Carp must be Australia's most hated fish. Introduced to the River Murray about 20 years ago, they have become such an environmental threat that they now bear the nickname of "river rabbits."
Carp are bottom feeders, stirring up mud, engulfing invertebrates in their large mouths and dislodging weeds, which float to the surface and die. This makes the fish particularly troublesome in the shallow waters of wetlands that border the river.
The Murray's flow was always episodic, and these wetlands periodically flooded and drained; a natural cycle that prompted native trees to germinate and native birds and fish to breed. The erection of locks and weirs ended that cycle, and the river is now a series of ponds. Only a fraction of its normal flow remains, its mouth has all but closed, and many of its wetlands are now dustbowls.
Some, however, are being reclaimed. River water is diverted to flood them artificially, then drained months later to simulate droughts. But carp come with the water. Some wetland access points are fitted with carp screens, which admit carp no larger than a finger. These die when the wetlands are drained and before they can grow large enough to cause problems, but excluding tiny carp means excluding most native fish as well. Other gates, built with larger mesh, admit larger carp.
Civil and Environmental Engineering students at Adelaide University have been researching a series of deterrents which they hope may selectively prevent carp from entering wetlands under rehabilitation.
"We feel that we might be able to use one of our deterrents to exclude or at least reduce carp movement into wetlands, and that one of these behavioural methods might not affect native fish so much," said Amy Ide, one of the team of Honours students.
The researchers are examining a range of options, including light, sound, bubble curtains and physical barriers. A giant flume in the department is flooded with water, and carp introduced to it, their behaviour monitored as each test is run.
"We found that light can be a barrier to carp movement, and it has the potential to be a barrier to them moving into wetlands," said Ms Ide. So does sound. Through an underwater speaker the researchers subject the fish to a range of sounds. "After testing frequencies from zero to a thousand Hertz (cycles per second), we found that carp were deterred most by 20 Hertz," said Nadine Kelly. "We think that maybe native fish won't be deterred as much by this frequency, so it might prove to be a species specific barrier," she said.
Tim Champion has been exploring the effectiveness of bubble curtains. "These have been tried overseas with some success to prevent fish from entering the openings of hydroelectric stations," said Mr Champion. "We thought we'd try to see how carp responded to them, and they do seem to act as a deterrent to their movement," he said.
Startle reactions among the carp showed that these deterrents are more effective when they are turned on suddenly, rather than being constantly applied. They can also be used together, and appear to enhance each other in repelling carp. But bubble curtains, lights and hydrophones rely on electricity, and their effectiveness is limited in regions without a power supply.
The researchers were encouraged to find that, of all the deterrents, a simple low physical barrier proved the most effective, the prototype being nothing more sophisticated than chicken wire. A physical barrier's effectiveness may well relate to the bottom-dwelling habit of carp, and be less of a deterrent to native fish which swim more freely the open water. It is this demand for selectivity that makes the application of these deterrents so challenging.
The researchers have also been developing an electronic detector that can register the movement of fish through a point of entry to a wetland. It consists of three electrodes running across the waterway. As a fish passes over them, the electric field between them is affected, revealing the direction and speed of a fish's movement as a trace on a monitoring computer screen. A wave generator at one end of the flume can even create choppy conditions for the trials.
"The detector lets us work out what time of day or night fish are moving in and out of these wetland structures so that we can get a better ideas of when, and at what time of year, to implement these control devices.," said James Cox. "We won't want the devices to be turned on all the time, and there are going to be particular periods when carp are moving," he said. "If we can work out when those times are, we can activate the deterrent devices then."
Using the detectors in conjunction with other devices, even to activate them, may prove especially valuable where electricity supply is limited to the small output available from batteries or solar devices.
"If we had these detectors just a small distance before the deterrent devices they could be triggered to turn on and it might startle the fish more, as we saw with the bubble curtain," said James Cox. "It worked a lot more effectively if we turned it on just as the fish were approaching and, with a detector, it would be quite simple to do that automatically, he said, "But we need to find that it doesn't have the same effect on native fish."
Materials provided by Adelaide University. Note: Content may be edited for style and length.
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