海角大神

Reverberating through the ice shelves and gyres of the Southern Ocean are the undersongs of the largest animal that has ever lived on this planet, the Antarctic聽blue whale. Telling tales of the hunt for krill, of navigation and seduction, these tunes can carry for hundreds of miles.

And the world is listening: Moored around Antarctica is a loose ring of passive acoustic monitoring devices, or PAMs, deployed by various academic institutions. Released by oceanographic research vessels, the devices sink to the seafloor where they record a remote and often hostile realm that is practically out of reach of scientists. After about a year, a returning ship plays a coded message that trips a wireless trigger and frees the PAM recording to the surface.

鈥淭he idea here is that these animals are really rarely encountered on oceangoing voyages through visual surveys. There鈥檚 a small number of sightings of them, but they make really loud noises that can be detected over really large ranges,鈥� says Brian Miller, a marine mammal acoustician at the Australian Antarctic Division in Kingston, Australia.听

After industrial whaling annihilated 99% of the blue whale population during the 20th century, the International Whaling Commission banned hunting in 1966. The critically endangered Antarctic blue whale is the largest of the species, with some reaching 110 feet in length and 330,000 pounds. Scientists estimate that about 3,000 individuals remain. And although blue whales may be well known, they are not well understood.听

PAMs are an excellent method to get to know them better, Dr. Miller says. 鈥淚t seems like listening for these animals might be a better option for monitoring their recovery and their status ... than spending lots of money to send lots of ships with visual observers to look for them.鈥�

Both sexes produce what researchers label 鈥淒鈥� calls such as these, which are believed to be used for social contact, especially during foraging. The noises here are sped up eight times. Headphones are recommended. (Credit for this and subsequent audio:聽Brian Miller/Australian Antarctic Division)

Courtesy of Dr Kate Stafford/Oregon State University

PAM devices tune into a world that is anything but silent. Sounds collected from this instrument will be stored in an acoustic library developed by scientists to better understand the blue whale.

When a PAM recording returns to its laboratory, analysts plumb its depths for sounds that can help determine the blue whale鈥檚 migratory paths and population, as well as new insights into its life. And yet, a year鈥檚 worth of sound also requires ... a year鈥檚 worth of listening.

To solve the prohibitive task of deciphering such a huge data set, cetologists have allied with computer scientists to develop custom software algorithms 鈥� instructions based on the patterns of previous whale sounds scripted into computer code 鈥� that can rapidly swim through oceans of data to home in on whale calls. Considering the constrained resources and diverse skills required for such work, these bespoke software packages may be only partially effective 鈥� or are poorly distributed among researchers.

So, to pool resources, unite international efforts, and present a broad range of recordings to test automated algorithms for the study of Antarctic blue whales (and fin whales, another species of concern), the scientific community has been broadening its efforts. The Acoustic Trends Working Group 鈥� funded through the International Whaling Commission鈥檚 Southern Ocean Research Partnership 鈥� has developed the .听

Dr. Miller, who served as project coordinator and lead author of the paper announcing the library, worked with colleagues to gather 2,000 hours of acoustic data from five nations, representing five separate years and instruments in each of the four circumpolar Antarctic regions. Next, a coterie of highly skilled analysts with extensive experience in identifying whale calls made 105,161 annotations identifying sound types, durations, and frequencies. Divided into hourlong blocks, the recordings were converted into standard .wav audio format and coupled with annotated text files.听

Fin whale 鈥渄ownsweep鈥� notes: These repeated sounds form a song produced only by males during the breeding season.

While blue whale calls can exceed 180 decibels 鈥� louder than the roar of jet airplane 鈥� most of their sounds are infrasonic, meaning the frequency of their sound waves falls below the threshold of human hearing. So, to 鈥渉ear鈥� a blue whale, analysts watch for its sounds by feeding recordings into spectrogram software, which in turn renders sound signals into two-dimensional representations of waves across their computer screens.

鈥淚t is quite tricky,鈥� says Emmanuelle Leroy, a bioacoustician who helped develop the聽digital library, about the annotating process. Variation in marking the boundaries and frequencies of calls is common between annotators and even for a single person when repeating work, she says. Improperly marking a sound can result in algorithms detecting false positives or missing whale sounds altogether.听

Because the library aims to provide the 鈥済round truth鈥� data set for scientists the world over to test their algorithms against, the team developed strict annotation parameters. Dr. Leroy says she enjoys looking at acoustic data, but the time she spent was taxing. 鈥淥f course, you dream of [annotating] during the night,鈥� she laughs. 鈥淵ou see .wav vocalizations everywhere.鈥�

Background interference, such as the weird whining and cracking of polar ice, is common in the recordings but also important in training algorithms to cut through the clamor of the sea and identify targeted sounds. 鈥淚t comes down to diversity. If you want to analyze diverse data sets, you鈥檝e got to train your algorithms on diverse data sets,鈥� says Dr. Miller.

The acoustic library is already being used by researchers at the University of Concepci贸n in Chile to compare blue and fin whale vocalizations over time and at different sites. And Shyam Madhusudhana, a postdoctoral fellow at Cornell University鈥檚 Lisa Yang Center for Conservation Bioacoustics, is using the聽library in the development of a global baleen whale sound detector, whereby researchers can simply drop in their own data sets to rapidly identify calls. He is applying deep-learning computer software to construct the detector.听

Courtesy of Dr Kate Stafford/Oregon State University

To listen for cetaceans, a hydrophone is deployed in the Southern Ocean. Scientists around the world are collecting deep-sea sounds to track the critically endangered blue whale.

Already successful in automatic speech recognition, deep learning mimics the way humans learn. Employing layers of increasingly complex computing system nodes, called artificial neural networks, the technique synthesizes data in the form of continuous examples 鈥� as opposed to linear instruction 鈥� to arrive at an answer. This method, which should more accurately identify whale calls across distance and throughout turbulent soundscapes, requires huge caches of training data. Hence the whale acoustic library, which, Dr. Madhusudhana says, is 鈥渧ital for this field.鈥�

Unlike other cetaceans, such as the humpback whale with its assortment of higher-frequency moans, shrieks, and bellows 鈥� and which Dr. Miller calls 鈥渢he jazz improv singers of the whale world鈥� 鈥� blue whales produce sounds that are simpler and can be reduced to two classifications with a medley of underlying units and phrases. Only males sing, presumably to attract a mate, whereas both sexes produce downsweeping 鈥淒鈥� calls, thought to be used for social contact. Dr. Miller refers to the blue whale sounds as the 鈥渞hythm section.鈥�

The repetitive聽鈥漐鈥� call:聽There are only a few sounds in the Antarctic聽blue whale repertoire. This call (named 鈥淶鈥� for the look of its sound pattern rendered visually) rolls into a song sung only by males.听

And yet for all its weight and sonance, the animal remains an enigma. 鈥淏lue whales are really mysterious,鈥� says Dr. Miller. 鈥淗undreds of thousands of them were killed during whaling, and we learned so little about them. ... We still, to this day, don鈥檛 know where the breeding grounds of the Antarctic blue whales are.鈥�

Dr. Leroy, who is now on R茅union Island in the Indian Ocean researching the vocal repertoires of local cetaceans, agrees about the mystery. While she says that acoustics offer vital tools for understanding the seasonal and geographic movements of the Antarctic blue whale, there is still much to learn.听

鈥淲e are blind,鈥� says Dr. Leroy. 鈥淲e see the songs, but we are blind.鈥