A hydrophone. Image: by wildestanimal via Getty Images
Scientists have recorded the bustling sounds of underwater life in coral reefs off the coast of Goa, India, as part of a new study that revealed insights into marine biodiversity and detected a strange “buzz” call from a mysterious unidentified animal.The research is part of a broader effort among marine biologists to use underwater sound recorders, known as hydrophones, to capture the soundscapes of aquatic wildlife around the world. Hydrophones can passively and non-invasively record underwater acoustics for days at a time, enabling scientists to eavesdrop on marine life to learn about their behavior, movements, and their response to environmental changes.
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To better understand the rich reef ecosystems of Goa, a team of researchers at the Indian Council of Scientific and Industrial Research’s National Institute of Oceanography (CSIR-NIO) placed a hydrophone near Grande Island at a depth of about 65 feet. Over the course of several days, the instrument captured hundreds of recordings of the choruses of “soniferous” (sound-making) fish, the high-frequency noises of shrimp, and the rumblings of boats passing near the area.The results unveiled fascinating details about this vibrant ecosystem that will be presented on Wednesday during a meeting on the International Quiet Ocean Experiment in Woods Hole, Massachusetts. The research also appeared in a new study published by the Journal of the Acoustical Society of America.“Our research, for the longest time, predominantly involved active acoustics systems in understanding habitats (bottom roughness, etc., using multibeam sonar),” said Bishwajit Chakraborty, a marine scientist at CSIR-NIO who co-authored the study, in an email to Motherboard. “By using active sonar systems, we add sound signals to water media which severely affects marine life.”
“We realized that using passive acoustics, i.e., using hydrophones to record underwater soundscapes can enable us to record the underwater sounds and conduct our studies to learn more about the environment without adding any sound to the media that may impact the quality of life of underwater organisms,” he added.The underwater soundscapes revealed that fish that feed on tiny ocean creatures called plankton clearly synced up their songs to the cycles of the Moon’s phases. With the help of machine learning, the researchers were also able to match hundreds of different calls to marine species, including drums, grunters, perches, and shrimp.“We carried out our investigation in Goa due to the presence of an active coral reef system which is easily accessible and conducting research here was logistically easy as well,” said Chakraborty. “We had good knowledge about the background of the area and biodiversity as well. Initial study by marine biologists in this area has suggested the availability of about 115 fish species here. Out of these 115 species, 55 are soniferous fishes, and the sound recordings of 21 of these fish species were available.” In addition to these known fish species, the team overheard a weird buzzy call from a mystery animal that they couldn’t identify, but sounds similar to another unknown chorus that was recorded off the coast of North Carolina.
“The spectral, temporal data reveal structural parameters of the sound data which suggest that the sound belongs to a fish (biophony),” Chakraborty said. “However, the lack of a centralized repository of archived sound recordings acts as a major hindrance and does not allow us to confirm the type of fish species based on the data.”“Unidentified sounds can provide valuable information about the richness of the soundscape,” he added. “As the nature of animal sounds indicates their behavior, we need to record and understand these sounds.”Indeed, many hydrophone studies have reported these tantalizing sounds with no clear origin, which is one of the advantages of tuning into these ocean broadcasts. Annie Innes-Gold, a PhD student at the Hawai'i Institute of Marine Biology, has compiled many of these unexplained sounds into this YouTube channel.In this way, hydrophones can expose phenomena in the ocean that might escape other observational methods, which is particularly important in an age of major human pressures on marine ecosystems, such as overfishing, plastic pollution, and climate change. That’s why scientists are working to build a Global Library of Underwater Biological Sounds that will help to monitor the soundscapes of the changing ocean, and the response of life to these challenges, according to a 2022 study in Frontiers in Ecology and Evolution.
“At a time when global biodiversity is in significant decline and increasingly impacted by climate change, there is a need to document and understand as many sound sources in the ocean as possible, potentially before they disappear,” the study said.To that end, Chakraborty and his colleagues hope to see the deployment of hydrophones around the world to collect these valuable recordings, while also developing machine learning methods that can more efficiently analyze and identify underwater sounds. “Acoustic monitoring will enable us to know more about the climate and its related long-term changes in the physical and biological condition of the underwater environment/organisms,” Chakraborty said. “For example, as Coral reefs are the hub of underwater biodiversity, conducting acoustic studies here are extremely essential to know the state and health of the reef and its inhabitants.” “Marine life is directly impacted by change in temperature, climate conditions etc. as they are mostly ectothermic by nature (with the exception of mammals) which loosely translates to, that they regulate their body temperature based on the surrounding temperature,” he concluded. “Therefore, in such cases, climatic changes are bound to have an impact on them, which in turn will affect their behavior patterns. This makes it extremely imperative to study these behaviors/change in behaviors to have an in-depth understanding of climate and its impact on underwater organisms.”Update: This article was updated to include comment from researcher Bishwajit Chakraborty.