The principal organotins of concern are compounds of tributyltin (TBT). However, triphenyltins have also been used in antifoulant paints.

Tributyltin oxide (TBTO) is the commercially available active ingredient. TBTO is poorly soluble in water, varying between <1 and >100 mg l^-1, depending on the pH, temperature and anions present in the water. In seawater and under normal conditions, TBT exists as three species (hydroxide, chloride and carbonate) which remain in equilibrium. The log Kow is 3.54 for seawater indicating that it partitions to the organic solvent and adsorbs strongly to particulate matter (WHO 1990).

TBT compounds have been registered as molluscicides, antifoulants (on boats, ships, quays, buoys, crab pots, fish nets and cages), wood preservatives (see Section B33), slimicides on masonry, disinfectants, and as biocides for cooling systems, power station cooling towers, pulp and paper mills, breweries, leather processing and textile mills (WHO 1990).

There are four main ways in which TBTs can enter the marine environment:

• during the application of the antifouling paint to boats or cage fish farm nets;
• leaching from paint on the hulls of vessels or from cage fish farm nets;
• when paint is removed;
• when paint remnants are discarded.

Controls on the use of TBT in antifouling paints were introduced in 1986 when the sale of TBT-based paints was banned. In 1987, the use of TBT-based paints on boats under 25 m and mariculture equipment was also prohibited. Further controls are proposed. These measures have reduced the potential routes of entry into the marine environment and successfully reduced environmental concentrations (Waite et al 1991). However, because TBT is strongly adsorbed to organic matter, the sediments retain concentrations of TBT which can be mobilised when the sediment is disturbed.

TBT is found in the water column, sediments and the biota. Higher levels have been reported close to pleasure boating activity, especially in or near marinas, boat yards and dry docks (WHO 1990). In Scotland, high levels have been reported in sea lochs where cage fish farming occurs. WHO (1990) and Waite et al (1991) reported concentrations in the water column, sediment and biota but these were for measurements taken in the late 1980s and WHO (1990) indicated that 'older' measurements may not be comparable with >newer= measurements because of advances in analytical techniques.

Michel and Averty (1999) report concentrations of TBT in the water column at marinas and commercial and military harbours on the French coast of the English channel in 1997 ranging from 0.0017 to 0.0877 mg l^-1 and 0.0023 to 0.280 mg l^-1 respectively. The majority of these measurements are above the EQS in the UK (0.002 mg l^-1).

Recent monitoring of TBT in the marine environment has been undertaken by using biological effects monitoring. Gibbs et al (1987) conclusively linked the condition of imposex in gastropod molluscs to the presence of organotins in the environment. This phenomenon was first observed in dogwhelks. The measurement of the degree of imposex in this species has developed into an effective monitoring technique for organotins which is cheaper and more sensitive than measuring chemical determinands (MPMMG 1998). This technique was included in the National Monitoring Programme and results were presented from Northern Ireland for 1994 in MPMMG (1998). Dogwhelks from sites within Belfast Lough showed clear signs of the imposex phenomenon, indicating the continuing presence of organotins. For the North Sea coast of the UK, similar results were derived from the TBT Imposex Survey of the North Sea (Harding et al 1997 cited in MPMMG 1998). This survey indicated that imposex remained widespread throughout the North Sea and English Channel from Shetland to Land's End. Harding et al (1998) report results for Western Coastal areas (including the UK west coast and coast of Northern Ireland and the Irish Republic) sampled in 1997 and concluded that the effects of TBT pollution could be observed over most of the area. Only at sites very close to sources of pollution (commercial and fishing harbours) could severe imposex effects be observed and at sites where TBT was once used (marinas and cage fish farm locations) measures of imposex could not be distinguished from background.

CEFAS (1998) report concentrations of monobutyltin (MBT), dibutyltin (DBT) and TBT in the livers of porpoises and grey seals found stranded around the coast of England and Wales between 1992 and 1996. Concentrations of total butyltins (the sum of MBT, DBT and TBT) ranged from 22 to 640 mg kg^-1 wet weight in porpoise and 3 to 22 mg kg^-1 wet weight in grey seals. DBT was the dominant form of organotin in the liver.

The main removal process for TBT in the water column is through adsorption onto particles. WHO (1990) suggested that between 10 and 95% of TBTO was estimated to undergo particle adsorption.

Progressive disappearance of adsorbed TBT is due to degradation. Biodegradation by micro-organisms and metabolism by higher organisms are probably the dominant processes. Biodegradation depends on environmental conditions, such as temperature, oxygenation, pH, level of mineral elements, the presence of easily biodegradable organic substances for co-metabolism and the nature of the microflora (WHO 1990). It also depends on the TBT concentration being below the lethal or inhibitory threshold of the micro-organisms. TBT is broken down into DBT and MBT. Half-lives for TBT in the environment vary widely (WHO 1990).

