Scientists at NIWA and Auckland University have discovered that the fouling of vessels by marine creatures is greatly increased by the underwater sounds generated by the vessels themselves.
Marine fouling caused by the growth of marine creatures on hulls is a huge cost to the shipping industry due to the increased drag on ships' hulls. There are also implications for marine biosecurity as organisms found on the hulls of vessels may be transported all around the world. Millions of dollars are spent each year on attempts to control fouling on commercial vessels.
It is known that sound triggers the larvae of many coastal organisms to settle more rapidly. For instance, fish and crab larvae are attracted toward the underwater sound of waves breaking on coastal reefs and noises produced by other reef-dwelling organisms during feeding.
Sound travels long distances through water. It is not affected by water currents, wave action or the clarity of the water so it provides a much more reliable settlement cue for marine organisms than sight or chemicals. In some areas, underwater sound from human activities in the marine environment is increasing by two to four times each decade, largely as a result of increased international vessel traffic.
Scientists conducted laboratory experiments to find out whether the sounds that ships and their equipment make underwater are attracting fouling organisms, such as mussels.
"We recorded the noise generated by a range of different vessels while they were in port in Wellington, including NIWA's Tangaroa and commercial vessels such as log transport ships, container ships and cruise ships," says NIWA biosecurity scientist Dr Serena Wilkens.
The scientists put a hydrophone (an underwater microphone) into the water next to the ships' hulls while they were berthed in port, and recorded the sound intensity and frequency from the ships' generators. Generators still run when ships are in port and they appear to produce a large amount of underwater noise.
In a controlled laboratory environment at the Leigh Marine Laboratory, the ship sounds were played to mussel larvae at two different intensities using submersible speakers. Another group of larvae were not exposed to noise and were kept in silent tanks. The larvae were at a pre-settlement stage, which means that they were swimming in the water, looking for a place to settle and attach. The experiment ran over several hours and larvae were monitored to detect when they had settled onto the bottom of their tank.
"We found that the mussel larvae exposed to the high intensity vessel sound settled and metamorphosed a lot quicker than the ones in the silent treatment; significantly quicker," says Dr Wilkens.
Dr Wilkens says that based on analysis of the data, the scientists can draw the conclusion that the vessel noise caused a higher percentage of mussel larvae to settle, and caused it to happen a lot quicker.
The mussel larvae settled very quickly - within a few hours. This is within the timeframe that the larvae would be exposed to the noise from a generator in a vessel in port. The results from this research indicated that underwater sound produced by vessels may be an important factor in promoting hull fouling by mussels.
The NIWA and Auckland University scientists are hoping to suggest ways of reducing the underwater noise produced by ships, such as dampening or eliminating sound production by vessels or where possible switching to shore-based electrical supply, while berthed. The results of their research were published recently in Biofouling, a world-leading journal for researchers working to understand and prevent fouling.
This research was funded by NIWA, the University of Auckland, and the Glenn Family Foundation.
