Sir

One of the most toxic substances ever deliberately put into the sea, the marine-paint additive tributyltin (TBT), has potentially entered its final chapter. An International Maritime Organization (IMO) convention aimed at phasing out its use on vessels came into effect in September, a year after the minimum requirements for ratification were reached. However, will this International Convention on the Control of Harmful Antifouling Systems on Ships see an end to the most efficient antifoulant ever produced?

Since the 1960s, TBT has been used as a biocide in antifouling paints on the hulls of vessels. But the discovery of toxic effects in non-target species, especially molluscs, led to the global restriction of this chemical, in the late 1980s, to vessels that were more than 25 metres in length. These restrictions seemed to be successful, with molluscan populations returning to the levels they were before TBT usage began, in all but heavily polluted areas.

However, the IMO sought a complete ban on TBT. For this to be ratified, 25% of member states — representing 25% of the world's shipping tonnage — needed to be signatories. These double requirements were met when Panama signed in September last year, ending a quest that had lasted more than a decade.

Why was acceptance so slow and the signatory states so reluctant? For a start, there are no suitable alternatives to TBT. Copper, the next best realistic alternative, does not have the widespread biocidal properties of TBT and requires more frequent application, meaning more time spent by the vessel out of the water and greater expense.

Another reason is that most of the world's shipping sails under 'flags of convenience'. This Law of the Sea loophole means that shipping companies can register their vessels in countries where costs are lower and shipping laws, including restrictions on TBT, are only loosely policed. Motivation to sign the antifouling convention is limited in the case of countries providing these 'flags'. Now that the convention has finally come into effect, it may still prove difficult to get the signatory states to implement and enforce domestic legislation.

Regrettably, TBT's effects may continue to be felt for some time to come.