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Using sound to see the ocean


Scientists at NOAA’s Seattle-based Alaska Fisheries Science Center are making waves—sound waves, that is.

Researchers in the AFSC Fisheries Acoustics Program left Kodiak, Alaska, on Monday, June 13 for a 2-month acoustic survey in the Gulf of Alaska.

NOAA Fisheries Service scientists have been conducting large-scale acoustic-trawl surveys in the Bering Sea and the Gulf of Alaska for over three decades to assess distribution and abundance for walleye pollock and other species of fish.

The estimates are used with other information to set annual catch limits, thus ensuring sustainable management of Alaska’s fisheries.

“Acoustic-trawl surveys provide a great way to estimate the abundance of animals such as walleye pollock,” said Chris Wilson, program manager for the acoustics group. “But we are always looking for ways to improve the process so that we can provide the best estimates possible.”

Wilson said the program’s research to improve both the acoustics and trawling aspects of the survey focuses on development of new survey tools and methods as well as testing the validity of assumptions that are traditionally made when conducting acoustic-trawl surveys. He provided a couple of examples to illustrate his point. 

To help determine the identity of the animals that are detected with acoustics during a survey, trawl information is needed, and Wilson explained that, “in recent years we have also benefited by using more acoustic frequencies than we did in the past to help tease out more information on the identity of the animals that we measure acoustically.” 


Seeing the Bubbles Graph

The researchers have also learned a great deal about how pollock may respond to underwater noise from their NOAA survey ships. 
“If fish are frightened by the noise of our ships and move out of the ship’s path, we can’t count them,” said Wilson. “We’ve done numerous experiments during our surveys to understand whether the fish actually respond differently to our two survey ships, and what we’ve learned is that the fish aren’t as predictable as we expected.” 

During some surveys and situations, the fish react more to the noisier vessel, but not always. 

Wilson said his group is also doing a lot of work to understand how well their survey trawl catches pollock of different sizes.
“This is very important because if the acoustics are ‘seeing’ all of the different sized pollock but the trawl isn’t ‘seeing’ or catching all sizes equally, there can be problems in accurately estimating the abundance.” 

Members of his group recently developed a stereo-camera system that fits onto the back of a modified trawl, to estimate the sizes and species of many different animals as they pass through the net and in front of the cameras. 

“The really neat thing about this device,” said Wilson, “is that, unlike traditional nets, we can now determine much more accurately what particular kinds of animals are found at what depths and alongside what other animals. And, because we can leave the net open, the animals simply pass unharmed out the back of the net after we get their photos!”

Wilson said the data collected with the stereo camera system will really help scientists better interpret their acoustic data. 

“Seeing” gas bubbles
NOAA scientists have been using acoustics to study fish and other marine life for more than 30 years.

When an explosion sank the Deepwater Horizon oil drilling rig in the Gulf of Mexico on April 20, 2010, killing 11 workers and spilling millions of barrels of crude oil into the gulf, NOAA scientists and academic partners saw another possible application for their acoustic research.
NOAA’s Alaska Fisheries Science Center acoustics expert Alex De Robertis was among the NOAA acousticians who in collaboration with colleagues Tom Weber and Larry Mayer from the University of New Hampshire quickly mobilized to the Gulf of Mexico to see if fisheries acoustics techniques could be useful in an oil spill situation.

“Other than for a few, small, controlled experiments in Norway involving the release of oil, there really was no precedent for this research,” said De Robertis. “It’s really important to get out during an event like this to learn as much as we can.”

Scientists were able to begin acoustic monitoring about a month after the explosion until after the well was capped. It didn’t take long before De Robertis and his colleagues determined that acoustically, they could clearly see gas bubbles.

“Using high-frequency sound waves, we were able to clearly see the rising oil plume in the upper 200 meters. This may ultimately lead to an accurate method of measuring oil flow rate to the surface, which could be used during any oil spill event anywhere in the world, including the Arctic,” said De Robertis.  “Using lower acoustic frequencies, we could see naturally occurring gas plumes rising from the ocean floor at depths of over two kilometers.”

During the Gulf of Mexico oil spill, acoustics were also used to monitor the integrity of the well head once it was capped, as well as to monitor what is known as the ‘Deep Scattering Layer’, a diverse deep-water community of sound-reflecting fish, shrimp and squid. De Robertis said this acoustic research also provides initial information for future acoustic measurements of these animals in the Gulf of Mexico.