Assessing the impact of antifouling compounds in the marine environment. Lessons to be learned from the use and misuse of biological indicators of TBT contamination

The condition known as imposer in whelks has been used widely as a biological indicator of tributyltin (TBT) contamination in the marine environment. It has provided valuable information on the extent and impact of contamination. It has also been used successfully in monitoring changes in levels of contamination since regulations, limiting the use of TBT-based antifoulants to vessels >25 m in length, were introduced. However, its misuse has also generated misleading information. There have been at least four reasons for this. First, sampling has been biased on 'hot-spots' of contamination, giving a false impression of the severity of the problem. Second, imposer is not, as has often been assumed, a specific response to TBT. Third, insufficient attention has been given to the longevity and habits of indicator species. Fourth, predictions of extinctions of species, which have been based on assessments of imposer, have not been fulfilled. It is argued that, while biological indicators should play key roles in assessing the impacts of pollutants, rigorous protocols are needed.