Organotins are chemical compounds that are organic derivatives of tin. A major source of organotins in the marine environment is the antifouling paints applied on ship hulls. Organotin compounds (chemically, they are characterized by the presence of at least one covalent carbon-tin bond) have been used as biocides in antifouling paints applied on hulls of ships and boats and, in general, on surfaces in prolonged contact with seawater to control the growth of fouling organisms.
As organotins are highly effective against most fouling organisms, their application resulted in enormous savings to the shipping industry. In fact, organotin-based, in particular tri-substituted organic compounds of tin (tributyltin, TBT), paints are described as the most effective antifouling paints ever devised for use on boat hulls. TBT has also been described as the most toxic substance ever introduced into the marine environment, especially because it is directly released into the aquatic environment. The high toxicity of TBT, together with its tendency to get accumulated in marine organisms, can adversely affect marine organisms, particularly mollusks and gastropods.
Environmental impacts of TBT use
The first evidence of environmental damage by TBT appeared in aquaculture farms in Arcachon (France), where, from 1975 to 1982, oyster production was severely reduced due to lack of reproduction and appearance of shell calcification anomalies in adult oysters. Decline in gastropod population has been registered worldwide, which is a direct consequence of the induction of imposex effect by TBT, an effect characterized by superimposition of male sexual characteristics on female organisms. Imposex has been shown to occur extensively in the North Sea, Atlantic Ocean, and Mediterranean Sea in Europe as well as along the coasts of USA, Japan, India, Australia, Chile, and so on. Apart from inducing reproductive failure, TBT also produces various shell anomalies such as shell thickening, gel formation, and chambering. Shell thickening that has been observed in TBT contamination can also result in various developmental changes. It acts as an inhibitor for many enzymatic systems and can disturb the energy metabolism of bivalves. Several studies show that TBT tends to accumulate in the liver, blubber, hair, and various other tissues and organs of certain species of marine mammals. TBT accumulates through the food chain, resulting in the occurrence of this compound as well as its breakdown products in fish, squid, shellfish, and in top predators such as whales, dolphins, seals, and fish-eating birds. Contaminated fish, oyster, mussel, and clams may pose substantial threat to human health. Prolonged exposure to organotins may cause liver and kidney damage in human beings.
Regulating TBT use
Consequential environmental impacts of TBT led to national regulation of TBT in many countries. By late 1980s, most of the European countries had restricted the use of organotin-based antifouling paints. This problem was brought to the notice of MEPC (Marine Environment Protection Committee) of the IMO (International Marine Organization) in 1988. After long deliberations, the AFS Convention was adopted on 5 October 2001. The Convention prohibits the use of harmful organotins in antifouling paints used on ships. It has entered into force on 17 September 2008.
However, there are certain gaps in effective implementation of the AFS Convention and /or in controlling impacts of organotins. Though the use of organotin compounds in antifouling paints is prohibited, safer and effective alternatives with global approval are not available as yet. Under the current form of the AFS Convention, restrictions on organotin compounds are applicable to ship hulls or external surfaces only. Regulation of organotin-based paints on fishnets and cages as well as organotin contamination through ballast water still needs to be regulated through the AFS Convention.
Similarly, there is a need for an international mechanism to prohibit sale and production of TBT-based paints at the global level. In the absence of domestic regulations, these paints may find market in many developing countries, which are not yet signatories to the AFS Convention. Further, organtoins are used extensively as heat and light stabilizers in the manufacture of polyvinyl chloride (PVC) polymers and as industrial catalysts for polyurethane and silicone elastomers. They are also extensively used in industrial and agricultural biocides such as fungicides, bactericide, miticides and insecticide as well as in surface disinfectants and preservatives for wood, paper, textiles, paints and some electrical equipments. Though antifouling paints is the major source of organotins in the marine environment, organotins from some of these applications also find their way in the marine environment. No international regulations exist for use of organotins in such applications at global level.
Furthermore, clear technical guidelines for the disposal of organotin-contaminated waste generated during dry-docking activities of ships are required. Similarly, there needs to be a clear law assigning responsibility of waste disposal to the generator of the waste, that is, the flag state or the original owner state. Adequate capabilities are lacking for monitoring of TBT application on ship hull in port states and flag states that belong to developing or less developed nations. Regular monitoring surveys are required on TBT contamination in these developing countries. Similarly, efforts for capacity building of authorities handling inspection, such as port state control officer, are required at the international level. To ensure that the port states of the developing world are not unduly pressurized during inspection of ships, a mechanism at the international level needs to be given serious consideration. Finally, a constant dialogue between science and policy is required to make competent and informed decisions, for effective implementation of these decisions and for communicating updated information and capacity building of relevant stakeholders.
Detailed information on organotins in the marine environments is published in a special issue titled ‘Implications of organotins in the marine environment and their prohibition’ of the Journal of Environmental Management (Feb 2009; Vol 90).