Listening and Learning in Cordell Bank National Marine Sanctuary
By Samara Haver
Imagine life in the deep, dark ocean where you cannot easily see or smell, but need to find food and companionship, and navigate from place to place while avoiding predators. Your eyes and nose would not be much help; instead, you would need to rely on sound to make sense of your environment.
In the ocean, light is rapidly absorbed and scattered by the upper layers of water, while sound travels very efficiently. Have you ever been in the upper deck of a baseball stadium and been able to see a player hit a ball before the audible crack of the ball-off-bat reaches you? In air, the speed of light is much faster than the speed of sound. If the baseball stadium were on the seafloor, it would probably be too dark to see the batter at all, but you would hear the ball hit over four times faster than you did on land. Marine animals have evolved to have an increased reliance on sound, and one of the best ways for us to learn about their underwater lives is by listening.
The Acoustic Environment
Using underwater microphones, called hydrophones, researchers can collect years of data to discover the “soundscape” of an underwater environment. A soundscape includes all of the sounds in a particular location at any time. For example, depending on the time of day, the soundscape of your neighborhood might include the rumble of cars and trucks driving by, wind rustling the leaves, or birds singing. There is a lot to be heard in every soundscape because humans, weather, and animals all generate sound.
Humans and many other animals rely on sound to communicate, and those that live underwater often rely on sound more heavily than land animals. In fact, many marine species evolved to rely on sound for all aspects of survival, including essential life functions such as finding food, navigation, finding mates, and for social interaction. Whales in particular are known to produce species-specific sounds or vocalizations that vary in amplitude (sound volume) and frequency (pitch). Through years of listening, marine researchers have learned to distinguish between these different sounds to identify specific species of whales without seeing them.
Microphones Under the Ocean
Although researchers have been using hydrophones to listen to whales since the 1940s, the invention of faster and smaller computers has revolutionized the way scientists record animal vocalizations. At first scientists could only record sounds from the platform of a ship, but now sophisticated instruments can be deployed in the ocean to record continuously for months, and sometimes years. This technology is now being used at multiple national marine sanctuaries around the United States to learn about whales and other sounds in the ocean, including Cordell Bank National Marine Sanctuary. In 2015, researchers deployed a passive acoustic hydrophone in Cordell Bank National Marine Sanctuary, close to the boundary with Greater Farallones National Marine Sanctuary, to listen for sounds from whales and vessels in the two sanctuaries. Watch this video to see footage from the hydrophone deployment and hear examples of different sounds in the ocean.
Whales Need to Hear
Whales are found worldwide, often in areas heavily used by humans. Human activities in the ocean are very noisy, creating sound through ship engines and propellers, oil exploration and drilling, and commercial fishing. Just as you have to talk louder to be heard in a crowded room, whales must also adapt to noises in their environment (student activity). Researchers at Cordell Bank National Marine Sanctuary were especially interested in finding out which species of whales could be heard in the sanctuary and when human-generated noise, such as from shipping lanes to and from San Francisco, might make it harder for whales to communicate and therefore survive.
Documenting a soundscape is a critical step for resource managers to understand which mammals are in an environment and when, as well as what their environment sounds like throughout the year. It is important that humans take steps to protect whales, including limiting sounds that can interfere with their communication, because they provide essential services to keep our ocean and planet healthy, improve fisheries, and support ecotourism.
What the Hydrophone Heard
By analyzing acoustic recordings from the hydrophone in Cordell Bank National Marine Sanctuary in 2015, 2016, and 2017, researchers discovered that blue whales (Balaenoptera musculus), fin whales (Balaenoptera physalus), and humpback whales (Megaptera novaeangliae) could be heard in the sanctuary. Blue whales and fin whales could be heard in the fall, winter, and spring, and humpback whales were heard throughout the year. Whales could be recognized through their songs even in months when they weren’t seen at the surface. Previously, blue whales had only been sighted in the summer and fall, and prior to having acoustic recordings, we were unaware that they were present in the winter and spring as well. This shows that when researchers rely solely on visual observations to track whales, they only see the small part of the animal’s life that is spent at the surface.
See if you can follow the sounds in the spectrogram below while listening to the associated audio clip!
In addition to monitoring whale vocalizations, the researchers also found that vessels traveling in the shipping lanes generated enough noise to be heard throughout Cordell Bank National Marine Sanctuary and much of Greater Farallones National Marine Sanctuary. By adding acoustic monitoring to the suite of whale research efforts in Cordell Bank National Marine Sanctuary, scientists are now better equipped to inform sanctuary managers about when harmful interactions between whales and vessels are more likely to occur. In turn, sanctuary managers are able to use the new information to take action to protect whales when they are most vulnerable to fatal collisions with vessels or excess noise.
Findings from the first two years of acoustic data collection were recently published in the Journal of the Acoustical Society of America. The hydrophone is still actively recording in Cordell Bank National Marine Sanctuary (2020), and as more acoustic data are collected, researchers will be able to evaluate changes in the soundscape over time. Also, many other national marine sanctuaries are currently recording underwater sound. These nation-wide hydrophones will make it possible for researchers to compare differences in how whales and human activities impact soundscapes at different locations across the United States.
Listening to Stakeholders
Monitoring the soundscape of Cordell Bank and Greater Farallones national marine sanctuaries to identify trends in human-generated noise is a conservation priority for NOAA. Tracking acoustic conditions and sources of noise provides critical information to sanctuary managers to inform decisions about habitat conservation. For example, the Vessel Strikes and Acoustic Impacts report generated by a Sanctuary Advisory Council joint working group details such recommendations.
In combination with monitoring marine mammal sightings and changing ocean conditions, passive acoustic data collection helps scientists and managers assess big-picture biological resources and anthropogenic impacts within sanctuaries. These research efforts are essential to our broader goals of sustaining healthy environments for animals and people throughout the sanctuary system.
Samara Haver is a NOAA Dr. Nancy Foster Scholar and Ph.D. candidate at Oregon State University.