The sampling map in the Mekong Delta (•: Sampling area; 1: Duyen Hai, Tra Vinh, 2: Cu Lao Dung, Soc Trang, 3: Tran De, Soc Trang, 4: Hoa Binh, Bac Lieu, 5: Dong Hai, Bac Lieu, and 6: Dam Doi, Ca Mau).