Oral presentation

Deltamethrin, fenitrothion, and hexaconazole concentrations were determined in water and sediment samples collected from 22 30-day old intensive white shrimp Litopenaeus vannamei ponds in Soc Trang, Bac Lieu, and Ca Mau province. Water and sediment samples from 3 main water supply canals were also collected in provinces. Stocking densities of shrimp in ponds ranged from 60~110 shrimp m-2. Acute toxicity tests were conducted on the white shrimp L. vannamei with the sizes of post-larvae and juvenile (30 days of culture). Twenty shrimp were randomly assigned to each treatment and aquariums were aerated continuously to keep oxygen at adequate levels. Test solutions were renewed every 24 h in accordance with the static renewal method for toxicity tests. The experiment was conducted for 96 h and no feed was supplied to the shrimps during the test period. Safe levels and median lethal concentrations (LC50) were calculated following the described by Parker (1973) and APHA (1995). The study was carried out from 2012 Sep. to 2013 Dec.

The results showed that the concentration of fenitrothion residues were not detected in water samples collected from 18 ponds. The concentrations of deltamethrin residues in the water samples collected in Soc Trang ranged from 0.01~0.16 ppb while hexaconazole residues were 0.04~0.16 ppb. In Bac Lieu, the concentration of deltamethrin residues ranged between 0.03 and 0.17 ppb, and hexaconazole ranged from 0.12~0.32 ppb. Whereas, the water samples collected in Ca Mau showed the lowest concentration of deltamethrin and hexaconazole residues and only one pond had deltamethrin with a value of 1.13 ppb. Deltamethrin, fenitrothion, and hexaconazole pesticide residuals were not found in water supply canals. In the sediment samples, the highest concentration of deltamethrin residue was found in Soc Trang with a value of 8.0 ppb, while the highest concentration of fenitrothion and hexaconezole residues were found in Bac Lieu with the values of 9.04 and 27.0 ppb, respectively. For the L. vannamei post-larvae, safe levels of deltamethrin, fenitrothion, and hexaconazole were 0.023, 0.146, and 0.256 ppb, respectively and 96h-LC50 values were 0.231, 1.53, and 2.35 ppb, respectively. For the 30-day old L. vannamei juvenile, safe levels of deltamethrin, fenitrothion, and hexaconazole were 0.039, 0.201, and 0.506 ppb and the mean 96h-LC50 values were 0.83, 2.06, and 5.30 ppb, respectively. The obtained results revealed that concentration of deltamethrin residues in water were higher than safe level and showed the most toxic. The concentration of deltamethrin, fenitrothion, and hexaconazole residues in sediment of ponds were exceed safe levels, especially several ponds reached 205 times for deltamethrin, 45 times for fenitrothion, and 53.4 times for hexaconazole.

Keywords: Acute toxicity, deltamethrin, fenitrothion, hexaconazole, pesticide residue, white shrimp Litopenaeus vannamei