Tributyltin antifoulant hazardous to aquatic ecosystems

The pesticide tributyltin (TBT) is a highly toxic chemical used as a general lumber preservative and slimicide (a chemical toxic to bacteria and fungi). However, the use of tributyltin in antifouling paints on the hulls of ships and boats poses one of the greatest environmental concerns, leading to regulatory aftion in a number of countries, including Canada.

Antifouling uses of tributyltin began in the 1960s and 1970s. Its extreme toxicity to some organisms became evident in the mid 1970s. The organism most sensitive to the substance is the female dogwhelk Nucella lapillus, a small marine snail. At concentrations as low as half a nanogram (ng) per litre, female dogwhelks start to grow a penis which eventually blocks the oviduct, preventing reproduction, and leading to population declines and local extinction. This phenomenon is called "imposex," the imposition of male sexual characteristics on females of the species, and it is one of the more dramatic examples of the so-called "endocrine disruption" phenomena. In sediments, tributyltin is also extremely toxic, with population-level effects in several organisms noted at concentrations as low as 40 ng per gram dry weight.

France was the first country to restrict the use of tributyltin in antifouling paints after French scientists observed, as early as 1975, that growth rates for oysters in some Atlantic coastal areas were declining. By the early 1980s effects on oysters in Arcachon Bay in France were linked to tributyltin, and in 1982 France banned the use of TBT-containing antifouling paints on vessels less than 25 metres in length.

Many other countries, including Canada in 1989, adopted the same regulations and another one stating tributyltin can be used on vessels greater than 25 m in length, if the release rate is less than 4 micrograms of tributyltin per square centimetre of hull surface per day. Antifouling coatings in Canada are regulated under the Pest Control Products Act, administered by Health Canada.

Canada is one of several countries monitoring the effectiveness of tributyltin regulations. Environment Canada's National Water Research Institute (NWRI) conducted a survey of water and sediment from across Canada in 1993-1994. The main conclusion was that the regulation had been only partially effective, having some effect in reducing tributyltin concentrations in freshwater, but not in sea water.

There was less effect in reducing TBT concentrations in sediment, where it likely persists longer. In many locations the concentration was high enough to cause acute and chronic toxicity to aquatic and benthic (living on the water bottom) organisms. In some areas, tributyltin from contaminated sediment may transfer back into the water column. It also appeared that large harbours handling ships legally painted with tributyltin antifouling paints continued to experience significant contamination. This situation was substantially the same in 1997 in some of the hotspots which NWRI continues to monitor.

It is expected that the International Maritime Organization will ban all antifouling uses of tributyltin by 2006, and possibly earlier. The organization and member countries are exploring several replacement antifoulants. One of these, Irgarol 1051, has been registered for use (with copper-containing formulations) in some European countries for several years, and registration is expected soon in the United States, and possibly Canada.

Irgarol 1051 is fairly persistent in water, and there are concerns that concentrations in some European coastal waters may pose hazards to sensitive phytoplankton. NWRI has conducted research on the persistence and fate of Irgarol 1051 in water, and is undertaking baseline studies of large Canadian harbours to assess the potential impact of Irgarol 1051 leaching from ships painted in other jurisdictions.

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