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

Abstract This study, investigated the ability of Paraquat (herbicide) and Dichlorvos (insecticide) to elicit avoidance response in population of Guppy fish, Poecilia reticulata and predict the population immediate decline (PID) of P. reticulata when exposed to both agro-chemicals. 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 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 mg/L) and Dichlorvos (0, 1.6, 2.6, 3.0, 3.2 and 4.0 mg/L). Their distributions were examined at 20 min intervals for a 3-h period (n = 9 observation periods). The results from 96-h FS were does-dependent with highest percentage mortality being 85.7% for Paraquat and 78.6% for Dichlorvos in respective highest concentrations. The 3-h NFS exposure showed statistically significant concentration-dependent spatial avoidance. The P. reticulata avoided the lowest concentration of Paraquat (2.5mg/L) and Dichlorvos (1.6 mg/L) 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 mg/L). The 3-hr AC50 value revealed that the guppies were more sensitive to Paraquat (0.37 mg/L) than Dichlorvos (1.73 mg/L). The PID was driven by the avoidance behaviour (NFS) of the guppies rather than mortality (FS). The FS heretofore overestimate environmental risk assessment of contamination due to mandatory exposure of mobile organisms to contaminants. Rather than the traditional FS systems, NFS should be adopted and incorporated in typical bioassays for better predictive results.