TBT-pollution in the Gulf of Thailand: a re-inspection of imposex incidence after 10 years.

Imposex in neogastropods was used to determine the relative TBT distribution in the Gulf of Thailand in 2006. To identify the imposex prevalence, 8757 specimens, belonging to 22 species from five families of neogastropods, were collected at 56 sites in 13 sample areas. These areas were located between the Bight of Bangkok in the north and the Malaysian border in the south. A contamination model was developed to compensate for differences in TBT sensitivity among species and to make comparisons among areas. At every area imposex was found in neogastropods. The highest incidence was in the east side of the Bight of Bangkok off Si Racha and Pattaya and in the southern part around Pattani. The same areas showed the highest frequency of imposex in 1996. While the frequency of imposex appeared to have slightly decreased in these areas, increases were found elsewhere. In Pattani Province this could be related to dumping of highly polluted harbour sediments in the shallow coastal area. The overall frequency of imposex in the Gulf of Thailand significantly increased from 1996 to 2006.