Dogwhelks are a good indicator of the contamination of the hazardous substance called tributyltin (TBT) which used to be used in anti-fouling paints on the hulls of large ships. Dog whelks can be used to assess the levels of TBT as it has been shown that high levels of this pollutant in the water has led to deformities in the reproductive organs of female dogwhelks. These deformities are known as imposex and the larger the concentration of TBT in the surrounding waters then the more significant the deformity of the dog whelk. This can eventually lead to the decline in populations due to the pollutant affecting the dogwhelks ability to reproduce.
Although not specifically referred to in the WFD, the measurement of imposex is the practical implementation of the use of an indicator species under the general benthic invertebrate quality element.
The method statement published in October 2008 for the first cycle of river basin planning has now been updated for the second cycle, linked below.