Noisy reefs help young fish find their home

Young reef fish use the chorus of sounds made by other fish to find and settle in suitable habitat, but damage to reefs from storms and coral bleaching affects these sounds and thus the ability of juvenile fish to find a home.
Researchers compared the effects of sounds of intact and degraded reefs on juvenile fish behavior; they found that soundscapes of degraded reefs lacked the volume and complexity of those of intact reefs and attracted far fewer juveniles.
Limiting future bleaching by reducing carbon emissions that lead to warmer seas is considered key to the survival of coral reefs.

Fish talk. They boom, buzz, croak, and chatter. They even create a dawn chorus similar to birds. Shrimp vocalize too, by snapping and popping.

A reef’s soundscape—the variety of sounds reef-dwelling animals collectively make—reflects the abundance and diversity of the reef’s inhabitants.

Healthy reefs have a loud and complex soundscape, which larval and young reef fish that start life as eggs floating on ocean currents use to find and settle in suitable habitat. Like an orchestra missing the horns or string section, reefs degraded by storms and coral bleaching lose both the volume and complexity of their song, making them less inviting to this next generation of reef fish.

A diversity of fish species on a coral reef in Sabah, Malaysia. A healthy coral reef supports many species of fish, as well sea turtles, soft corals, and invertebrates such as starfish, squid, and shrimp. Image credit: Sue Palminteri/Mongabay.

Studies have shown that coral structure and presence of living corals are important for survival of reef fish communities, and a diversity of reef fish, in turn, support coral survival and facilitate their recovery from devastation. Parrot fish and other herbivores eat the algae that can cover and kill corals; sharks, groupers, and other carnivores maintain stable populations of the herbivores, and nearly all become food for other organisms, which retains nutrients in the system.

We know that coral bleaching – the loss of energy-producing algae from corals when water becomes too warm – kills some corals and thus simplifies and degrades the structure of reef habitats. A multi-national team of researchers examined how changes in soundscape caused by reef degradation might affect the behavior of reef fish, particularly juvenile fish that spend their early days in open water before finding and settling on a reef.

They published their findings last month in the Proceedings of the National Academy of Sciences of the United States.

A degraded reef coral reef at Lizard Island, Northern Great Barrier Reef, Australia. The area was heavily degraded over the last five years by severe coral bleaching and tropical cyclones, both of which are increasing in frequency and severity worldwide as sea temperatures rise. Image credit: Tim Gordon, University of Exeter.

Fish need sound to move around

Many reef fish actually spawn (reproduce) in open water. Open water has fewer predators, and the currents disperse the larvae and fertilized eggs to a variety of places. Larvae of many reef fish species live and develop in the water column, descending to the sea floor habitats at night when they rely on sensory cues to find a safe place to settle.

Once they develop into juveniles, these fish use sound and other cues to find and eventually settle on a coral reef, where they remain into adulthood.

The researchers examined the responses of larval and juvenile fish to three different soundscapes by recording and playing back the sounds of an area of Australia’s Great Barrier Reef before and after the area’s worst period of degradation in its recorded history. They recorded 10 reefs in November 2012, while they were still intact, and again in November 2016, after two major cyclones and a global mass-bleaching event.

A hawkfish at home on the reef in Sabah, Malaysia. Image credit: Sue Palminteri/Mongabay.

Listening and learning

In 2012, at each of 10 reef sites, they suspended an underwater microphone, called a hydrophone, above the sea floor and connected it to a digital recorder and battery to record the sounds below the surface. They repeated the recordings in 2016 at the same sites and times (just after sunset) to obtain matched pairs of “pre-degradation” and “post-degradation” recordings.

The researchers analyzed four aspects of each sound recording—its complexity, richness, sound pressure (volume), and the rate of snaps from the claws of shrimp and other invertebrates. At each reef site, they compared these four indices from before and after the storm and bleaching episodes.

They found that the 10 reefs were quieter after the devastation, with fewer numbers and types of sounds, indicating a drop in the number and variety of creatures at all the sites.

Lead author Tim Gordon, a marine biologist at the University of Exeter, said in a statement: “It’s heart-breaking to hear. The usual pops, chirps, snaps and chatters of countless fish and invertebrates have disappeared. The symphony of the sea is being silenced.”

A second degraded reef coral reef at Lizard Island, Northern Great Barrier Reef, Australia. With few fish, degraded reefs lack the complex symphony of sounds of a healthy reef that attract young fish to a reef. Image credit: Tim Gordon, University of Exeter.

The research team then played back these recordings, plus a recording of the (quiet) ambient sound from the open ocean, in two different artificial underwater habitats positioned far from a real reef in order to test the reactions of young fishes to the three soundscapes.

They floated sets of light traps at 10-20 meters (33-66 feet) depth to investigate sound preferences of free-swimming fish larvae.
They created sets of artificial patch reefs from the rubble of dead coral on sand flats in 2-5 m (7-16 ft) water depth to study how juvenile fishes select and settle on a reef site.

The researchers identified the fish that turned up at each of the simulated habitats, releasing them later the same day.

This schematic outlines how degradation caused by bleaching and cyclones changes the soundscape of a reef and, in turn, its functionality. A quieter, simpler soundscape makes reefs less attractive to young fishes, potentially reducing the populations of important algae-grazing species. Fewer fish and reduced grazing may exacerbate degradation, preventing recovery. Image credit: Tim Gordon, University of Exeter.

Letting the fish decide

They found that young fish of all types preferred the sounds of a healthy reef to those of either a degraded reef or open ocean. More fish came to light traps and artificial reefs where the pre-degradation sounds were played than to sites playing sounds of a degraded reef. In fact, their responses to the degraded reef soundscapes were roughly the same as their reactions to the sounds of the open ocean, where they cannot settle.

The researchers suggest that either the juvenile fish need to be closer to a degraded reef to hear its sounds, and so the quieter degraded reefs may be audible to fewer juvenile fish or that the larvae and fish simply prefer the healthy reef soundscape.

They propose that the changes in the soundscapes may have resulted from a smaller number of vocalizing animals in the degraded reef’s fish community; from changes in the animals’ behavior, such that they call less frequently under less desirable environmental conditions; or some combination of these two processes.

Highly structured corals of healthy reefs provide shelter for fish, turtles, and a wide range of invertebrates. Image credit: Sue Palminteri/Mongabay.

Regardless of mechanism, the sounds of disturbed reefs attracted far fewer juvenile fish than sounds of intact reefs. The concern among scientists is that the loss of strength and diversity of the soundscapes in degraded reefs and the corresponding decline in young fish settling there may prevent degraded reefs from recovering and negatively affect the overall health of these reefs.

Co-author Steve Simpson, Associate Professor in Marine Biology & Global Change at the University of Exeter, explained: “If the reefs have gone quiet, then the chances of the next generation of fish recolonizing the reefs are much reduced. Without fish, the reefs can’t recover.”