Building a Fence out of Sound

This highly specific noisemaker drives seals away. 

Published January 21, 2016

Off the Scottish coastline, farmers cultivate salmon in large ocean pens. These netted areas teeming with fish are within sight, sound, and smell of wild—and hungry—harbor seals. From a seal’s perspective, salmon farms are likely idyllic places: captive buffets just begging to be burgled. But in truth, salmon farms are not easy pickings: in many places, including Scotland, thieving seals are often punished with a bullet to the head. Dead seals don’t steal salmon.

Wherever human herds meet wild carnivores, conflict arises. And in these battles of life versus livelihood, it is almost always the predator that loses. Thomas Götz, a marine mammalogist at the University of St Andrews, thinks there’s a better way to resolve these conflicts: loud noises.

Using blaring horns and sirens to spook predators isn’t an entirely new idea. But typical devices—known as “acoustic deterrents”—have only limited utility: most animals eventually habituate to the sounds. In some cases, acoustic deterrents actually cause problems of their own, contributing yet more noise pollution to an already cacophonous soundscape. Even worse, most acoustic deterrents aren’t specific to the problem species, and wind up driving away non-target animals, too.

Götz thinks that acoustic deterrents can benefit from a more targeted approach.

In a test, Götz wanted to see if he could use short, unpredictable, brief, and isolated bursts of specific sounds to target only seals—like a dog whistle in reverse. The idea, he says, is to activate a seal’s startle reflex. It wouldn’t be just better for the seals, he suggests, but by reducing the overall amount of artificial sound it would be better for the entire marine community.

To test the approach, Götz and his team monitored seal depredation at a salmon farm off the west coast of Scotland for an entire season. Then, they deployed their acoustic deterrents and watched for another season. They also monitored seal depredation at two other farms for shorter periods of time (one had an acoustic deterrent and one didn’t).

With their acoustic deterrents in place, Götz found that the number of salmon disappearing down the throats of hungry seals dropped by 91 to 97 percent, depending on the site: a clear sign that these noise deterrents work. But perhaps as importantly, the scientists also found that their more targeted deterrents didn’t seem to affect harbor porpoises or otters. Theirs is a seal-specific anti-theft device.

George Mason University researcher Andrew Wright points out that while reducing conflict between seals and fishermen is valuable, “these animals will suffer some consequences to achieve this.” If the seals have become accustomed to finding an easy meal, their ability to survive and reproduce might suffer while they expend energy hunting elsewhere.

Still, it’s probably preferable to the more broadband acoustic deterrents currently in use, and definitely preferable to the death penalty. “We feel that our deterrence method could address the ethical and conservation concerns associated with these lethal removals,” Götz says.

He also believes that similar methods could be used elsewhere, including on land. The startle reflex is quite old, from an evolutionary perspective, and has been found in every mammal ever studied. “It should be possible to elicit similar responses in other carnivores, such as wolves or big cats,” he says.

If it works, Götz says, then it would enable the wolf, lion, tiger, or seal to continue contributing productively to its own ecosystem, while keeping safely away from ours.