies per concentration of 6 compartments in quadruplet) were exposed to a gradient of Paraquat (0, 2.5, 5.0, 7.5, 10.0 and 15.0 mgL-1) and Dichlorvos (0, 1.6, 2.6, 3.0, 3.2 and 4.0 mgL-1). Their distributions were examined at 20 min intervals for a 3-h period (n=9observation periods). The results from 96-h FS were dose-dependent with the highest percentage mortality being 85.7% for Paraquat and 78.6% for Dichlorvos in their respective highest concentrations. The 3-h NFS exposure showed statistically significant concentration-dependent spatial avoidance. The P. reticulata avoided the lowest concentration of Paraquat (2.5mgL-1) and Dichlorvos (1.6 mgL-1) by 68.3% and 48.3% respectively. The avoidance increased significantly (p < 0.005) to 75%, for the highest concentrations of both test chemicals (15.0 and 4.0 mgL-1). The 3-hr AC50 value revealed that the guppies were more sensitive to Paraquat (0.37 mgL-1) than Dichlorvos (1.73 mgL-1). The PID was driven by the avoidance behavior (NFS) of the guppies rather than mortality (FS). The FS heretofore overestimate the environmental risk of contamination due to mandatory exposure of mobile organisms to contaminants. Rather than the traditional FS systems, NFS should be adopted and incorporated into typical bioassays for better predictive results.