Tuna is attuned to tin

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  • Published: Jan 14, 2004
  • Author: Steve Down
  • Channels: Gas Chromatography
thumbnail image: Tuna is attuned to tin

Keeping your bottom clean can be a problem, especially if you are in the middle of the ocean and you are a boat. The traditional method uses anti-fouling paint that contains tin compounds but this is being banned due to widespread aquatic pollution. The extent of this contamination is difficult to measure but a new study has proposed that levels of tin compounds in the skipjack tuna can be used as a monitor.

Whether you are piloting a small pleasure craft in the local harbour or a mighty ocean-going vessel, you want the passage through the water to be as smooth as possible. The motion of the ship is affected by several factors. Choppy or rough seas will slow the boat down but so will the growth of barnacles and algae on the hull. It has been estimated that that fouling of hulls can create so much turbulence for the moving ship that fuel consumption is increased by up to 30%.

The most common remedy has been the application of anti-fouling paints to the hulls of boats. In the early days of sailing, lime and arsenic were used as coatings but in recent times, the active ingredient is generally a simple organic chemical called tributyltin (TBT). Various estimates suggest that 75-90% of the world?s boats have been treated with TBT. It is an effective agent that prevents attachment of aquatic organisms but it has an unwanted side effect. TBT slowly leaches into the water, killing other sea life and entering the food chain.

In these times of increased pollution awareness and responsibility, the dangers of TBT have been recognised and the International Maritime Organisation agreed a convention in 2001 banning the use of organo-tin compounds in anti-fouling paints. The new convention was agreed by 25 States representing 25% of the world's merchant shipping tonnage.

Even when TBT usage declines, we need to know how this affects TBT levels in the waters and marine life. Clearly, this is a global problem and difficult to investigate but a multi-national research project has enlisted the help of a common fish. The skipjack tuna is distributed in offshore waters all over the world, making it an ideal candidate as a live biomonitor of TBT pollution.

The work was led by Shinsuke Tanabe and colleagues from the Centre for Marine Environmental Studies at Ehime University in Japan and involved seven other organisations (listed below) around the world. Tuna were collected from the North Pacific Ocean and from waters near various Asian countries, the Seychelles and Brazil. Organo-tin compounds including TBT were extracted from the various organs and tissue of the fish and analysed by gas chromatography with flame photometric detection (GC-FPD) after conversion to their propyl derivatives.

The results are published in Environ. Pollut. 2004, 127, 1. Among all butyltin compounds, TBT was the dominant pollutant found. The tuna liver proved to be a reliable indicator of the amount of TBT present, regardless of the sex and length of the fish, or its migration path in the sea. The amounts found in fish from a particular region will reflect the pollution levels in the water where they were caught.

Based on these criteria, the researchers discovered pollution on a global scale from butyltins, most of which was TBT. In particular, high levels were found in fish from the waters around Japan (up to 400 ng/g or parts-per-billion) and other developing Asian countries such as the Philippines and Indonesia.

The total amounts of tin from all types of compounds were also determined in the fish, using inductively coupled plasma mass spectrometry (ICP MS). These figures revealed that anthropogenic butyltins are the major source of tin in tuna, making up nearly 90% of the total tin present. Since most of the butyltins found are TBT, cutting out their use will have a colossal effect on aquatic tin pollution.

Other types of tin-less anti-fouling paints have been developed. Some of these contain copper, which can also be released into the water, so their use must also be questioned.

A group at Cornell University is experimenting with polymer-based anti-fouling paints which are dual functional. They minimise adhesion but also enable the hulls to become self-cleaning when the boats are in motion.

In the meantime, the expected fall in levels of TBT in the marine environment can be followed using the unusual sampling medium of skipjack tuna.

The research centres involved were:

Dirty propellor
Why anti-fouling measures are required

Skipjack tuna
Skipjack tuna collecting TBT (Image: US Fish and Wildlife Service)

The global distribution of skipjack tuna
The global distribution of skipjack tuna - shown in red ( Image: Florida Museum of Natural History)

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