You may have had the harrowing experience of accidentally pressing your car’s panic button, suddenly turning a quiet parking lot into an ear-piercing nightmare. When used intentionally, this tool is a crime deterrent, sending the bad guys running while you push through the loud sound to travel safely to your destination.
Researchers at the University of Minnesota Duluth are developing a car alarm, but for dams along the Mississippi River. These alarms will also deter bad guys – invasive fish called bigheaded carp. Bigheaded carp are already near La Crosse, WI and pushing northward up the Mississippi River. Invasive carp compete with native fish for food and can cut native populations by one-third. Some carp weigh more than two bowling balls and can jump 9 feet out of the water, threatening boaters with concussions.
Boats pass through gates in dams called locks, which carp secretly pass through as well. Noise may be one way that river managers can stop invasive carp from passing through.
In a recent study, researchers investigated the soundscape – like landscape, but with sound – of a lock along the Mississippi to hear the natural sounds underwater.
Sound might be especially helpful to stop carp while letting native species pass through. Carp are from a group of fish, called Otophysans, that have a great sense of hearing. This is because their swim bladder is directly connected to their inner ears. This connection makes them especially sensitive to certain sounds and compelled to move away. Future research will need to find sounds that target carp’s specialized hearing to reduce the impact on native species.
By monitoring sounds, the scientists learned how loud different types of boat traffic are in the lock. They can use this information later to make sure fish deterrents are loud enough that the carp hear them over background noise.
The team found that the normal background noise underwater in front of the lock gate is 111 decibels (dB re 1 μPa). Commercial vessels towing cargo produce 137 dB. Decibels are logarithmic, so every 10 dB increase is a tenfold increase in volume. Acoustic carp deterrents need to be louder than the water they’re in, especially during vessel passage.
Sound moves differently underwater than through air, and human ears evolved to hear best above water. Because only 1% of sound transfers from water to air, its unlikely humans will be able to notice the deterrent's noise.
The locks along the Mississippi might get louder in the coming years, but unlike your accidental car alarm, that’s a good thing for invasive species prevention.