The croak-like sounds heard off the coast of New Zealand in the 1980s may be a form of communication. Mysterious repetitive sounds emanating from the depths of the ocean may have frightened some, but in the 1980s they offered a fascinating glimpse into the underwater soundscape.

In July 1982, New Zealand researchers recorded unknown sounds as part of an experiment to characterize the soundscape of the southern Fiji Basin. The sound consisted of four short, croak-like bursts, which inspired the sound’s name “Bio-Duck”.

“The sound was so repetitive that at first we couldn’t believe it was biological,” said researcher Ros Chapman from the University of Victoria. “But when we talked about the data with other colleagues in Australia, we found that these types of sounds were quite common in New Zealand and other parts of Australia.”

They agreed that the sounds must be biological. Chapman recently presented his work on the analysis of mysterious sounds at the 187th meeting of the Acoustical Society of America.

Disclosure of confidential data

“I became involved in data analysis of the experiment in 1986,” Chapman said. “We found that the data contained a golden vein of new information about many types of sounds in the ocean, including the sounds of marine mammals.”

“You have to understand that this type of ocean noise research was in its infancy at the time. As we examined the data further, it became clear that we were learning something new about sound in the ocean every day; it was a really exciting time for us,” he said.

However, the sounds were never positively identified. There are theories that the sounds were made by Antarctic baby whales, as the sounds were also recorded in Antarctic waters in later years, but there was no independent visual evidence of whales making the sounds in the New Zealand data.

Underwater communication theories

No matter the animal, Chapman believes sounds can be a conversation. The data was recorded by an acoustic antenna, a hydrophone system towed behind the ship. The uniqueness of the antenna allowed the researchers to determine the direction the sounds were coming from.

“We found that there were often several different speakers in different parts of the ocean and they were all making these sounds,” Chapman said. “The most surprising thing was that when one speaker spoke, the others remained silent, as if they were listening. Then the first speaker stopped talking and listened to the answers of the others.”

The study, as well as the waveform and spectrum of recordings during the session, will provide further evidence that it is a conversation between several animals.

“In my mind it was always an unanswered question,” Chapman said. “Maybe they were talking about dinner, maybe the parents were talking to the kids, maybe they were just commenting on the crazy ship going back and forth and pulling that long rope behind it.”