The Sound of Science: What oyster reefs may be (literally) telling us about the environment

Mangrove habitat photo
Oyster reef near the mangrove habitat. PHOTO CREDIT: Florida Oceanographic Society.

By: Vincent Encomio and Hilde Zenil Florida Oceanographic Society

Jacques Cousteau described the ocean as “The Silent World”. In truth, the ocean is anything but silent, and can be a cacophony of snaps, pops, clicks, whistles, squeaking, croaking, drumming and even songs. Many marine animals, from whales to fish to shrimp, produce sounds to communicate with each other, or incidental sounds due to movement and feeding. These intentional and non-intentional sounds can convey important information about animals’ behavior, location and abundance, especially in areas where visual observations are difficult.

At the Florida Oceanographic Society (FOS), we were interested in seeing if biological sounds, or bioacoustics, could be used as a potential tool to monitor local habitats, especially oyster reefs. A study in New Zealand showed that different reef habitats could be differentiated by their bioacoustic signals. Another study in Japan described a decrease in snapping shrimp sounds in coastal bays that experienced low oxygen events. The characteristic “snap” of these shrimp is produced by the formation and collapse of a tiny bubble when the shrimp closes its enlarged claw. The snap may play a role in communication, aggressive behavior or even capturing prey. Furthermore, snapping shrimp sounds dominate shallow, warm water marine habitats across the world, and are so prevalent that they interfere with the sonar of naval vessels. The fact that a biological function affected by environmental stress, could be “heard” was intriguing to us. Could snapping shrimp be a marine version of the “canary in the coal mine?”

We had also been begun working with Dr. Grant Gilmore, one of the leading fish biologists in this part of the world. Dr. Gilmore has done extensive research in fish bioacoustics, particularly to characterize spawning aggregations of fish such as spotted sea trout and snook. Male fish produce calls to attract mates, with each species producing distinctive calls. We had enlisted Dr. Gilmore to aid us in utilizing bioacoustics as a monitoring tool for our oyster restoration projects. It was Dr. Gilmore who asked the seminal question: What does an oyster reef sound like?

Hilde listening to the oyster reef. PHOTO CREDIT: Florida Oceanographic Society

Addressing this question could not have occurred at a more opportune time for us. In 2009, Martin County received a grant from NOAA to restore over 25 acres of oyster reefs in the St. Lucie and Loxahatchee Rivers. A major goal of this restoration project was to provide habitat for not only oysters, but also other species. The FOS received funding from Martin County to assist in the monitoring effort of this project, and we (FOS and Dr. Gilmore) proposed to use passive acoustics to monitor these reefs. Several species common to oyster reefs, such as oyster toadfish, gobies, mud crabs and snapping shrimp are known to produce sound. We hypothesized that newly constructed oyster reefs would have a different bioacoustic signal than natural reefs, and that those differences would be related to the pattern of abundance on those reefs, i.e. more animals, more sound. We were also monitoring any changes in sound levels that could occur as a result of changes in water quality. The St. Lucie River periodically experiences devastating freshwater releases from Lake Okeechobee. Could bioacoustics be a rapid and non-invasive way of monitoring the health of these new oyster reefs?

Snapping shrimp up close. PHOTO CREDIT: Florida Oceanographic Society

We deployed hydrophones (underwater microphones) at several restored and natural oyster reefs. The amount of sound produced at some of these sites was staggering. Sitting in a boat on a calm day, one would never realize the thunderous din underneath. Much of the oyster reef “soundscape” was dominated by snapping shrimp. We found differences in sound between restored and natural reefs, with the restored reefs having surprisingly higher levels of sound. Direct sampling of these reefs found that there were more animals on the restored reefs than the natural reefs, confirming our bioacoustic measurements. Why did the restored reefs have greater amounts of animals? We hypothesized that the restored reefs would create more space for colonization. Like a new neighborhood of empty houses, residents, especially the snapping shrimp, quickly moved into these restored reefs. We did not, however, observe any effects of freshwater discharge events, as conditions were quite dry during the study. We will continue to monitor these sites in the future to determine if sound can be an effective biological indicator water quality.

This project has been an exciting avenue of research for the FOS, and was Hilde Zenil’s Master’s Thesis project at Florida Atlantic University. She has presented her work at several scientific meetings, receiving several awards for her research. We hope to continue this work and utilize bioacoustics as a tool to monitor not only oyster reefs, but also other nearshore habitats. We hope the “clamor” generated by this oyster restoration project will continue to grow louder!

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