September 9, 2019

How Does Underwater Sound Impact Sharks?

News Source: Forbes

Author(s): Melissa Cristina Márquez

As summer wraps up in the northern hemisphere, it is no surprise that sharks were once again a hot topic in the media. We share our ocean with this large predator and while most beach trips are uneventful, unfortunately, there are fatal encounters during these human-wildlife interactions. Although statistically very rare, this is seldom comforting to those who have witnessed or know or someone who has been bitten or killed by a shark. Trying to peacefully co-exist with this wildlife has led scientists to try to develop non-lethal shark mitigation strategies, and one non-lethal strategy is to take advantage of their senses, like sound.

“Sharks possess an impressive array of senses, including smell, vision, hearing, touch, hydrodynamic detection and electroreception. One could take advantage of these acute senses to deter sharks from an area purely based on the manipulation of their sensory cues,” explained study lead Dr. Lucille Chapuis of the University of Exeter. Chapuis and her team set out to test if some sounds could act as acoustic repellents.

Sound is an important stimulus underwater, as it can spread in all directions quickly and further than any other sensory cue. This can allow animals to detect where struggling prey is located or the general direction of danger, for example. While the impact of noise on marine mammals has been widely studied, the effect of sound on the behavior of sharks has not been investigated since the 1970s. “Sharks are inherently a difficult group of animals to study. They are quite rare to encounter in the wild, making it difficult to find enough individuals on which to base a statistically sound study,” Chapuis told Forbes. “This type of fieldwork is also expensive, and difficult to secure funding for. [Until] the last decades, we did not have the technologies which allowed us to remotely observe sharks in their natural environments, while not altering their behavior by observing them from a boat or by diving next to them.”

Using a baited underwater camera rig to record the responses of eight species of sharks (seven reef and coastal shark species and the white shark, Carcharodon carcharias), Chapuis and her team focused on two sets of sounds (orca calls and artificially created ones) and used silence as their ‘control test.’ Marine mammals, especially dolphins like orcas, rely on communication while swimming and hunting. Orcas are known to hunt on large sharks, like great white sharks, and that is why the team chose this sound to playback to sharks in the wild. According to Chapuis, some shark tourism operators even claim that sharks disappear from their primed sites when a pod of orcas is observed in the area. “It has never been properly tested, but it could mean that sharks hear the orca calls and respond to the threat by vacating the area.”

The second set of sounds were created by Chapuis on a computer and consisted of different intensities and frequencies in the range of shark's hearing sensitivity at a chaotic rhythm. Representing an unfamiliar and unnatural acoustic signal for these sharks, the team thought it could trigger aversive behavior. The experiment found that the effects of the sounds varied depending on which species of sharks they targeted. The results: “Reef and coastal sharks, like grey reef sharks or lemon sharks, for example, were less present from the area where either of the two sets of sounds were playing, rather than when no sound was played at all. For the few sharks which were still observable while sounds were playing, we found that they were interacting less, and less aggressively, with our experimental rig, than when there was silence. However, great white sharks behaved a bit differently. The artificial sound [influenced] their presence but not the orca calls. And the great whites, if present, were interacting as much with the rig, irrelevantly on sounds being on or off.”

So, the white sharks didn’t react to the acoustic stimuli from their own predators? What could this possibly mean?! “The orca calls we used for our playbacks were recordings from a population of orcas which are known to feed on large fishes like sharks, and while they were potentially hunting some at the time they were recorded,” explained Chapuis. “However, they were recorded in South Australia from a local population. Orcas have a very complex acoustic different repertoires depending on where they live and on what they hunt. When we played these calls to the South African great white sharks, it is possible that it did not mean anything to them, i.e. did not represent a threat. I would like to repeat the experiment with recordings from South African orcas!”

The team was surprised to see that even a relatively low sound level could affect the behaviour of these sharks. “This is actually concerning because there is an increasing amount of man-made noise invading the ocean: shipping, building, oil and gas surveys all produce loud noise which overlaps with sharks' hearing sensitivity. If a small speaker can disrupt the behaviour of a reef shark, what are the effects of blasts coming from seismic surveys?”

This new study highlights the critical need for more studies on the effects of man-made noise in sharks in our increasingly noisy world. For more information, you can read the full report in the journal Nature.