Sediment samples were collected during the dry season in the Tha Chin River (March; n ¼ 19) and in the Mun River (JanuaryeFebruary; n ¼ 30), and during the wet season in the Tha Chin River (June; n ¼ 12) (Fig. 1). In addition, six reference sediment samples were collected from a location in the Mun River isolated from the aquaculture farms. This area was assumed not to be impacted by anthropogenic activities and was considered as the ‘non-polluted’ reference site (Fig. 1). For a description of the antibiotic sample collection methods see the Supporting information.