Using sound to gather ecosystem information
This blog post comes from guest blogger, scientist Alexis Rudd.
It’s 2:00 am and I’m on the NOAA Ship Oscar Elton Sette, locating dolphins in the dark. The water around the ship is inky black and smooth, and I am inside staring at a computer screen. So how am I locating these dolphins? With sound.
Sound is a very important tool on this research cruise, and we are using it in two ways. First, we are “passively” listening to the sounds in the ocean to locate and identify dolphins and whales. In addition, we are “actively” sending out sounds from the ship, which bounce off of marine organisms and echo back to the ship.
The passive acoustics is conducted using something called a hydrophone array. A hydrophone is basically an underwater microphone, and an array is a set of hydrophones. Our hydrophones are attached to a cable, which streams out behind the ship, like a 300 m long tail. Each hydrophone records underwater sounds. On this cruise we are specifically interested in the sounds of dolphins and whales. Dolphins make three main types of sounds: whistles, which are used for communication, clicks, which are used for echolocation and foraging, and “buzzes” which are really just sped-up clicks. Buzzes are especially interesting, because they are usually used when dolphins or echolocating whales are closing in on their prey.
Caption: An example of the spectrograms of sounds recorded by our hydrophone array. Picture from scientist Alexis Rudd.
Thus far, we have recorded sounds from at least 6 species: pilot whales, spinner dolphins, striped dolphins, bottlenose dolphins, rough-toothed dolphins, and sperm whales.
Caption: Left, pilot whales spy hopping. Right, a striped dolphin. Photos by scientist Chad Yoshinaga.
Nighttime has been especially exciting for the last couple of days. For example, in the last 7.2 miles we have traveled, I have not stopped hearing dolphin clicks and whistles for more than a minute. However, on other nights we can travel hours and hours without hearing a single whistle.
For each of these groups of dolphins or whales, we use physics and math to calculate their location. Sound travels through the water at a specific speed, depending on the density, salinity, and temperature. When a dolphin or whale makes a sound, some of the hydrophones on the array are further from the dolphin than others. As a result, each hydrophone records the dolphin sound at a slightly different time. Using this time difference and basic high school trigonometry, we can calculate the location of the dolphin that made the sound.
While with passive acoustics you can figure out the location of an animal making a sound, with active acoustics you can figure out the size of an animal, using the characteristics of its echo. Different animals echo more loudly at different frequencies of sound. Frequency refers to how high pitched a sound is. For example, fingernails on a chalkboard are a very high frequency, and the rumbling of thunder is a low frequency. Smaller animals echo better with high frequency sounds, and larger animals with lower frequencies. Using different frequencies, we are able to measure the depths, densities, and compositions of the plankton and fish in the ocean. We can compare these layers to oceanographic information such as the topography of the ocean floor, sea surface temperature, and currents.
Caption: An example of data gathered using active acoustics and the various features detected. Figure by scientist Adrienne Copeland.
The combination of the use of active and passive acoustics is excellent for the looking at multiple aspects of an ecosystem, from oceanography to prey distribution to top predators like whales and dolphins. Understanding how these multiple processes work together is an essential part of understanding the ecosystem.