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Noisy fish? Lake Superior underwater noises include ships, wind and ... eelpout

Researchers on the Blue Heron, the research vessel of UMD's Large Lakes Observatory, get ready to deploy an underwater microphone to record sounds 75 feet under the surface of Lake Superior. They picked up some interesting results. Photo courtesy UMD.

Jay Austin and his University of Minnesota Duluth research team were trying to study drifting ice sheets on Lake Superior when they ran into a problem.

Data from the underwater Doppler recording devices was so polluted by background noise that they couldn't determine any results.

Bummer for the ice research. But like any good, inquisitive scientist, Austin was intrigued.

What the heck was making all that racket underwater in Lake Superior?

In the case of the fouled ice data, it was ice crashing into itself during the big-ice winter of 2013-2014. But Austin was curious: Are there are other sources of sound under the lake as well? There are reams of research on underwater acoustics in oceans worldwide. But when the UMD physics professor and Large Lakes Observatory researcher started to search for data on underwater sound in large lakes, Austin found nothing.

"It wasn't that it was hard to find. It was nonexistent," Austin said of Great Lakes acoustics research. "There aren't any whales or submarines ... or dolphins out there in big lakes, so apparently no one thought it was interesting enough to look at."

So Austin decided to take a listen for himself. Last summer a Large Lakes Observatory team set an underwater microphone about six miles out from the McQuade Road boat landing on the North Shore. The microphone hung about halfway down in 150 feet of water for a week.

When the recorder was retrieved, data revealed three distinct sounds never before reported in Lake Superior.

The first was nearly constant, peaking and ebbing in lower intensity. The second was shorter in duration and had much higher peaks.

Wind, Austin discovered, created the lower level sounds. Data from surface wind meters on nearby buoys correlated almost perfectly with the variations in sound underwater. The higher the wind, the louder the sound underwater.

Not rocket science, Austin figured, but interesting. The findings showed you could effectively measure wind speed even when surface gauges weren't possible simply by recording underwater sound.

The second sound, louder and shorter duration, turned out to be passing ships. The microphone picked up freighters going in and out of the Twin Ports harbor at both Duluth Entry and Superior Entry. The sounds correlated with GPS tracking data that showed when ships passed by.

Eelpout 'clicks'

The third sounds researchers discovered were clicks — short but intense bursts of sound. There were only 70 of them recorded during the weeklong experiment, but they were very noticeable.

This time Austin did find research to explain what he was hearing: Burbot, a prehistoric fish in the cod family that looks like a cross between a catfish and an eel. Also also called eelpout, burbot are known to emit clicking sounds.

"Again, it's interesting. I'm not sure what will come of it," Austin said, saying he was unsure if the clicks were truly fish communicating or just random noises they made.

A study funded by Canada's Department of Oceans and Fisheries published in 2014 found that burbot in a pen deep underwater in Canada's Great Slave Lake indeed did seem to communicate, or at least intentionally emit sounds, especially at mating season. The acoustic profiles of those sounds were nearly identical to those "clicks" recorded under Lake Superior.

Peter Cott, lead researcher in the Canadian study, described the sounds as "songs of the burbot." Cott's team of researchers said the deepwater fish that live in very low light may have evolved to communicate by sound because they can't see each other well.

Austin isn't sure that's what he was hearing from Lake Superior burbot in the summer. But there was one curious aspect: Whenever the ships passed nearby, the clicking stopped. There were no clicks at all when the noise of ships was present, which shouldn't have happened if the clicks were truly random.

"That's interesting because it makes it seem like they (burbot) are reacting to a physical presence. It seems to suggest a behavioral response (to ships going by) that they can control the sound," Austin said. He's finishing up his data analysis before passing his research on to marine biologists for them to decipher and, if they choose, pursue further.

Winter data coming

Austin presented his research as a poster at the annual conference at the Acoustical Society of America in Boston last month. He's writing a manuscript that will be submitted to peer review later this year to determine if the research will be formally published.

There may be more to come. Last winter researchers left a microphone underwater for months, retrieving the recorded data this spring. They had hoped to hear the sounds of ice forming, breaking up and crashing against itself, but the warm winter didn't allow for much Lake Superior ice. They are still downloading the winter data and don't yet have results, but will soon.

Austin is quick to say he's not sure if anything will come of his sound research or if it will go any further.

"It may well end up that this is the end of it, that's there's really nothing to learn from this. But someone has to look at it first to determine that," Austin said.

He compared the sound research to his landmark climate research from a decade ago when he was the first to go back and graph temperature data from Lake Superior buoys taken between 1979 and 2006 — data that showed a remarkable warming trend of the big lake's surface water, warming faster than almost any other climate science had shown. The data had been available for years, but no one had looked at it closely.

"If you look at the arc of my research career it's a series of sort of stumbling into things," he said.

Austin isn't thinking his sound research will rival the climate work. But you don't know if you don't try, he noted.

"I tell my students, don't assume that because it's something simple that it's already been checked out," Austin said. "If it seems interesting, follow it through. You never know where it will go."

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Underwater waves? Lake Superior has them

A more than $1 million research project is underway on Lake Superior as scientists from three universities look at what causes underwater waves and what impact those waves have on the big lake's ecosystem.

Scientists from the University of Minnesota Duluth's Large Lakes Observatory are joined by cohorts from Oregon State University and Scripps Institute of Oceanography in the project funded by the National Science Foundation.

Sam Kelly, a UMD associate physics professor and Large Lakes Observatory researcher, is heading the effort.

The observatory's research boat, the Blue Heron, has been out this past week deploying dozens of monitoring devices to measure temperature and wave dynamics below the surface. They aren't currents, but internal, or underwater waves that move based on the water density and temperature layers, or stratification.

As that energy is transferred further below the surface, it causes underwater waves that roll up and down. That underwater wave action disperses nutrients and cold or warm water up and down in the water column.

Waves on the surface of a big lake or ocean can only grow up to a few feet tall at most before breaking down, because water is heavier than air. Underwater waves are made of deep, cold, dense water rising into layers of shallower, warmer, lighter water. They can reach huge size — hundreds of feet in the ocean; no one knows yet how big they are in Lake Superior.

Experts say turbulence from these waves can generate thousands of times more mixing in deep water than than previously thought and that's moving scientists to rethink global climate models and how ocean warmth is dispersed.

The underwater turbulence is likely having an impact on the lake's ecosystem, but no one knows yet how much. It also may be transferring energy, via warmer water, to depths where warm water hadn't been present before. Lake Superior's surface waters have been warming rapidly in recent decades.

Underwater waves are being studied in oceans, but because of tides and other factors, have been hard to pin down. It's hoped the Lake Superior research will help broaden understanding of the phenomenon since there's no tide to deal with.

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