An exhaustive literature review on the toxicity of organotins to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (Zabel et al 1988, WHO 1990). The most sensitive groups of organisms have been identified.

TBT is very toxic to algae, molluscs, crustacea and fish. TBT has been identified as an endocrine disrupting substance (Environment Agency 1998) with observable effects in gastropod molluscs and suggested effects in marine mammals (CEFAS 1998).

The development of the motile spores of a green macroalga was impaired by exposure to TBT (5-day EC50 of 0.001 mg l^-1) and this stage was considered to be the most sensitive. The growth of a marine angiosperm was reduced at TBT concentrations of 1 mg kg^-1 of sediment but no effect were observed at 0.1 mg kg^-1.

Marine molluscs have been observed to undergo a number of changes in response to exposure to sub-lethal concentrations of TBT, including shell deposition of growing oyster, gonadal development and gender of adult oysters, settlement, growth and mortality of larval oysters and other bivalves and to cause imposex in female gastropods (e.g. Smith 1981, Waldock 1986, Zabel et al 1988, WHO 1990). Credible adverse effects have been observed in saltwaters organisms in laboratory tests at concentrations as low as 0.01mg l^-1 (Zabel et al 1988). The NOEL for spat of the most sensitive oyster species Crassostrea gigas has been reported to be in the region of 0.002 mg l^-1 (WHO 1990).

Imposex is a condition where the female gastropod develops a penis and a vas deferens which in severe cases block the genital pore causing reproductive failure and premature death. WHO (1990) report a NOEL for the development of imposex in female dogwhelks of below 0.0015 mg l-1 of TBT. Imposex effects have been observed in whelks Buccinum undatum and Neptunea antiqua collected from the west coast of Scotland in 1997. These effects were observed 10 years after the ban on the use of TBT on small craft and cage fish farm nets and the effects were attributed to TBT accumulated in the sediment. No sediment concentrations of TBT were measured and the degree of imposex was not considered to be affecting the fishery potential of these species (Poloczanska and Ansell 1999).

Copepods are the most sensitive group of crustaceans, with 96-hour LC50s ranging from 0.6 to 2.2 mg l^-1 (WHO 1990). TBT reduces reproductive performance, neonate survival and juvenile growth in crustaceans.

The toxicity of TBT to marine fish is highly variable, with 96-hour LC50s ranging from 1.5 to 36 mg l^-1 with larval stages more sensitive than adults.

The EQS for TBT is 0.002 mg l^-1 in the water column.

Rees et al (1999) and Waldock et al (1999) studied the epifaunal and infaunal benthic communities respectively in the River Crouch, SE England in relation to decreasing environmental concentrations of TBT in water and sediment between 1986 and 1992. In both cases, changes in faunal composition could not be conclusively linked to decreasing TBT concentrations but faunal diversity increased at sites in the upper estuary where TBT levels were greater in 1987 and had been significantly reduced by 1992.

More recent studies have been conducted to determine the extent of organotin contamination in the tissues of marine mammals. TBT is bioaccumulative and the ability of cetaceans to metabolise TBT is considered to be low (CEFAS 1998). Various studies have highlighted butyltin accumulation in a range of cetacean species from Japan and the north Pacific Ocean. Recent data from CEFAS on the contamination of porpoises and seals from around England and Wales have demonstrated that low-level organotin contamination of marine mammals occurs around the UK (CEFAS 1998). However, CEFAS was unable to assess the significance of their findings and stressed that further information was needed on the threats facing these animals from possible toxic and bioaccumulation effects.

TBT bioaccumulates in organisms because of its solubility in fat. WHO (1990) reported BCFs of up to 7,000 in laboratory investigations with molluscs and fish and higher values have been reported from field studies. Uptake from food is more important than uptake directly from the water.

Potential effects include:

• toxicity to algae, invertebrates (especially molluscs) and fish of TBT in the water column at concentrations above the EQS of 0.002 mg l^-1;
• toxicity to algae, invertebrates (especially molluscs) and fish of triphenyltin and its derivatives in the water column at concentrations above the EQS of 0.008 mg l^-1;
• the presence of TBT is monitored by the degree of imposex in female gastropod molluscs rather than by direct measurements of concentrations of organotins in the water column or the sediments. Where imposex is detected within a European marine site, a precautionary approach should be adopted in the control of TBT and activities associated with its use (e.g. pleasure boating, cage fish farming);
• accumulation of TBT in sediments which could induce imposex in gastropod molluscs after several years;
• bioaccumulation of TBT in the food chain posing a potential hazard to birds and Annex II sea mammals;
• endocrine disruption impacts of TBT in invertebrates, fish and potentially Annex II sea mammals.

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