Hearing the economic ripples of a global pandemic

A world of sound lies beneath the ocean’s surface—the bellowing calls of gargantuan whales, the sharp clicks of dolphins hunting for squid, the deluge of rain on the ocean’s surface, the rumble of an earthquake.

An underwater microphone, or hydrophone, installed on MBARI’s cabled ocean observatory deep beneath the surface of Monterey Bay is constantly listening to the underwater soundscape. It streams sounds to shore year-round. Our researchers can listen live and study the sound information it gathers in real time. Importantly, we also archive the acoustic data to uncover patterns over time.

The hydrophone at MBARI’s cabled observatory has been instrumental for studying the ocean soundscape. This underwater microphone continuously records the sounds of the ocean, and an acoustic archive has revealed trends over time, like the reduction of low-frequency noise during the COVID global pandemic. Image: © 2016 MBARI

While most of the sounds the hydrophone hears come from natural sources, like animals or geological processes, noise from human activities is present too. The deep growls from shipping traffic can permeate the ocean’s soundscape. These thundering grumbles ebb and flow with pulses of vessel activity.

The COVID global pandemic upended life around the world and disrupted global economic activity. But it also enabled a rare opportunity to measure the relationship between shipping traffic and the underwater soundscape.

Taking a closer listen to audio logged by MBARI’s hydrophone in 2020 revealed a drop in low-frequency sound in the spring and summer—a change researchers attribute to a decline in shipping traffic during the COVID pandemic.

The Monterey Accelerated Research System, or MARS, cabled observatory sits approximately 20 kilometers (12.5 miles) from offshore shipping lanes, providing researchers the opportunity to “eavesdrop” on vessels as they pass through the Monterey Bay National Marine Sanctuary to ports to the north (San Francisco Bay) and south (Southern California).

Mapping total hours of vessel presence using Automatic Identification System (AIS) records revealed the “arteries” of maritime shipping off the coast of central California. The black dot marks the location of the MARS cabled observatory, and the white arc represents a 165-kilometer (103-mile) radius around the hydrophone connected to the cabled observatory. (Data from San Francisco Bay were excluded from this map and study because this traffic is not relevant to sound recordings logged at the MARS observatory.) Image: John Ryan

Most marine acoustic recording systems can only be deployed for a relatively short period before battery and data storage capacity become depleted. MBARI’s cabled observatory provides a constant connection to
shore, removing the limitations on power and data to allow continuous listening.

Examining data archived from 2018 to 2020 proved critical for revealing how the marine soundscape off the central California coast changed during the COVID pandemic.

Between January 2020 and June 2020, low-frequency noise levels decreased—but was this a typical seasonal change or a phenomenon unique to 2020? The research team compared recordings for each month of 2020 to the same month in previous years. If noise levels during 2020 were persistently lower than previous years, this would indicate 2020 was indeed unusual.

Compared to 2018 and 2019, low-frequency noise was significantly reduced in 2020. In fact, in June 2020, the acoustic intensity was nearly halved. Noise levels began to rebound in July 2020.

To determine if the drop in low-frequency noise evident in the hydrophone recordings was in fact caused by a reduction in ship traffic, researchers leveraged two independent data sets.

Using data from U.S. Customs and Border Protection, personnel from the U.S. Maritime Administration (Department of Transportation) documented the total number of port calls for all ports along the California coast and the total gross tonnage that those ships were carrying. These data provided a statewide view of maritime shipping.

The research team leveraged Automatic Identification System (AIS) data from the U.S. Coast Guard to understand changes within earshot of MBARI’s hydrophone. These data provided a way of tracking the movement of vessels, including their speed and their location, to focus on the Central Coast region specifically.

The port and AIS data confirmed that the significantly reduced low-frequency noise during February through June 2020 occurred alongside unusually low port activity across California and reduced transits by large vessels in the Monterey Bay region. 2020 had the lowest levels of shipping activity and the lowest levels of noise among all three years—it was clear that as shipping activity declined in 2020, the soundscape grew quieter.

The MBARI team is part of the SanctSound research collaborative spearheaded by the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Navy to understand the soundscapes of national marine sanctuaries across the country. The four-year research project started in 2018 and brought together numerous scientific partners to study sound within seven national marine sanctuaries and one national monument in waters off the east and west coasts and Hawaii.

The results of this study in the Monterey Bay National Marine Sanctuary provide an example of how underwater sound can be used to monitor the local consequences of global-scale changes in human behavior, allowing researchers to further consider the implications of such changes for the marine animals that call this place home.

Thanks to data processing by MBARI engineers, the mountain of acoustic data collected by MBARI’s hydrophone is also accessible to other research teams.

Pacific Ocean Sound Recordings makes that trove of acoustic data accessible to researchers worldwide via the Registry of Open Data on the Amazon Web Services (AWS) cloud. MBARI’s Pacific Ocean Sound Recordings dataset comprises a 6.5-year archive of audio recordings that began in July 2015. The archive grows continuously as active recording adds to it. Researchers and students around the world, who otherwise may not have access to high-quality ocean acoustic data, can tap into this data resource and explore the ocean’s world of sound.

The original recordings were acquired at a very high sample rate of 256,000 samples per second (256 kHz). These recordings—almost 50,000 hours of sound—are available on the AWS cloud. However, many research applications can be well served by lower resolution recordings with a much smaller data volume. For this purpose, the dataset includes daily files of recordings decimated to lower sample rates (16 kHz and 2 kHz). Beyond making all of the data products available and keeping them updated, this project provides examples for working with the data in the cloud, applying signal processing and machine-learning tools. These tools are essential to enable effective data sharing and use.

The Monterey Bay National Marine Sanctuary is home to at least 36 marine mammal species, including humpback whales (Megaptera novaeangliae), that rely on sound to communicate. Understanding the underwater soundscape—and how it changes with human activities—is critical to protecting these species of special conservation interest. Image: © 2015 Knute Brekke

The total volume of the data is 150 terabytes and growing. Such a massive amount of data would be too much for a person to analyze, so machine-learning services from cloud providers like AWS help with data processing.

Data and technology from many other MBARI projects are also publicly available. MBARI is eager to share its expertise with the global scientific community—from oceanographic data collected by ongoing monitoring efforts to detailed maps of the deep seafloor to identification guides for deep-sea animals.

The hydrophone on MBARI’s cabled observatory also provides valuable opportunities for outreach. MBARI streams live underwater audio to the Soundscape Listening Room. Listen for the bellowing moans and groans of whales, the quick clicks and clacks of dolphins, the grumbles of earthquakes, and even the pitter-patter of raindrops on the ocean’s surface. The listening room also includes archived natural sounds from marine mammals and earth processes.

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