Wondered Resting Cargo Vessel Noise Levels At Anchor?

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Kelsie Murchy led a group including Fisheries and Oceans, Cowichan Tribes and B.C. Conservation Foundation, says an article published in Vancouver Sun.

Increased Four-Fold Globally

Those bulk carriers anchored in Burrard Inlet, English Bay and at ports up and down B.C.’s coasts are not resting as peacefully as they might seem to be.

It’s estimated commercial shipping has increased four-fold globally over the past 20 years and the noise those vessels create while in motion has been well-studied.

Measuring Underwater Soundscape

But Kelsie Murchy, a doctoral student at the University of Victoria, wondered how much noise cargo vessels made while at anchor with the engines off and propellers at rest.

She and fellow researchers from UVic, the federal Fisheries and Oceans Department, B.C. Conservation Foundation and the Cowichan Tribes set out to measure the underwater soundscape made by bulk carriers anchored in Cowichan Bay.

Better Understanding

Kelsie Murchy, a Ph.D. student at UVic, led a team studying the noise made by cargo vessels at anchor in Cowichan Bay, hoping it leads to a better understanding of how the underwater soundscape affects marine life.The results were surprising.

Suprising Results

“Their engines weren’t turned on, which is where you’ll typically get most of your noise from a moving vessel,” Murchy said.

“But when the carriers are anchored the motors aren’t running, the propellers aren’t moving, they’re all shut off.”

Things Like Generators

“However, they’re still running generators to keep all the systems of the vessels on.

We studied bulk carriers, their engines weren’t on, they weren’t moving, but there are things like generators and they created a substantial amount of noise.”

Up to 40 per cent of the noise a ship typically makes while in transit, in fact.

Measuring Sound

The team measured sounds from anchored cargo ships ranging from two to eight decibels louder than the surrounding ambient sound, depending on how far away an anchored ship was, compared with a moving ship creating about 20 decibels of noise.

And while a few decibels might not sound like much, consider that sound doubles for every increase of three decibels because decibels are measured on a logarithmic scale.

Drastic Changes

“The underwater soundscape has been dramatically changing over the last few decades, with shipping being a major contributor,” Murchy said.

“We now have another noise source related to shipping that could be impacting marine species.”

Water Is Denser Than Air

Because water is denser than air, sound moves four times faster through water.

If that doesn’t seem to jive with your experiences of sound being muffled after you’ve dived into the lake, that’s because our ears aren’t adapted for listening underwater.

A Different Apporach

But marine animal ears are and some animals hear high frequencies better, some hear lower frequencies better.

“Our ears aren’t built to hear well underwater,” she said.

“Whales, dolphins, they’re definitely way better at hearing sound underwater compared to us.”

And not just marine mammals, but invertebrates and fish, as well.

“Our salmon, for example, they can’t hear those high frequencies,” said Murchy, whose doctoral research in Victoria focuses on the impacts of underwater noise on Chinook.

Array Of Soundtrap

To measure the underwater soundscape, the researchers deployed an array of Soundtrap hydrophones throughout Cowichan Bay, where cargo vessels regularly anchor prior to heading to the Port of Vancouver.

The hydrophones were positioned during the fall of 2019 and 2020 and the results, published on July 26 in Marine Pollution Bulletin, showed sound pressure levels increasing by up to eight decibels.

The Results

“These results demonstrate the change anchored carriers can have on underwater soundscapes and is an important step in understanding the potential impact these vessels may have on marine organisms and important ecosystems,” the report says.

Size Of Carrier Matters

The size of the bulk carrier at anchor also influences the degree to which volume increases, the study says, with larger carriers (by gross tonnage) creating more noise at anchor than smaller carriers do.

The Opposite Reaction

“And it’s interesting that noise from these anchored carriers is louder at the bow instead of the stern, which is opposite of what we observe when they are in motion,” Murchy said.

Quiet Times In Harbours

To the team’s knowledge, no previous research had examined noise produced from bulk carriers at anchor.

The data gleaned could perhaps lead to quiet times in harbours during times marine life is communicating or foraging.

First Steps

“Our study didn’t evaluate that, but it’s kind of that first step to beginning to look at how our marine species and marine ecosystems are being impacted.”

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Source: Vancouver Sun

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