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Scientists from Scripps Institution of Oceanography at the University of California San Diego were able to "hear" the impacts of a marine heat wave and even economic slowdowns by analyzing 15 years of ocean sounds recorded off the coast of Southern California.

The recordings, collected between 2008 and 2023, allowed researchers to hear whales moving north in response to a marine heat wave that began in 2014 as well as the massive decrease in noise from container ships during the 2008 financial crisis.

The findings, published in the , show that listening to the sea can be a tool for monitoring ocean ecosystems and even human economic activity.

Sound travels far and fast in the ocean. The loud, low-frequency songs of giant whales can be heard up to 1,000 kilometers (620 miles) away, and sound travels roughly four times faster in seawater than in the air. This is why whales evolved to use their voices to communicate underwater. Unfortunately, human activities, such as global shipping, are also filling the oceans with noise that can make it harder for whales to communicate.

The researchers behind the study are part of the Scripps Marine Bioacoustics Research Collaborative, where, according to study co-author and Scripps postdoctoral researcher Natalie Posdaljian, "the question we are always asking is what can sound tell us about the health of the ocean or what is happening in the ocean."

In 2023, the study authors delved into recordings collected by six underwater microphones that were secured to the seafloor off the coast of Southern California in 2008. Since their installation, these microphones have created the longest acoustic timeseries on the California coast.

Long-term sound recordings are exceptionally rare and valuable at a time in history when climate change is rapidly altering ocean conditions. For creatures like whales that spend much of their time underwater, tracking their movements can be challenging, but sound offers a wide-reaching, cost-effective window into their activities.

To focus their analysis of the sounds recorded in various locations in the Southern California Bight, the team chose four specific frequency ranges that correspond to different sources of sound in the ocean.

The study analyzed 18–25 Hertz (Hz) for fin whale calls, 40–48 Hz for blue whale calls, 63 Hz for ship noise and 800 Hz for wind-generated sounds. Once they had isolated these frequency bands, the team compared the patterns they found with climate data, ship tracking information, wind measurements and major economic events to identify relationships between ocean sounds and environmental changes.

The study revealed dramatic shifts in whale behavior during the 2014–2016 marine heat wave known as "the Blob": whale calls decreased by up to 50% at southern sites while increasing at northern sites near the colder water of the California Current.

Ship noise levels crashed by over 40% during the 2008–2009 financial crisis and still hadn't returned to pre-recession levels by 2023. The recordings even captured daily "rush hours" in the ocean, with ship noise peaking at 3 a.m. and 2 p.m. Pacific Time as vessels arrived and departed the Ports of Los Angeles and Long Beach.

The study also found a relationship between whale activity and a climate phenomenon called the (PDO), which is a bit like a longer-acting version of /La Niña that occurs in the North Pacific instead of near the equator.

The PDO is associated with shifts in ocean temperature patterns, alternating between warmer and cooler phases roughly every 20 or 30 years, though recently the intervals have been shorter. Fortunately, the dataset included a phase change of the PDO that revealed that during the cool phase, fin whale calls were 30% more frequent than during the warm phase.

The findings show how ocean soundscapes can serve as a near real-time monitoring system for marine ecosystem health, providing early warning of species displacement and habitat shifts due to climate change and increasingly frequent marine heat waves.

The connection between economic events and ocean noise also demonstrates yet another dimension of humanity's pervasive effects on the natural world.

"We can actually hear climate change and economic shifts in the ocean when we use this type of analysis," said Posdaljian.

The study also underscores the importance of long-term monitoring.

"If we just had sensors in the water during or after a marine heat wave, we wouldn't be able to establish a baseline for how the ocean normally sounds," said Vanessa ZoBell, a Scripps postdoctoral researcher and lead study author.

"That baseline creates a basis for comparison and seeing how things change can give us ways to identify and protect areas that could serve as refuges for marine life in the future."

"Current funding challenges are a big threat to this type of long-term monitoring, which depends on consistent support," said Posdaljian. "If funding is interrupted, we will lose valuable data, and with it the ability to detect long-term trends in how the ocean is responding to climate change and human activity."

ZoBell and Posdaljian recommend expanding acoustic monitoring to other ocean regions to track global patterns of marine life moving in response to climate change. Remote locations that are harder or more costly to access or monitor via other means could especially benefit from the passive nature of sound collection.

The study authors also suggested shipping companies and ports could use soundscape data to measure the effectiveness of vessel quieting technologies and speed reduction programs aimed at reducing ocean noise and the incidence of .

"You can learn so much just by listening," said ZoBell.