All sailors know that a boat’s dirty bottom will knock knots off your speed, increase fuel consumption and clog impellers – but now, according to a study in New Zealand, foul-bottomed pleasure boats are also being held responsible for introducing invasive pests and alien species into marine ecosystems.

A report by MAF Biosecurity, a New Zealand government agency, highlights the case of the Australian Sea Squirt, Eudistoma elongatum, which has run rampant in the waters of the Northland area of New Zealand. It was believed to have been introduced by hitching a ride on the hulls of ocean-going ships and yachts.

The agency has encouraged Kiwi boat owners to regularly clean their hulls, especially before sailing long distances. A spokeswoman for Biosecurity, Lesley Patston, said “Ensure your antifouling is in good condition and reapplied as recommended by the manufacturer. Particular attention should be paid to areas such as the keel, intakes and outtakes, propellers and shafts, rudders and casings. These are prime areas for harbouring pests. Finally, check your boat is clean before you move location. If it’s not, clean it!”

Australia already has rigorous measures in place, requiring newly arrived vessels to have a clean, uncontaminated bottom. The discharge of contaminated ballast water from ships, which is a major problem throughout the world, is being tackled by the signing of an international agreement forcing ships to treat water before it is discharged. In the UK, ships have been held responsible for the spread of the Zebra Mussel, which has become widespread in recent years.