Fourteen sampling sites were selected along the Mekong river. The first sites is the Golden Triangle, (20o 20' 8'' N, 100o 04' 48'' E) Chiang Saen district, Chiang Rai province. The end of the sampling sites is at the Kaeng Sa-phue, (15o 14'
12'' N, 105o 14' 63'' E) Phibun Mangsahan district, Ubon Rachathani province (Figure 1, Table 1).