Noise pollution in the ocean isn’t just a nuisance; it has grave consequences for the survival of some marine organisms. A recent study reveals that ship noises make young eels stressed and when confronted with predators, they are less likely to fend off attacks due to impaired escape behaviors, known as antipredator responses.
During exposure to harbor ship noise, young eels were less responsive when faced with a looming predator and showed slower escape behaviors than eels exposed to sounds of the harbor only. And when pursued in a simulated predator chase, they were caught faster than eels exposed to harbor-only sounds.
While humans have explored the oceans for centuries, ship traffic now is greater than ever before, largely because of international trade. Commercial shipping activity—transporting the myriad of consumer goods we have become increasingly reliant on—pervades the oceans. Many of the goods transported by ships may make our lives easier, but these unfamiliar man-made noises can pose a threat to marine organisms. In some cases, the effects could mean the difference between life and death.
In the vast oceans, many organisms rely on sensing of sound for various daily activities, such as finding prey, communicating with others, moving around, and many others. Little is known about the impact of human-created noises on the behavior of marine organisms. Although some studies have shown that ship noises affect communication in dolphins and whales and the behavior of fishes, they don’t tell us how man-made sounds directly affect their survival.
Now, a team led by Dr Stephen Simpson from the University of Exeter, UK, investigated the behaviors of young European eels, Anguilla anguilla—a critically endangered species whose populations were decimated over the past few decades—in response to simulated predation threats when exposed to recordings of ship sounds from three UK harbors. Young eels, around 8 to 12 centimeters in length—known as glass-stage eels—are transparent, and found in the coasts of Europe, where they reach after traveling for a year across the Atlantic Ocean through routes frequented by ships.
For the experiments, the researchers initially played a recording of ambient ocean sounds without any ship noises for 2 minutes in the eel tanks. Then they randomly played 2-minute recordings where half of the tracks had the sound of a ship passing through a harbor and the other half lacked ship noises.
During each track, the researchers tested eels for their response to two predation scenarios: ambush and pursuit. For ambush threats, characterized by a ‘sit-wait-strike’ action, they presented a model fish mounted on a pendulum, mimicking an approaching predator, and recorded any startle behavior by the eels, exhibited by a change in swimming direction, and the time it took for this change. Pursuit predation refers to a ‘chase and catch’ situation, where eels are chased through a maze with a handnet until captured. These experiments were also carried out in the open water to recreate the eels’ real-world settings.
Eels that were exposed to additional noise sounds from harbors fared worse in their antipredator responses. They were 50% less likely to startle in response to an ambush predator than those exposed to recordings of the harbors without ship noise. Among the eels that did startle, a quarter of them were slower than their counterparts exposed to harbor-only sounds.
Predators are more likely to ambush eels when they are seeking refuge at the sea floor. Successful startle responses may help eels disengage from the grip of predators. But failure of startle responses triggered by potential predators shows their inability to detect a predator attack, and slower reactions means that eels are more likely to be seized by predators—resulting in death.
For pursuit predation experiments, eels exposed to ship noise were caught twice as fast compared with their counterparts from the harbor-only sound treatments. This is bad news because the longer it takes for predators to chase the eels, the better their chances of survival by getting more time to spot and scurry into crevices and other hiding places, or if they are lucky, the predator decides to give up the chase.
After the playback of ship noises, eels also showed a reduction in lateralized behavior—the number of turns they took in their preferred direction at symmetrical junctions in a maze—which represents their spatial performance. Highly lateralized behavior shows vigilance by inspecting the area for predators. Poor spatial performance of the eels shows they were not making proper use of the space available to them implying impaired decision-making abilities, and this could be why they were caught quicker in the pursuit experiments, the researchers suggest.
Even worse, eels exposed to ship noises showed signs of stress as they had higher ventilation rates—the rate at which they pump water through their gills—and oxygen usage than the eels exposed to harbor-only sounds. Stressed eels could be distracted by the noise, the researchers surmise, interfering with their ability to recognize and detect predators.
Noise can have far-reaching impacts on animal behavior in the ocean because sound waves can travel faster and longer distances, posing an additional threat to organisms that will face increasingly acidic oceans in the coming decades due to greater absorption of carbon dioxide from the atmosphere.
But experiments on predators are needed, the researchers state, to see how they react to man-made noises such as that of ships, to predict the combined effect of noise on predator-prey interactions.
The findings may eventually help guide the shipping industry to mitigate the impact of ship noise on these critically endangered European eels.
Simpson, S. D., Purser, J. and Radford, A. N. (2014), Anthropogenic noise compromises antipredator behaviour in European eels. Global Change Biology. doi: 10.1111/gcb.12685