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Boats leave behind more than a wake. Research has shown that the sound of boat motors causes humpback whales to fall silent and porpoises to flee. A recent paper concludes that even fish embryos get agitated by noise.

“The embryos know what the reef normally sounds like, so when a boat drives overhead they know that something’s changed,” says the paper’s lead author, Eric Fakan, a researcher at James Cook University in Australia, who studies noise pollution. That change causes a metabolic response that scientists are just beginning to understand.

In the study, Fakan and his team discovered that fish embryos exposed to boat motor noise appeared to grow faster. And this could leave the nearly hatched larvae at a disadvantage during their vulnerable first days.

To learn exactly how boat noise affects embryos, the researchers collected freshly laid eggs from two species of coral reef damselfish: fire clownfish and spiny chromis. They followed individuals first as egg-bound embryos and then as freshly hatched larvae, swimming into life with their external yolk sacs still attached.

The scientists incubated the eggs in tanks, exposing them to one of two sound treatments. The control group listened to a recording of reef sounds the researchers had collected from a secluded spot on the Great Barrier Reef. The experimental group listened to the same recording along with a recording of a two-stroke engine, a common design mechanically similar to the motor that drives a chainsaw.

Throughout the experiment, the researchers monitored each embryo’s heartbeat and rate of yolk consumption. In both species, the embryos in the experimental group had heart rates about 10 percent higher than those in the control group. In both the control and experimental groups of fire clownfish, yolk sac consumption was the same. But the spiny chromis embryos that were exposed to the din of a motor burned through their energy reserves faster, resulting in larvae with yolk sacs about 13 percent smaller upon hatching than those in the control group. The same spiny chromis larvae also had bodies that were five percent larger than normal.

Biologists usually think that large larvae have a better chance of survival—Fakan calls the assumption a “universally understood rule of thumb”—but in this experiment, larger bodies came at the expense of energy reserves.

The researchers think the faster metabolism and faster growth happen because the embryos’ sensory organs develop before their stress response systems, creating a window of time during their first days when the embryos can perceive and identify threats but cannot respond to them.

“At this stage, organisms can’t produce stress hormones. They can’t balance things out,” says Fakan. “If you and I were stressed, our bodies would produce stress hormones.” Instead, they appear to respond by reallocating energy for normal development to speed up the growth process.

“When you have an activity consuming energy, it’s taking that energy from something else,” says George Iwama, assistant director of the Institute for the Oceans and Fisheries at the University of British Columbia and an expert on fish stress who was not part of the study. He speculates that the unusual patterns of growth the researchers observed are detrimental to crucial developmental milestones.

While the findings are alarming, this line of research also offers concrete solutions to protect the fish. For instance, a paper out last year shows that quieter four-stroke engines, which are mechanically similar to automobile engines, do half the sonic damage to fish embryos as two-stroke engines.

“Global warming is hard to fix,” Fakan says, “but anthropogenic noise seems like something easier to grasp.”

As the consequences of noise pollution become clear, coastal communities can pressure state and local regulators to take simple, concrete measures to protect their waterways.