What’s That Sound in the Salt Marsh?
By Joel Davies, Conservation Technician
A public dock overlooking the salt marsh between Bald Head Island and Middle Island.
If you have ever spent time wandering the salt marshes on Bald Head Island, you may have heard distinct sounds without a traceable origin (listen above!). Pops or snaps bounce around in seemingly every direction at sporadic intervals. If you were ever curious as to who or what makes these sounds, you would not be alone! Staff here at the Conservancy have a list of running theories to try and answer this question. Some believe the opening and closing of oyster shells to be responsible, while others think it’s not a creature at all, but rather holes made from fiddler crabs filling with water as the tide fluctuates. In any case, none of these theories have ever been fully tested before, so our Conservation Technician Joel Davies took it upon himself to try and find an explanation.
The first thing to figure out was when and where the sounds were most prevalent. Joel visited a few different spots around the island during peak low and high tides. In general, it seemed that the popping was more frequent during lower tides, and was more often present near oyster patches. However, this was not the case for every spot he visited, as some areas had lots of activity with little to no oysters.
The SwiftOne ARU we used to record the mysterious popping sounds in the salt marsh
The next step was to get a recording of the sound, so it could be visualized and compared against known marsh animal calls. To do this, Joel utilized an autonomous recording unit (ARU) named the “SwiftOne.” This device is normally used to track bird calls, as it can be fastened to tree trunks and left alone for long spans of time. However, it proved to be just as useful for picking up popping noises. Joel left the recorder out undisturbed for full tidal cycles in areas where he initially observed popping sounds to see if there was some consistency in the noise being made.
To visualize the recordings, Joel put them into the ARU’s companion program “Raven Lite,” which can create spectrograms, or visual representations of sound frequencies through time. Each dark vertical line represents one pop observed from the recording.
A spectrogram Raven Lite created of the popping sound
A snapping shrimp viewed through a microscope – these tiny but mighty shrimp are only an inch or two long!
After the criteria was established for what the sounds looked like, it was time to find creatures that made similar noises. During oyster sampling in October, one creature was caught that seemed like a close match: the snapping (or pistol) shrimp! These organisms have obviously asymmetrical claws, with the larger one able to snap to stun prey and communicate with other shrimp. This snapping motion creates a cavitation bubble underwater, which is responsible for the popping sound you might hear while observing these shrimp (listen below!). This particular species tends to reside in sheltered areas like oyster reefs.
When these sounds are visualized, it looks similar to the initial recording.
Spectrogram of a snapping shrimp’s “snap”
While more experimentation needs to be done to rule out other variables and/or hypotheses, the snapping shrimp seems like a viable contender to solve this sound conundrum. As the weather (and water) starts to warm up again, hopefully these shrimp will come back out and display their unique behaviors for us!
