Underwater sound quest
At the end of 2012, an unprecedented scientific expedition took place at the Maurice Lamontagne Institute in Mont-Joli, Quebec. Fisheries and Oceans Canada researchers deployed an underwater acoustic observatory station in the Gulf of St. Lawrence that continuously captured the noise radiating from ships.
Combined with maritime traffic map data, these recordings will be used to build a model to assess noise pollution in Fin Whale and Blue Whale habitats. It was a unique experiment that may become a benchmark and inspire other researchers around the world.
Behind the apparent underwater silence lurks deafening noise in the St. Lawrence and elsewhere. “Shipping noise has increased 10-fold since 1960,” says Yvan Simard, who leads the team behind this innovative scientific quest.
Research scientist at the Maurice Lamontagne Institute, and Fisheries and Oceans Canada Chair in underwater acoustics applied to ecosystem research at ISMER-UQAR, he says that this noise pollution, largely caused by shipping, disrupts the lives of marine species, especially their communication. Validated in 2015, his model could be adapted and used to simulate the impacts of noise on the acoustic environment of marine mammals along shipping routes to the Arctic.
A 3-phase project
"The project’s main objective is to map the noise radiated by merchant shipping throughout the year in the Gulf of St. Lawrence. After one year, a 300-map atlas of maritime traffic density was produced. The acoustic observatory's recordings were sent to the numerical analysis and processing laboratory. These two types of information will now be combined in shipping noise simulation models to build an atlas representing the underwater acoustic conditions of marine mammals at given locations and times," says Yvan Simard.
First, the research scientist had to estimate the acoustic signatures of merchant fleet ships transiting the St. Lawrence River toward the Great Lakes. These types of sound samples did not previously exist.
To be representative of maritime traffic, the acoustic observatory therefore recorded sounds in the centre of the 10-km-wide shipping route toward Quebec, in a portion where the Lower Estuary of the St. Lawrence is 50 km wide. At this location, the Laurentian Channel is 350 m deep. St. Lawrence Seaway traffic is sparse compared to traffic in the Strait of Gibraltar or the English Channel, for example, making it easier to isolate the sound of each ship and obtain the individual signature.
"At the same time, we need to be aware of the composition and density of traffic in the St. Lawrence. To do this, we obtained the navigation monitoring information through the AIS (Automatic Identification System) from the Canadian Coast Guard. With this data, we knew the identification number and type of each ship, as well as its length, position, speed, the time at which it passed by, etc.," says Yvan Simard.
The research scientist also says, "By combining the two sets of information, we were able to assign an acoustic signature to each ship passing above the acoustic observatory. We had the necessary information to develop and validate the model of the noise radiated by ships in the ecosystem at various times of year, locations and depths, for example."
Collecting acoustic signatures
Once the location for the acoustic observatory was selected, it still had to be installed! No small task. Imagine: the acoustic observatory consists of three AURAL hydrophones (Autonomous Underwater Recorder for Acoustic Listening) aligned vertically in the water at regular intervals and held in position by a 280-metre anchor line off Mont-Joli.
"The approach is quite simple, but it requires solid expertise to design all the acoustic equipment, minimize vibration at source, and install it on site," says Yvan Simard. Fisheries and Oceans Canada has this expertise. The AURAL hydrophones, developed by Yvan Simard and his colleague Yves Samson, are recognized worldwide for their technical quality and versatility (2009 Federal Partners in Technology Transfer Award).
Operating completely independently, the acoustic observatory recorded all sounds for a period of one year and stored them on the hydrophone hard drives. The research scientists also performed maintenance after six months to recover various information including one terabyte of stored data.
Then the analysis was initiated. "Using signal processing algorithms, we are able to recognize and extract the net signature of a ship in accordance with international standards," says Yvan Simard.
Did you say net signature? "In fact, because the hydrophones are always on, everything is recorded, including whale vocalizations used to determine their habitat use, echolocation activities, and even noise produced by wind, waves or ice. To establish the net signature of ships, the interference must be cleaned when necessary," says Yvan Simard. The algorithms make it possible to eliminate the background noise.
Beyond "cleaning" the signal, getting a ship's true acoustic signature is another part of the challenge. This involves recording the sound at the ship's cruising speed, preferably from the side (according to ANSI's international standards) because sound radiated by a ship is not propagated evenly in all directions. The sound recorded at the stern of a ship is different from the sound at its bow, for example.
After a year on site, the acoustic observatory was dismantled in November 2013 and should be redeployed elsewhere at a later date.
Why do we need a model?
Underwater noise made by humans is increasing. The change in sound levels in the environment has doubled every decade since the 1960s due to increased global shipping and economic activity. Yet for marine life, the acoustic environment is vital. "When you change it, marine mammal communications, for example, are masked, like a cry lost in a noisy crowd, or the transmission range decreases. Their diet may also be affected, since they detect prey by echolocation," says Yvan Simard.
Now validated, the model developed by the research scientist can be used to determine the effects of shipping routes to the Arctic on underwater populations, estimate how wildlife is affected by sound changes generated by drilling operations or adopt noise management regulations.
To learn more
To listen to underwater sounds made by humans and animals:
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