For the last five years, an underwater microphone deployed by the Monterey Bay Aquarium Research Institute, positioned on the Monterey Bay seafloor, several hundred feet beneath the surface of the ocean, has been recording the sounds of sea -- including the spooky songs of blue whales.

While analyzing the tremendous wealth of data, researchers noticed blue whale songs follow a seasonal pattern, according to a study published Thursday in the journal Current Biology.

Recordings from the summer months, when the blue whales were visiting the Pacific and spending their days eating krill, revealed a preference for nighttime singing. But the audio recorded during the fall and winter demonstrated a reversal -- silent nights and days filled with song.

"This was a very striking signal to observe in such an enormous dataset, and led us to ask the questions: what drives these population-level patterns in song, and do these patterns indicate changes in blue whale behavior through the seasonal cycle?" lead researcher William Oestreich, a doctoral candidate in biology at Stanford University, told UPI in an email.

To answer those questions, Oestreich and his colleagues turned to tags, which helped the research team track the diving, movement, feeding and singing behaviors of individual whales.

"We found that individual blue whales that are feeding and have not yet started migrating south sing primarily during the night, whereas blue whales which are migrating sing primarily during the day," said Oestreich, lead author of new paper.

"By analyzing data from these tags, we discovered that the nighttime tendency for singing that these whales display during their months of feeding is driven by a tradeoff between singing and feeding behaviors within a 24-hour day," he said.

During the day, krill are often found densely packed deep beneath the ocean surface. To take advantage of this concentration of food, blue whales spend most of the daylight hours during the summer diving and eating. At night, the krill rise to the surface and spread out, and as a result, whales have more time to sing.

"Once these whales begin migrating south, however, they feed very sparsely -- and often not at all," Oestreich said. "Because there is no longer a tradeoff between this deep water feeding behavior and singing, the individual whales are able to sing throughout the daytime during migration."

Scientists aren't sure why migrating blue whales tend to stop singing at nighttime.

The acoustic signature discovered by Oestreich and his research partners will help marine biologists and conservation scientists track blue whale migrations.

Blue whales, the largest mammals on Earth, rely on the large krill populations found off the west coast of North America to fuel their trek south to their breeding grounds off the Pacific coast of Central America. According to Oestreich, understanding their season movements is key to protecting blue whale populations.

"We are now better able to monitor when these blue whales are migrating in relation to changes in the ecosystem they inhabit," Oestreich said.

The newly analyzed acoustic recordings have also offered researchers new insights into why blue whales sing. The data showed the whales tend to sing more and more as they prepare to migrate, which lends support to theories that singing is mostly the domain of male whales and used to attract mates.

It's possible blue whale songs serve multiple purposes, and researchers hope to continue using a combination of tracking data and hydrophobic recordings to better understand how and why blue whales deploy their vocal tools.

"Now that humans are able to determine whether these blue whales are feeding or migrating just by listening, it begs the question: do blue whales listen and use this signal as well?" Oestreich said.

"These animals live in low population densities over vast stretches of the Pacific Ocean with variable conditions and food supplies, meaning that making a 'well-informed' decision about when to give up on feeding and start migrating south is difficult based on one's immediate surroundings," he said.

It's possible the songs of individual whales encode information that help their relatives make better decisions about when to feed and when to begin their trek south to breed, but Oestreich said more research is needed.

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