Gray’s Reef National Marine Sanctuary

Ocean Sound Monitoring

The Gray's Reef National Marine Sanctuary (GRNMS), located off the coast of Georgia is one of the largest live-bottom reefs in the southeastern United States, where rocky ledges are colonized by sponges, soft corals, and other invertebrates, providing a biologically rich habitat in an area otherwise characterized by large swaths of sandy seafloor. Cherished for its biological diversity, visitors travel to GRNMS to recreationally dive and fish outside of the Research Area, which are activities that were the focus of previous sound monitoring in the sanctuary. Multiple sites in the sanctuary were part of the Sanctuary Soundscape Monitoring Project, a 5-year (2018-2022) system-wide monitoring project to understand the diversity of sounds in sanctuary waters.

Current ONMS ocean sound monitoring and analysis is maintained at one site (GR01) that is characterized by rocky ledge live-bottom and located in an area open to recreational activities. The site supports high abundances of fish and protected species, such turtles and marine mammals, and functions as a migratory corridor for many pelagic species. Long-term sound monitoring is being utilized at GRNMS to track seasonal patterns in fish choruses, snapping shrimp activity, North Atlantic right whale presence, and human visitation, which are collectively driven by the large seasonal temperature swings observed in this dynamic and biologically productive sanctuary.

To see historical monitoring sites that are no longer active, please visit Sanctuary Soundscape Project data portal.

Summary of monitoring sites

Long-term Monitoring Site	Primary Monitoring Purpose	Oceanographic Setting	Depth (m)	Sampling Rate (kHz)	Known Biological Sounds	Vessel Traffic Setting	Latitude	Longitude	StartDate	Total Recording Days
GR01	A historical monitoring site with complex reef habitat that supports high densities of fish and is a destination for recreational fishing.	coastal-shallow	17	48	Multiple species of fish choruses (Spring, summer, and fall)	Open to fishing, anchoring prohibited	31.4	-80.9	Jan 2019	1519

Ocean Sound Conditions

Soundscapes vary across years, seasons, and even within a day. Differences are driven by shifts in wind and weather patterns, migration and behavior of animals, and patterns in human activities. We track how sound levels change in different frequencies using standardized soundscape metrics to understand these changes. Seasonal and annual percentiles of all the data, a measurement of data spread with the middle 50% falling between the 25th and 75th percentile, are used to define typical conditions.

What are the seasonal patterns across frequencies?

Season is often a main driver of soundscape differences: wind and weather patterns shift, species migrate or change behavior, and humans change their marine activities. As shown on the graphic(s) below, we can visualize the differences by looking at the variation across frequency (pitch of the sound in Hertz) shown on the x-axis and intensity (how loud the sound is in decibels) shown on the y-axis. The colored lines represent regionally-specific oceanographic seasons, vertical shaded bars are the frequency ranges that a sound source of interest can be heard, and the black lines bound the expected range of modeled sound intensity when only wind noise is present.
Here are some questions to consider when viewing the graph(s) below:
(1) Which season has the highest sound levels?
(2) Are there peaks in the sound levels for any of the sounds of interest?
(3) Are the low-frequency sound levels outside the expected range for wind noise?

Seasonal median sound levels (sound intensity measured by mean-square pressure in microPascal per Hertz) across a range of frequencies (~10 to ~24,000 hertz) at a given monitoring site, with annual recording effort represented by a bar graph underneath. Each season is a different colored line. Solid black lines show modeled ambient sound levels from wind at 1 m/s (lower line) and 22.6 m/s (upper line) based on hydrophone depth. Frequency bands indicative of a biological or anthropogenic sound source of interest are highlighted in semi-transparent gray and labeled; peak frequency for dominant fish species are labeled with vertical dashed lines. Credit: Megan McKenna and Emma Beretta/CIRES and NOAA

Processing raw audio files to calibrated sound levels (i.e., soundscape metrics) involves multiple steps to get a specified time frequency data product. All soundscape metrics visualized in the soundscape inventory reports are averaged from hybrid millidecade sound levels, calculated using either MANTA or PyPAM software packages. Both softwares calculate mean PSD (dB re 1 µPa^2/Hz) per minute in 1-Hz wide bins using Welch’s method with a Hann window, FFT length equal to the sample rate, and 50% overlap. Following calibration based upon instrument specific sensitivities, PSD values per minute were further processed in hybrid millidecade spectral densities, which are an efficient means of storing PSD spectra from high sample rate audio files using 1-Hz values up to 435 Hz, and then millidecade wide PSD values up to one half of the sample rate (Martin et al. 2021).

PAMscapes was used to calculate hourly median hybrid milli-decade bands (loadSoundscapeData(ncFile, keepQuals = c(1,2)) and binSoundscapeData(hmddata, bin = "1hour", method = c("median")). The hourly data were then matched with an estimate of wind speed at that location using matchGFS in PAMscapes (Global Forecast System (GFS) weather model). Long-term condition plots in these reports were generated by averaging the hourly median hybrid millidecade results within different time constraints (i.e. annual, seasonal) and percentiles (e.g., 25% and 75%) for each hybrid millidecade frequency band.

