Underwater Hitch Hikers on teh Bottom of Boats
Yacht owners know that removing fouling organisms from the hull of their boat can be expensive and laborious. If left unchecked, these growths significantly increase drag on the boat, affect its performance in the water and, ultimately, its fuel consumption. What many yacht owners may not know, however, is that by regularly maintaining their boat, they can prevent the spread of harmful marine species that have been introduced into Australian waters from overseas.
There are around 170 exotic species in Australian waters, many of which are thought to have arrived on the hulls of ships. Some of these species pose a significant threat to Australia's native marine biota as well as to aquaculture. The earliest invaders arrived on the hulls of wooden sailing vessels more than 100 years ago. Despite significant changes in ship design and speed, exotic species are still being found on the hulls of modern cargo and fishing vessels. But no-one has looked at the hulls of recreational vessels.
As many as 900 private yachts enter Australian waters each year and moor alongside local vessels in marinas and sheltered harbours. The arriving yachts can also carry exotic marine species on their hulls. Given appropriate environmental conditions, the exotic species can be introduced and establish in the new locations. There are about 16,000 recreational vessels, more than 6 metres long in Queensland. These local vessels can help to spread the invaders around the Australian coast.
This was demonstrated in March 1999 when large numbers of the highly invasive black striped mussel, Mytilopsis sp., were found on hulls of recreational vessels as well as on permanent structures in three Darwin marinas. It appears that the black striped mussel was introduced to Darwin by an overseas yacht five months prior to its discovery in the local marinas. At present, there are no quarantine requirements for hull fouling on private yachts (the Darwin region is a recent exception).
The black striped mussel is native to the eastern Pacific Ocean but has invaded the waters of Fiji, India, Japan and Taiwan. Its discovery in Darwin caused concern because it is closely related to the zebra mussel which was introduced from Europe into freshwater systems of northern America. The zebra mussel chokes waterways and pipes and occupies significant proportions of habitat formerly occupied by native biota. Removal of the zebra mussels and remedial engineering costs US $ 600 million annually.
Oliver Floerl from James Cook University in Townsville, Queensland, Graeme Inglis from the National Institute of Water and Atmospheric Research in Christchurch, New Zealand, and Vicki Hall from CRC Reef Research Centre Ltd, are trying to help yacht owners reduce the problem of hull fouling and the potential for spreading introduced species by examining factors that contribute to the growths on boats' hulls.
An important part of the research is to identify the interactions between the fouling communities on the boats and those living in the surrounding marina. The floating docks and pilings of an average-sized marina provide several thousand square metres of space for organisms to grow on, and may be the major source of species that foul resident boats.
The research shows that vessels moored within a given marina develop fouling communities that are characteristic of the assemblages that grow on pilings and structures in that marina. On average, the cover of fouling organisms on the boats increases by about 10% for every five months that they spend moored and uncleaned within tropical waters. This means that, despite the use of modern antifouling paints, marine organisms cover about half of the submerged surfaces of the boat within two years of being painted. The longer the boat goes without being repainted, the more species it shares with the surrounding marina and the greater risk it poses for spreading these species to other locations.
The researchers have found that resident fouling communities differ between marinas that are a relatively short sailing distance apart (~300 km). This means that boats travelling around the Australian coast can carry fouling species with them that may not occur in the marina at their destination.
The design of some marinas may even aggravate the problem. For example, many marinas are enclosed by solid breakwalls that protect the boats from wind and waves. Others, particularly those located in sheltered embayments or estuaries, lack breakwalls. Preliminary studies on water movement in the different types of marinas show that the breakwalls of enclosed marinas can create circular eddies that retain water for much longer than marinas without breakwaters. This can lead to an entrapment of the planktonic larvae of fouling species within enclosed marinas. Rates of recruitment by fouling organisms were found to be between 2 to 9 times greater in these enclosed environments than in unenclosed marinas, and between 3 to 19 times greater than in adjacent coastal environments.
These results have two major implications. For boat owners, they suggest that maintenance will be required more often in marinas with poor tidal flushing, because fouling is considerably greater in these environments. Second, because water circulation in enclosed marinas appears to trap marine larvae for a significant period of time, the spread of established introduced species from the marina may depend more on patterns of vessel movement than on larval dispersion by ocean currents.
With the help of marina operators and boat owners, this ongoing research will also test the effectiveness of different types of antifouling paints in tropical waters. The researchers are also compiling information on the movement patterns and maintenance of privately-owned vessels to identify risk factors for the spread of introduced species.
By Oliver Floerl, Graeme Inglis and Vicki Hall
Oliver Floerl is a PhD student in the School of Tropical Environment Studies and Geography at James Cook University. His research is supported by JCU, the Australian Quarantine and Inspection Service, and Akzo Nobel International Epiglass Paints.
Dr Graeme Inglis is a marine scientist with the National Institute of Water and Atmospheric Research in New Zealand.
Dr Vicki Hall is a marine scientist and former Postgraduate Coordinator at CRC Reef Research Centre Ltd.