Article: Kaplan, M.B. and T.A. Mooney. 2015. Ambient noise and temporal patterns of boat activity in the US Virgin Islands National Park. Marine Pollution Bulletin 98: 221-228. http://dx.doi.org/10.1016/j.marpolbul.2015.06.047.
Imagine a normal day in the office, you are typing away, the coffee maker is running, and your coworkers are chit chatting. All of these activities involve sound. Now, wouldn’t it completely disrupt your day if a generator started up and drowned out all these sounds, forcing you to yell at your coworker so they can hear you? I would say so! Well, that sort of situation can happen in the ocean too.
Noise in the Ocean
Most people think the ocean is an incredibly silent place. And while it is true that sound waves travel differently in water than air, there are still plenty of noises! Now, some of these noises are the status quo background sounds that organisms underwater are used to: waves, parrotfish munching away, whales communicating, and snapping shrimp, to name a few.
Unfortunately, humans can often act as the interrupting generator, creating disruptive sound with our activities. For example, we use sonar, drive pilings, dredge waterways, and operate boats of all sizes. The noises from these activities can impact the physiology and behavior of marine animals, from invertebrates to mammals. Some captive studies of fish have shown disruption of schooling and orientation behaviors as well as hearing loss when exposed to sounds small boats would create. Therefore, Kaplan and Mooney set out to examine the prevalence and characteristics of noise from small boats in the environment.
The researchers conducted their study on three coral reefs in the U.S. Virgin Islands National Park off the island of St. John: Tektite (TK), Ram Head (RH), and Yawzi (YA) (Fig. 2). This location attracts approximately a half a million visitors annually. Most visitors access reefs via boat. The coral reef system in this park is experiencing coral cover declines, likely from a combination of stressors including boat noise. They examined the daily, weekly, and summer trends in boat noise at these locations. They deployed acoustic recording devices for 4 months beginning in April of 2013.
They analyzed the data by listening to the sound recordings and visually examining long-term spectral average plots (see Fig. 3). They also calculated the median sound pressure levels (SPL) for 3 frequency bands: the low fish band (100-1000 Hz), high frequency snapping shrimp band (2-20 kHz), and full bandwidth (100-20,000 Hz). SPL is a measure of the pressure a sound wave exerts relative to atmospheric (or underwater) pressure. It is what our ears detect allowing us to hear, so you can think of it as audio volume, like a stereo.
Patterns of Noise on the Reefs
The authors were able to detect boats at the reefs throughout the study period (Table 1). Tektite had the most occurrences of boat noise and the highest proportion of recordings with boat noise. Only a quarter of the days the acoustic equipment was deployed were free of vessel noise. For comparison, at Ram Head and Yawzi, half of the deployment days were noise free.
On all of the reefs, there were fewer detections of boat noise between midnight and 4 am and a lull between 10 am and noon. Those likely align with when people are sleeping and enjoying a lunch hour. At Tektite, the most boat noise occurred around 8 am, 8 pm, and 10 pm—possibly due to generator use at moorings.
Sound pressure intensity (measured in decibels or dB) differed between recordings that contained boat noise and those that did not, corrected for reef and time of day. SPL was significantly higher with boat noise present, up to 10 dB and 7 dB higher during the day and night, respectively (Fig. 4). Recordings with boat noise had a greater energy content at low-frequencies, those below 1000 Hz, with power spectral densities (how the sound wave energy varies with frequency) as much as 20 dB greater at specific frequencies.
Results Summary: That is a lot of confusing terminology—for me as well! Basically, the scientists found that boat engines, even from small vessels, generate a lot of noise in reef environments. This is not exactly a surprise, but quantifying it is the first step in setting up future studies!
Why should we care about noise in marine habitats?
Human created noise is increasing around the world, including many marine areas. However, little is known about it or its impacts. We need to understand what we are exposing animals to and how they will respond. This study of small boat activity is a step on the way to that understanding. It can also be used to monitor and find patterns of human activity in the area, which can then be used if management of noise levels is needed. Acoustic monitoring such as this could also be utilized in estimating the ecosystem services of various marine habitats.
The documented boat noise has the potential to further stress coral reef habitats, especially as it may overwhelm communication between animals. The levels of sound are low enough that they should not produce hearing loss but the frequency and duration of the sounds could cause damage over time. There is great overlap between frequency bands for vessel noise and fish sounds and hearing (Fig. 5). Boat noise could mask sounds fish require and associate with recruitment, feeding, territorial behaviors, and reproduction. Check out this oceanbites post on noise impacts on dolphins! There are still many questions about noise, both natural and anthropogenic, in the marine environment! It will be exciting to find out what scientists learn next!