How are ocean sound conditions changing across years?

We can track changes in ocean soundscape conditions and its contributors by comparing annual sound levels. Efforts to reduce noise impacts to marine animals are underway on local to global scales. Strategies can include avoidance of times and areas when sensitive species are present to reduce vulnerability (e.g., a shipping lane), changing the operation of a potentially hazardous noise source (e.g., by slowing and therefore quieting vessels) or through the design and use of alternative, quieter sources (e.g., new, quieter ship design and/or technologies). The effectiveness of these approaches, and the scales over which they are effective, can be tracked through comprehensive monitoring efforts. A focused analysis is often necessary to tease apart the multiple drivers of ocean sound levels, however, annual summaries provide initial insights to overall patterns.
Here are some questions to consider when viewing the graph(s) below:
(1) Are levels lower in the most recent year of monitoring in any of the frequencies of interest?
(2) Are there peaks in the sound levels for any of the sounds of interest? Do they differ across years?

Annual median sound levels (sound intensity measured by mean-square pressure in microPascal per Hertz) across a range of frequencies (~10 to ~24,000 hertz) at a given monitoring site, with annual recording effort represented by a bar graph underneath. Each year is a different blue line, getting darker for every additional year of data. Solid black lines show modeled ambient sound levels from wind at 1 m/s (lower line) and 22.6 m/s (upper line) based on hydrophone depth. Frequency bands indicative of a biological or anthropogenic sound source of interest are highlighted in semi-transparent gray and labeled; peak frequency for dominant fish species are labeled with vertical dashed lines. Credit: Megan McKenna and Emma Beretta/CIRES and NOAA

Is the intensity (loudness) of sound sources of interest within typical range?

In some soundscapes, we can use specific frequencies as indicators for the presence of a source of interest (e.g. species presence and behavior). Monitoring sites are often chosen because a known source that we want to track is present and we can track this dominant sound energy contributor using their specific frequency or frequency range. For example, seasonal migration of humpback whales to Hawaiian Islands results in soundscapes dominated by their calling between 50 and 630 Hz. At sites near commercial shipping lanes, 63 and 125 Hz one-third octave level (TOL) are used as an indicator for ship noise (Haver et al. 2021)). At sites where snapping shrimp are present, 4,000 - 18,000 Hz can be used as an indicator of their sounds. Only frequencies that have been identified as reliable for tracking a source of interest are shown below.

Time series plot of daily median sound levels (sound intensity measured by mean-square pressure in microPascal per Hertz) for a specific frequency band(s) of interest at this site, separated by year. Background color shading (blue, purple, gold) indicates low (<25th percentile), typical (25-75th percentile), and high sound levels (>75th percentile) across this monitoring site's entire dataset at this frequency band(s) for comparability with annual medians, marked with horizontal black dashed lines. Pie charts on the right hand side of the graphic show the proportion of daily median sound levels that fell within each category for each year, following the same color-coding and percentile bins. Credit: Megan McKenna and Emma Beretta/CIRES and NOAA

Ocean Sound Indicators

We can use long-term monitoring of ocean sound to derive and track indicators of ocean conditions. These indicators track the status and trend of habitat condition, species presence, human-use patterns, and management activities. There are many analytical methods used to generate ocean sound indicators. Below are ocean sound indicators relevant to the sanctuary and available for condition tracking.

When do whales contribute to the soundscape, indicating their presence nearby?

GRNMS is located within the migratory corridor and only known calving ground of the critically endangered North Atlantic right whale (NARW). Between November and April, NARW migrate south from their foraging grounds in waters off New England to give birth in the shallow, coastal waters off of South Carolina, Georgia, and northern Florida.

In the middle of the GRNMS, a bottom-mounted hydrophone enables continuous monitoring of sound. Scientists at the Southeast Fisheries Science Center track the acoustic presence of North Atlantic right whales in this region from these recordings by listening for their calls. One of the most frequent sounds NARW make are called upcalls, which appear to be used for communication with one another. Detection of upcalls in acoustic recordings can reveal trends in right whale seasonal presence and migratory patterns over time.

In the plot below, the seasonal detection of right whales during their winter migratory and calving season are shown. The blue bars represent the number of days right whales were acoustically detected per week in GRNMS. Shaded gray areas indicate times when the hydrophone was not recording. Though infrequent, right whales were heard in GRNMS during their migratory and calving season. This likely indicates they were just moving through the GRNMS to and from calving grounds further south.

Bar plots showing the weekly acoustic presence of North Atlantic right whales at listening station GR01 in Gray's Reef National Marine Sanctuary. Data were analyzed during their winter migratory season from mid-November through mid-April annually from 2018 to 2025. Shaded regions indicate weeks with no recording effort. Credit: Amelia Johnson/NOAA.

The National Marine Fisheries Service (NMFS) maintains the analysis of acoustic presence of many whale species from passive acoustic monitoring data within U.S. waters, including ONMS sites. Visit the interactive website to explore spatial and temporal patterns in different species.