Long-term sound recordings in the Gulf of Mexico and oceans around the world have been at the forefront of oceanographic research, especially whale and dolphin research. On the final leg of the NOAA’s Southeast 2023 Vessel Surveys For Abundance and Distribution of Marine Mammals and Seabirds project, the acoustics team is recovering and deploying a variety of moored underwater recording instruments.
NOAA scientists and their collaborators have been deploying underwater recording devices throughout the region to collect ocean sound data for a variety of projects. They deploy these instruments in strategic locations to record sounds and provide information on the soundscape. This includes sounds from human activities, fish, and marine mammals that come through the area.
We deploy moored acoustic recorders for long periods of time to better understand the biology of acoustically active animals such as marine mammals. They also improve our understanding of ocean noise levels and how human activities impact them.
Melissa Soldevilla, Ph.D., an acoustician for NOAA’s Southeast Fisheries Science Center explained, “The recordings we bring back help our team to better understand distribution, abundance, habitat use, seasonal and longer-term movement patterns, and calling behavior of various marine mammals.”
Overall, the information obtained from these recordings will contribute to effective management, scientific discoveries and conservation efforts.
Recovering and Deploying Passive Acoustic Moorings for Monitoring Ocean Soundscapes
Each bottom-mounted mooring contains a stationary passive acoustic recorder that is anchored by a large weight to the seafloor and has flotation to hold the instrumentation up in the water column. Each bottom-mounted mooring is set out in a strategic location for several months or years.
During this last leg of the survey, our team went to a location near our planned track line to retrieve and redeploy a bottom-mounted passive acoustic mooring. This mooring is a part of the Ocean Noise Reference Station Network that began in 2014. It’s a collaborative effort between NOAA’s Pacific Marine Environmental Laboratory, NOAA Fisheries, NOAA’s National Marine Sanctuaries, and the National Park Service.
The network is made up of 12 recording devices that are stationed in the U.S. Exclusive Economic Zone with the aim to collect recordings of low-frequency ocean sounds. The sounds may include biological sounds, such as fish and baleen whale calls, natural sounds from the environment (e.g. wind, waves, and earthquakes), as well as human-made sounds from shipping vessels and other industries.
On this cruise, the team successfully retrieved a network mooring that had been recording for 2 years. We then deployed a new mooring that will remain in the water collecting data for another 2 years.
The new recording device is tethered between the weight and a float so that the recording device is held in the water column at almost 1,000 meters water depth. This places it in the Sound Fixing and Ranging Channel where ocean sounds can travel over great distances. After this survey is complete, our team will send the retrieved data to NOAA’s Pacific Marine Environmental Laboratory to save and analyze the sound recordings.
Jonny Reid, an acoustician, stated, “Retrieving the mooring and deploying the new one went really well. We worked as a team and were able to successfully complete the tasks in an efficient manner.”
The information collected from these long-term moorings will increase our knowledge and understanding of the sounds that contribute to ocean soundscapes, how noise levels are changing over time among sites within the network, as well as the acoustic behavior and biology of vocalizing marine mammals.
Deploying a Moored SoundTrap Array to Track Rice’s Whales
We successfully deployed another eight passive acoustic moorings, called SoundTraps™, to record sounds from Rice’s whales in the northwestern Gulf of Mexico. These recorders were deployed as part of a sparse array, with approximately 20 kilometers between moorings. The information they collect will help us understand how Rice’s whales are using this northwestern Gulf habitat.
Our acoustics team has known that Rice’s whales regularly occur in the northeastern Gulf of Mexico. Listening for their sounds with long-term passive acoustic recorders, we’ve recently discovered that they persistently occur in the northwestern Gulf as well.
“Their low-frequency calls can be detected over tens of kilometers. By deploying an array of SoundTrap recorders, we hope to detect the calls of individual whales on multiple instruments at the same time, which will allow us to localize and track calling Rice’s whales,” Soldevilla explained.
Examples of Rice’s whale calls:
- Audio of long-moan call
- Audio of long-moan call variant from the northwestern Gulf of Mexico
- Audio of downsweep sequence
Note: Audio has been sped up 1.25x to enable the human listener to better hear the very low-frequency sounds.
SoundTrap acoustic recorders are tethered just above the seafloor in water depths from about 100–400 meters. Acoustic recordings from a single mooring in the center of this study area and a sighting of a whale here in 2017 indicated this area represents important habitat for these rare whales. The acoustics team used this information to determine where to place the new SoundTrap mooring sites along the shelfbreak. The acoustic recorders will remain out at sea, collecting data for approximately 6 months.
Tony Martinez, chief scientist on the survey stated, “Our team was able to deploy eight SoundTraps in one day. We had about an hour between each station, so we needed to be efficient. It was a successful day!”
The team anticipates these acoustic instruments will record a variety of long-moan call types that Rice’s whales make in the northwestern Gulf. By acoustically localizing and tracking the calls, scientists hope to learn whether individual whales produce only one or multiple long-moan call types during an acoustic encounter and how many calling whales are around at one time.
This data will also provide information on how far we can hear their calls in the noisier waters of the western Gulf, and how the whales are moving around in this newly discovered habitat. This information will help us to understand how they may overlap with human activities that could put them at risk, and develop management actions to help in the recovery of this endangered species.