Secretive or rare marine mammals can prove problematic for scientists who want to take a head count. But a new model provides a different way for researchers to census these species — don't look for them, listen instead.
A team of researchers from the University of New South Wales in Sydney, Australia, have found a way to decipher the distinct calls of leopard seals in Antarctica, and translate them into population size.
"Because they have this really stylized acoustic behavior, we can turn numbers of seals into numbers of sounds," said ecologist Tracey Rogers, who helped create the model. "[We can] use this technique to monitor over a long period to see if their populations are changing…and where they are, whether that's changing or not."
Using acoustics, Rogers and her colleagues detected 10 times the number of leopard seals than had been counted using traditional visual surveys. "So it's not that they weren't there, they were there all the time, it's just we weren’t detecting them," Rogers said.
Hard to spot
Leopard seals can reach 10 feet long (3 meters) and weigh up to 750 pounds (340 kilograms). They are the only seals known to eat other seals, and their only predator is the killer whale. But what makes them tricky to count is their habitat. The giants live on pack ice, which is old sea-ice that formed elsewhere and later floated off with the wind and currents. The floating ice is very difficult to work in, and can make surveying the species challenging and expensive. In addition, the dark-grey seals are easy to miss.
"If they're not out on the ice, and they're in the water — the dark seals in the dark water — you don't see them at all," Rogers said.
These seals are also widely distributed, and don't live in packs like some other marine mammals — more like solitary tigers than a pack of wolves, Rogers said.
This dispersive nature also makes it hard to know what's going on with the seal population, in terms of size and location.
But because these animals are so spread out, they need to call to each other to know who is there and determine age and gender. This behavior helps them find a breeding partner.
"To actually find a mate they tend to sing and call to each other, so they tend to have these very stereotyped calling behaviors," Rogers said.
The researchers analyzed and figured out the different calling patterns to use in their model.
"We’ve cracked their communication code, so we can use their own communication patterns just like they’re doing," Rogers said.
The model also uses information about the loudness and frequency of the calls, and how far away they are. This sound data can thus reveal information about the animal's behavior, telling the researchers which areas the seals think are good habits and which aren't. Some of the calls can be detected over an area of 54 square miles (140 square kilometers), about twice the size of Manhattan.
The model still needs some improvement, Rogers said. The way sound travels underwater can be looked at in different ways, and the researchers want to test out different variations of the model.
After future tests, this model would be ideal for studying some other marine mammals that have behavior similar to leopard seals, such as the ross seal.
"They are even more rarely seen in the visual surveys than leopard seals, but acoustically, we were hearing a lot of them," Rogers said.
The work was presented April 19 at the Acoustical Society of America's 159th meeting in Baltimore, Md.