For baby cod, offshore wind turbines offer a seductive tone

This story was originally published by Hakai Magazine and appears here as part of the Climate Desk collaboration.

Offshore wind is one of the fastest-growing sources of renewable energy, and with its expansion comes increasing scrutiny of its potential side-effects. Alessandro Cresci, a biologist at the Institute of Marine Research in Norway, and his team have now shown that larval cod are attracted to one of the low-frequency sounds emitted by wind turbines, suggesting offshore wind installations could potentially alter the early life of microscopic fish that drift too close.

Cresci and his colleagues made their discovery through experiments conducted in the deep fjord water near the Austevoll Research Station in Norway. The team placed 89 cod larvae in floating transparent mesh chambers that allowed them to drift naturally, then filmed as they subjected half the fish in 15-minute trials to the output of an underwater sound projector set to 100 Hz to mimic the deep thrum put out by wind turbines.

When left to their own devices, all the cod larvae oriented themselves to the northwest. Like the closely related haddock, cod have an innate sense of direction that guides their ocean swimming. When the scientists played the low-frequency sound, the baby fish still had a northwest preference, but it was weak. Instead, the larvae favoured pointing their bodies in the direction of the sound. Cresci thinks the larvae may be attracted to the 100-Hz sound waves because that low frequency is among the symphony of sounds sometimes part of the background din along the coastline or near the bottom of the ocean where the fish might like to settle.

As sound waves propagate through water, they compress and decompress water molecules in their path. Fish can tell what direction a sound is coming from by detecting changes in the motion of water particles. “In water,” says Cresci, fish are “connected to the medium around them, so all the vibrations in the molecules of water are transferred to the body.”

Like other creatures on land and in the sea, fish use sound to communicate, avoid predators, find prey, and understand the world around them. Sound also helps many marine creatures find the best place to live. In previous research, scientists have shown that by playing the sounds of a thriving reef near a degraded reef they could cause more fish to settle in the area. For many species, where they settle as larvae is where they tend to be found as adults.

Even if larval fish are attracted to offshore wind farms en masse, what happens next is yet unknown.

Since fishers typically can’t safely operate near turbines, offshore wind farms could become pseudo-protected areas where fish populations can grow large. But Ella Kim, a graduate student at the Scripps Institution of Oceanography at the University of California San Diego who studies fish acoustics and was not involved with the study, says it could go the other way.

Kim suggests that even if fish larvae do end up coalescing within offshore wind farms, the noise from the turbines and increased boat traffic to service the equipment could drown out fish communication. “Once these larvae get there,” Kim says, “will they have such impaired hearing that they won’t be able to even hear each other and reproduce?”

Aaron Rice, a bioacoustician at Cornell University in New York who was not involved with the study, says the research is useful because it shows that not only can fish larvae hear the sound but that they’re responding to it by orienting toward it. Rice adds, however, that the underwater noise from real wind turbines is far more complex than the lone 100-Hz sound tested in the study. He says care should be taken in reading too much into the results.

As well as noise pollution, many marine species are also at risk from overfishing, rising ocean temperatures, and other pressures. When trying to decide whether offshore wind power is a net benefit or harm for marine life, says Rice, it’s important to keep these other elements in mind.

“The more understanding that we can have in terms of how offshore wind [power] impacts the ocean,” he says, “the better we can respond to the changing demands and minimize impacts.”