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Any surface placed in the ocean soon gains a cover of bacteria, algae and seaweeds, and animals such as barnacles, and this is known as fouling. Fouling is a serious problem for vessels large and small because it causes loss of speed and costly increases in fuel. Static structures such as pilings, the legs of offshore oil and gas platforms, and the cages of fish farms are rendered dangerously susceptible to wave damage by heavy fouling. Cleaning the hull of a modern merchant ship is an expensive and laborious task, and it is better to deter the organisms from settling in the first place. Anodic protection by attaching copper sheathing to the hull is termed ‘sacrificial’ because the protection it gives results from its slowly dissolving and introducing toxic ions into the water flowing across the hull. This was particularly effective in protecting wooden-hulled ships against infestations of teredo worm. Marine paints were then developed that also leached toxic substances, the most effective of which contained tributyltins (or TBTs). However, TBTs proved to be so persistent in the sea that they caused large and unacceptable changes in the faunas of ports and harbours, and began to spread further into the open sea. One of their effects (among many) was to cause ‘imposex’ in dog whelks (Nucella spp.), turning them all into sterile intermediates between male and female. The International Maritime Organization's International Convention on the Control of Harmful Anti-fouling Systems on Ships was adopted in 2001, and using marine paints containing TBTs is now widely banned, but unfortunately not universally so. Traces of TBTs are still found contaminating marine organisms, even ones we eat.

So why are animals like fish and whales rarely fouled? The answer seems to be because they are continually sloughing their skins so any fouling drops off; now new non-toxic paints are being developed that slough in much the same way.

M. V. Angel

Subjects: Maritime History.

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