Passive Acoustic Surveys
Passive Acoustic updates from the PIFSC Blog
Although we most commonly study cetaceans through visual surveys, we are only able to count them when they surface to breathe. To improve our abundance estimates and provide additional information, we eavesdrop on the sounds produced by cetaceans below the surface.
Why use passive acoustic monitoring?
Sound travels much further in water than light, and cetaceans commonly use sound rather than vision to communicate with each other and to find food. Just as cetaceans listen for each other and for their prey, we can listen in on their sounds to locate groups of whales and dolphins. Some cetaceans exhibit cryptic behavior and can be very sneaky, avoiding the boats and ships that we use to study them. Poor weather and rough seas can also make it difficult for the visual team to observe animals from the ship, but the acoustics team can still detect animal sounds in rough conditions. The act of quietly listening for marine mammal sounds is known as passive acoustic monitoring. By monitoring the ocean for cetacean sounds, the acoustics team can locate and track animals, including some that might otherwise go unseen by the visual team.
How does passive acoustic monitoring work?
An acoustic array is used to eavesdrop on vocalizing marine mammals. The acoustic array consists of a series of hydrophones (underwater microphones) that are towed behind the ship while it is moving along the trackline. Because the array is towed, it is also called a towed array. Sounds recorded by the hydrophones are immediately transmitted back to the ship. Scientists monitor the sounds by listening with headphones and watching a visual representation of the sounds known as a spectrogram because some sounds cannot be heard by the human ear. Using these two techniques, the acoustic team can determine when a vocalizing whale or dolphin is present and what species it is based on various characteristics of the sounds produced. Once animals are detected, the time difference of sound arrival to the hydrophones is used to locate and track the cetaceans as they move. Depending on the species and its location, the acoustics team may alert the visual team to the presence of animals in the area and help guide the ship to them.
What other things can we do with passive acoustics?
Although real-time monitoring and tracking of vocal cetacean groups is the primary mission of the acoustics team at sea, periodically we use other sensors to listen in on whales and dolphins. The towed array is designed to filter out very low frequency sounds so that dolphin sounds are not masked by ship sounds and the noise of the array flowing through the water. However, many baleen whales species produce low-frequency sounds, meaning we cannot detect these species using the array. When the visual observers identify baleen whales, we often deploy a sonobuoy to record the sounds from these whales. A sonobuoy has a hydrophone suspended from a cable with a VHF radio floating at the surface. Sounds heard by the sonobuoy hydrophone are transmitted back to the ship over the radio frequency.
Long-term acoustic recorders are also used to listen for whales and dolphins over periods of months to years in a specific area. High-frequency Acoustic Recording Packages (HARPs) record a very broad frequency range allowing us to monitor the occurrence of all vocal species for more than a year before they are recovered. HARPs need to be recovered for us to identify the cetaceans recorded. PIFSC maintains a passive acoustic network with several long-term monitoring sites.