3.6 ANTI-FOULING PAINTS

Anti-fouling paints are used on the hulls of ships to prevent the build-up of fouling organisms³⁵ Since the mid-1970s the most effective means of maintaining clean hulls has been by using anti-fouling paints that contain Tri(n-butyl)tin (TBT)³⁶ Prior to this, copper-oxide based paints were more commonly used³⁷ Both copper and TBT are biocides which are dispersed throughout the paint and are eventually released at the paint surface to inhibit settlement of organisms³⁸ The main advantage of TBT based paints over copper-oxide based paints is their longevity. TBT paints provide fouling protection for five or more years, while copper-oxide based paints only last from 2-3 years³⁹

PLATE 1
Fouling on a square intake near vessel stern

Source: CSIRO

PLATE 2
Hull fouling near vessel stern

Source: CSIRO
Modern TBT paints are said to be `self polishing' - a chemical reaction between the paint and the water results in a continual dissolving of the paint film, and a slow release of the biocide into the water⁴⁰

The benefit of TBT-based paints in preventing hull fouling, and therefore in reducing the transfer of exotic marine organisms, is counterbalanced by concerns about TBT's high toxicity. TBT is harmful to marine life at less than 1 part per billion; indeed, TBT has been described as the most toxic substance ever deliberately introduced into the ocean⁴¹ In various countries, including Australia, France and the UK, malformations in oysters have been attributed to TBT⁴² It is believed that TBT from the hulls of small craft, which tend to be present in large numbers in estuaries and small harbours near where shellfish are harvested, was the cause of these malformations, and of declining productivity in estuarine systems around the world⁴³ Consequently, most countries, including Australia, have banned the use of TBT paints on vessels less than 25m in length, and on marine and estuarine structures such as piers and buoys⁴⁴ In Victoria, the application and purchase of TBT paint requires written approval from the EPA, and must be carried out by certified applicators⁴⁵ Although Japan, New Zealand and the USA, have placed a total ban on TBT application, the Committee notes that ships coated with TBT paints are still permitted to visit these countries. There is pressure from the IMO to phase out the use of TBT paints around the world within 10 years⁴⁶

Copper-oxide based paints, though currently considered the best alternative to TBT paints, remain problematic. They may cause corrosion to aluminium hulls; have a shorter life; demand regular in-water cleaning of hulls; and, though less toxic than TBT, are still highly toxic to marine organisms⁴⁷ A promising option in the future may be the use of fouling release coatings. These are non-toxic, non-stick coatings (silicone based) which weaken the adhesive bond between fouling organisms and the hull, so that organisms are removed as the vessel moves through the water⁴⁸

A consultant's report to ANZECC has recommended that no further restrictions be placed on TBT usage in Australia until suitable alternatives are developed⁴⁹ It has been suggested that restricting the use of TBT to large vessels will limit environmental damage to areas around ports and ship yards⁵⁰

Having a small area affected by TBT paints may be a trade-off for introducing biological species that spread everywhere.

The Committee notes that in a 1989 study, TBT levels at various sites in Port Phillip Bay, Western Port Bay, and the Gippsland Lakes were considerably higher than levels known to be damaging to aquatic organisms⁵¹ The highest levels of TBT were found at marinas and slipways, and despite bans placed in 1989 on the use of TBT on the hulls of recreational vessels, monitoring in 1990/91 failed to demonstrate any decline in TBT levels⁵² A 1993 study of TBT levels in Port Phillip Bay, however, which used morphological changes in marine snails as a bioindicator of TBT contamination, showed that TBT had declined to negligible levels at all 10 locations surveyed⁵³

The Committee believes that, despite the effectiveness of TBT in preventing hull fouling, TBT does not offer a long-term solution to the problem of exotic marine organism transfer via hulls, due to problems of toxicity. The Committee recognises that current alternatives to TBT are not as effective, nor as durable, and that this increases the likelihood of hull fouling and the need for more frequent in-water hull cleaning, thereby increasing the risk of the translocation of exotic marine species.