Article: Tricas, T. and Boyle. K. (2014) Acoustic behaviors in Hawaiian coral reef communities. Marine Ecology Progress Series 511:1-16 DOI: 10.3354/meps10930
All animals use sound to communicate with one another and fishes are no different. Coral reef communities are composed of several different fish species who use acoustics to attract one another, warn off predators, and during courtship exercises. Coral reef fishes use acoustics in the low spectrum frequency (<50 Hz to several kHz), compared to humans whose vocal range is from 80Hz-1100Hz, but recording these sounds proves to be very difficult with the background noise of waves, wind, and multiple other species on the reef using a very similar acoustic band. More than 600 fish species occupy inshore and offshore coral reefs, but only a very small handful of fish acoustics have been recorded. Scuba divers have attempted to observe fish behavior and language, but only recently has the technological advance of recreational scuba equipment allowed divers to patiently listen and observe fishes. Rebreather scuba systems (Fig. 1) allow divers to have a longer bottom time and most importantly eliminate the loud sound caused by the release of bubbles from a diver’s regulator. Researchers from Hawaii set out to observe, record and archive reef fish acoustics to better understand fish language.
Divers set off to three separate coral reefs in Hawaii to record fish sounds. The reefs ranged from 8-40m in depth, with one deep dive extending to 80 m. All divers used a closed-circuit rebreather system as their air source, eliminating any exhaust bubbles, which would interfere with acoustic recordings, and allowed the divers to stay in the water for up to three hours. For a period of one year, the divers observed fishes’ behaviors and recorded the acoustic frequencies of the fishes using digital video camera equipped with an external hydrophone to record sound on a single audio channel. Later the video and sound footage was analyzed and categorized as either a single independent sound event (<1 second) or as a continuous sound “train” that consisted of single pulses in a series or as longer “growls” with <0.5 second intervals. In the cases of schooling fish, individual fish acoustics could not be identified, so a separate “schooling” category was established.
Results and Discussion
A total of 96 reef fish species were identified and produced 85 different sound types during different scenarios. Thirty-nine of the observed species displayed sound communication during agnostic (aggressive-submissive) interactions, compared to 39 species who used acoustics during courtship and mating rituals. Parrotfish proved to be the loudest chewers, showing the highest frequency of sound when scraping their teeth across coral substrates. Triggerfish and filefish also exhibited scraping sounds when eating benthic material, although with less vigor than the parrotfish. Several fish species showed a short pulsed warning sound in response to divers or potential predators (Fig. 2).
Sound plays a huge part in the social interaction of species. With this start-up database we are now able to understand and even relate to different fish species. Just like learning a new language helps us connect to different cultures, learning the acoustics of coral reefs and their inhabitants will help us better understand the dynamics of each the coral reef ecosystem.