Have whelks disappeared from Mull? Or, if they haven’t disappeared, they are playing a very good game of hide-and-seek – and for this they have good reason.
Whelks, not to be confused with winkles, live in the sublittoral zone and littoral fringe and cannot therefore survive the lower salinity of the intertidal zone – the best chance of seeing them is at the lowest tides. However, even when diving local wilderness shores I rarely spot one.
One theory is that Tributyltin (TBT), a chemical compound used around the world in ship hull paint, is killing them. For 40 years TBT was used as a biocide in marine anti-fouling paint (commonly known as bottom paint), which was applied to the hulls of ocean going vessels to reduce the growth of barnacles and other organic matter. It should be of no surprise therefore that it does what it says on the tin and the use of TBT negatively affects bio-diversity for that is its very purpose.
Bottom paint is a cheap way to stop barnacles, weeds and algae from growing on a ships hull as all of these can adversely affect a ships structure, hydro-dynamics and performance. The invention of the copper-bottom was introduced to British warships of the line with similar effect – indeed the phrase ‘copper-bottomed’, when used nowadays with reference to the reliability of a venture or investment, stands testament to the effectiveness of the process. Sir Humphrey Davy of the Royal Navy pioneered the lining of its wooden ship’s hulls, prone to fouling by barnacle growth, with copper plating during the 1880s. Britain was fighting the French, the Spanish and the Dutch navies at once and all three were heavily defeated – British ships were faster, more manoeuvrable and could stay out at sea for longer.
However, lining a ship with copper is expensive, never more so than today and so is painting a ship’s hull each year. So TBT was cheap and effective but the chemical leaches into the marine environment where it is highly toxic to a wide range of organisms and its pollution led to the collapse of whole populations of organisms.
The International Maritime Organisation now bans TBT compounds. These bans first started in the 1980s on boats less than 25 metres long and the use of biocide compounds in anti-fouling paint was completely banned in 2008 by the International Convention on the Control of Harmful Anti-fouling Systems on Ships. It states that ships cannot bear organotin compounds on their hulls or external parts or surfaces unless there is a coating that forms a barrier so that organotin compounds cannot leach out. TBT will most likely be present in the water column and sediment for up to twenty years because of its long half-life.
TBT anti-fouling paints are still being used in countries with poor regulation enforcement, such as in the Caribbean so I don’t expect Whelks are very happy over there. Over here the search for whelks continues.