A healthy coral reef creates quite the underwater racket. Reef croakers croak, damselfish woop-woop-woop, and clown fish sound thuds like a woodpecker at a tree.
But a new study reports that climate-change related disturbances can turn down the volume on coral reefs, causing fewer young fish to hear their way home. The results, published today in PNAS, point to disturbing trends. “You can hear the impact of climate change,” says lead author Timothy Gordon, a PhD candidate at the University of Exeter in the UK. But the findings also highlight the potential for sound as a reef-restoration tool.
Coral reefs face threats from climate change-related increases in the frequency and severity of cyclones, as well as mass-bleaching events, in which warming waters cause corals to expel symbiotic algae, their main energy source. Reef fish can help degraded reefs recover by grazing on the harmful macroalgae that would otherwise coat dead coral and prevent new coral from settling.
These algae-fighting fish start their lives as eggs on the reef and then get swept out to sea. “They spend up to several months of their life out in the open ocean,” explains Gordon. “There are fewer predators out there.” When it’s time to return home, the young fish rely on a variety of sensory mechanisms, including sound.
Gordon and colleagues wanted to know whether the soundscape of home might change with reef degradation. They recorded the nighttime soundscapes of 10 reefs around Lizard Island in Australia’s northern Great Barrier Reef in November 2012 when the reefs were relatively healthy, and then again in November 2016 after the reefs had been degraded by two cyclones and the most severe global mass-bleaching event on record. They found that the degraded reef sounds were far quieter and less acoustically rich.
“This [study] really shows the importance of having some baseline data,” says marine biologist Aran Mooney of the Woods Hole Oceanographic Institution, who specializes in the sensory biology and bioacoustics of animals. Ideally, says Mooney, the researchers would’ve had even more sound clips, to account for the wide natural variation in reef sound over time. But he adds that the study was only possible at all because the researchers had thought to listen in on the healthy reef in the first place.
Gordon and his team then used their recordings to test the effect of the altered soundscape on young fish navigating home. They set up groupings, each with three underwater speakers, that broadcast either the healthy reef recording, the degraded reef recording, or the sound of the open ocean as a control. Out at sea, they set up two groupings of these three recording types, with each speaker drawing fish into a light-filled chamber. The researchers used another grouping of three recording types on a sand flat closer to the actual reef, where the speakers drew fish to “reef patches” that the scientists constructed from coral rubble. For 18 nights, the team broadcast these sounds. Each morning, they identified and counted the fish that had settled in the traps and on the reef patches the night before.
Across fish species, both in open sea and on the sand flat, the healthy reef recordings drew larger numbers of young fish, 40 percent more on the reef patches. Gordon and his team will soon explore whether fish actually prefer the sound of a healthier reef or simply cannot hear their way to the much quieter degraded reef. Either way, Gordon finds the results of his current study concerning. If fewer reef-restoring fish are drawn to the soundscapes of degraded reefs, he says, this could create a destructive feedback mechanism—“reefs might get locked into degradation in this quieter state,” he says.
The study underscores the complexity of these ecosystems, says quantitative ecologist and coral reef specialist Stuart Sandin of the Scripps Institution of Oceanography. “As we alter some elements of the system, there emerge unexpected consequences,” he says. “While we may expect ‘life to find a way’ with fishes using new cues to find a place to call home, such adaptations are unpredictable and may not keep pace with human-induced environmental change.”
But the findings also offer a degree of hope that sound, if it is a key component of the degradation process, might be a new tool for drawing reef-restoring fish home. “The nice thing about sound is that you can easily manipulate it,” says Mooney. “This really lays the groundwork for a lot of future science.”
“Would it be a good idea to dupe fish by calling them into degraded habitats with fake healthy sounds?” Gordon wonders. He is currently researching this possibility.