Why does the US Navy listen to the sound of giant groupers and crayfish?

Whale skeletons lying around the coast of Fuerteventura in the Canary Islands are a stark reminder of the harmful effects of military sonar. Sonar systems from boats are believed to be one of the factors that cause whales to lose their direction and run aground, according to the BBC.

This marine-unfriendly technology could soon be replaced. Lori Adornato, project specialist at the military research agency DARPA, thinks that it is possible to use nature sounds to detect objects under the sea instead of sonar pulses.

'Now we treat all this natural sound as noise and try to get rid of it. But why don't we take advantage of these sounds to detect potential dangers,' said Ms. Adornato.

Illustration of a marine object detection system.

Outstanding advantage

The Marine Life Monitoring Fixed Sensors (PALS) project led by Ms. Adornato was developed to take full advantage of the natural activities of organisms in the aquatic environment to detect threats.

Aircraft-dropped sonars only work for a few hours over a small area. In contrast, a PALS system can cover a large area for several months. Therefore, PALS is capable of providing near-constant data.

Ms. Adornato says species that live only on a certain reef are the best 'guards'. PALS is funding a number of research groups that aim to find approaches to different reefs.

'You want to make sure your creatures are always there,' Ms Adornato said.

Laurent Cherubin, principal investigator of the Grouper Guard group at Florida Atlantic University, works with giant Atlantic groupers. This fish can weigh up to 300 kg and make loud noises to deter intruders.

The giant Atlantic grouper attracts many divers to Florida waters.

'Their voices are very loud and have low frequencies. The giant grouper is territorial and will emit loud calls to intimidate intruders,' Cherubin said.

A vocal grouper can be detected from 800m away. However, not all grouper calls have a threatening purpose. This fish also uses calls to attract mates and some other mysterious purposes.

Cherubin's research team focused on alarm calls directed at intruders. Distinguishing this cry is not easy, so the team set up machine learning algorithms to do the job. The machine learning system is trained by listening to thousands of recordings until it distinguishes between different types of chirping.

The algorithm can then be developed into software integrated into the small processors on audio receivers. A receiver can cover a reef, record grouper calls and track moving objects.

Biological sonar from shrimps

The PALS team's work at defense corporation Raytheon is closer to traditional anti-submarine sonar. However, the study here has a special point.

'We are trying to detect the echo caused by the object reflecting the crayfish noise. It works similarly to a traditional sonar system,' said Alison Laferriere, a researcher at Raytheon.

In other words, Raytheon's system works like normal sonars. But instead of using artificial sound waves, the system uses noise produced by shrimp.

The crayfish is known as the noisiest species in the ocean. The large claws of this shrimp can grip so quickly that it creates a vacuum bubble and intense sound to stun prey. Shrimps also use claws to communicate with each other.

'The sound produced by a shrimp percussion is very short-lived and can travel far. A click is much quieter than a traditional sonar source, but thousands of clicks can happen every minute,' Laferriere said.

A species of shrimp in the crayfish family.

The sound of shrimp knocking can change with the time of day and water temperature. However, a flock of shrimp percussion is never quiet. One of the biggest challenges facing Laferriere's team is dealing with the enormous amount of noise generated by the shrimp themselves and their reflections into the surrounding area.

Analyzing these reflexes is particularly difficult. Unlike traditional sonar, the location of the shrimp's sound source is not obvious. Again, the solution used is modern technology.

Laferriere's team has developed algorithms to analyze the sound and select a unique clamp. The algorithm calculates the position of the shrimp, then analyzes the reflected path of the sound and finds out where it is reflected.

The team had to create computer models to determine what types of sound are reflected back from stationary and moving objects. If successful, smart voice recorders will be installed on the ship and the ability to locate any target in the area.

Other PALS groups take a similar approach. Northrop Grumman researchers are working on another shrimp-based sonar system. A research team from the US Navy is looking at sounds from coral reefs.

The project promises to bring about a sensor network capable of covering large areas for long periods of time with most hardware provided by nature. Only recording equipment that needs replacement or repair.

Realization ability

"DARPA's approach would be a major breakthrough if successful," said Sidharth Kaushal, an expert at Britain's RUSI defense think tank. However, he doubts DARPA's ability to succeed because previous similar projects have failed.

German U-boat submarines have been found to affect luminescent plankton. But subsequent attempts to capitalize on this effect on a larger scale yielded little success.

'The efforts of both the Soviet Union and the United States during the Cold War went nowhere. Partly because they have no way of distinguishing a 'false positive', such as a passing whale, from a submarine,' Mr Sidharth said.

By contrast, Ms. Adornato believes that a combination of marine life and intelligent algorithms can provide a reliable solution.

'These observation systems are very low impact. They can be deployed to many different environments without disrupting the ecosystem,' Ms. Adornato said.

PALS has completed the initial testing phase. The second phase of the project will demonstrate the solution's viability in controlled trials. Adornato hopes PALS will be field tested by 2023 and delivered to the US Navy.

Using sounds made by marine life and understanding how they change will provide researchers with a low-cost, environmentally friendly method to monitor human activity underwater. PALS technologies can also be applied to scientific research.