The soundscape of a coral reef is a measure of habitability
Healthy coral reefs are acoustically rich places. They host a diversity of animals whose feeding, communication, and courtship rituals are audible for kilometers as a chatter of snapping clicks and pops. In recent years, the volume has faded because of the damage many reef systems have suffered from recent storms and climate-induced bleaching. For a damaged reef like the one shown here to recover, it must recruit young fish from the open ocean to replace ones that have died off. A reef’s acoustic cues guide the orientation, habitat selection, and settlement of many fishes. The degraded reefs are known to experience a lower settlement rate than healthy reefs. And researchers worry that a reef’s resilience could become inhibited by the altered soundscape.
Prior studies of the mechanism behind the reduced settlement have focused on visual and olfactory cues. University of Exeter marine biologist Stephen Simpson , his student Timothy Gordon , and their colleagues wondered whether a reef’s soundscape could act as a direct indicator of habitat quality. They compared soundscapes from lagoonal reefs around Lizard Island—a continental shelf in Australia’s Great Barrier Reef—at the same locations and under similar conditions in November 2012 and in November 2016. Between those dates, two cyclones had drastically reduced coral cover and altered the reef’s fish communities, and a subsequent 2016 heat wave bleached 60% of the reef’s coral. Not surprisingly, the acoustic complexity and sound pressure levels they found in 2016 were significantly lower than 2012 values.
To assess how the damage affected marine life, the researchers constructed artificial reefs from coral rubble. They then mounted loudspeakers underwater to broadcast either predegradation reef sounds, postdegradation reef sounds, or an open ocean (no reef) control at various locations just before dawn. For 18 consecutive nights, the team then measured the abundance and composition of fish caught in traps after each sound treatment. (Juvenile settlement is predominantly nocturnal behavior.) They found that the soundscapes from damaged reefs were less attractive to fish than from undamaged ones and lured 40% fewer juvenile fish than traps broadcasting a predegradation soundscape. What’s more, the degraded soundscapes were no more attractive to fish than open-ocean soundscapes. (T. A. C. Gordon et al., Proc. Natl. Acad. Sci. USA, in press .)
