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One fish, two fish, shallow fish, deep fish: Studying the behavior of deep-sea animals using sound waves

By Jessie Perelman, Safina Center “Kalpana Chawla ‘Launchpad’ Fellow”

Back in the early 1940s, researchers in southern California were experimenting with underwater sonar systems to try to detect submarines. Their sonar detection systems worked well in the shallow ocean depths near San Diego, but not in deeper waters beyond the continental shelf–where the sonar’s waves seemed to be stopped before they would normally reach the deep seafloor. There they unintentionally discovered the existence of a “phantom sound-reflecting layer” that hovered at a depth of 900 feet during the day and strangely rose to the surface when the sun went down, only to reappear the next morning. Mariners began to observe these layers, now called deep scattering layers (DSLs), on shipboard sonar systems as they sailed over deep waters. They often referred to DSLs as ‘false bottoms’ since they resembled the echoes of the seafloor when the true seafloor knowingly lay far deeper.

This example of an acoustic echogram shows a 2D image of sound scattering layers formed by aggregations of marine animals in the water column. Using fisheries sonar systems, these layers of animals can be tracked and researchers can observe changes in their behavior over space and time. Photo: Jessie Perelman

As it turns out, these mysterious echoes come from dense aggregations of marine animals that rise to the surface at night and descend back to mid-water depths hundreds to several thousand feet below the surface during the day. So many animals participate, in fact, that this repetitive phenomenon is has been deemed the largest animal migration on Earth, and it happens every single day. This immense vertical movement of marine creatures and the biological matter (biomass) they contain is now known to play a major role in ocean carbon cycling, with animals transporting carbon from the surface waters to depth as they eat and breathe.

Which animals are making this daily journey? Primarily small fish, invertebrates, and zooplankton. These animals that dwell mostly in the middle ocean layers rise to feed in nutrient-rich surface waters at night and hide in dark mid-waters (the ocean’s twilight zone) during the day to hide from visual predators like billfish and marine mammals.

Fisheries sonar systems and other acoustic sensors are frequently used to study DSL animals in a wide range of ocean habitats, from the tropics to the poles. Because it is often difficult and expensive for net trawls to collect organisms in these deep layers, acoustics provide a valuable and fairly easy way to observe them remotely, and many research and fishing vessels are equipped with acoustic instruments for this purpose.

How it works: An echo sounder (fisheries sonar) mounted on a ship’s hull sends a sound pulse referred to as a “ping” down into the water column at a relatively low frequency, most commonly 38 kHz for detection of fish. When the sound wave hits an object or animal whose density is different from the surrounding water, the wave gets reflected back to the ship as “backscatter” noise, allowing researchers to identify the depths, abundances, and movements of these observed animals.

With support from the Safina Center “Kalpana Chawla ‘Launchpad’ Fellowship,” I recently had the opportunity to travel to Wellington, New Zealand and work with a team of fisheries scientists at the National Institute of Water and Atmospheric Research (NIWA). In addition to learning about the field of fisheries acoustics, I got to begin working on a project using echo-sounder information to study the effects of mining-related deep-sea sediment plume on the behavior of animals in the water column.

The National Institute of Water and Atmospheric Research (NIWA) was established in 1992. Their mission is to “conduct leading environmental science to enable the sustainable management of natural resources for New Zealand and the planet.” Photo: Jessie Perelman

During several experiments over the past two years, a large metal platform with a pump that draws in sediment and releases it again—a device appropriately named a seafloor “disturber”—was used create a sediment plume over the Chatham Rise off the eastern coast of New Zealand, an area of potential interest to mine for the mineral phosphorite, as well as bottom trawling and long-line fishing activities. Like the seafloor disturber, these activities generate plumes of sediment above the seafloor, which may cause avoidance behaviors or distress in the animals residing here.

This seafloor disturber was used to kick up a sediment plume on the seafloor along the Chatham Rise, an area off the eastern coast of New Zealand. The disturber was slowly towed along the seafloor and created a sediment plume about 5-10 meters high to simulate one of the potential effects of deep-sea mining. Photo: NIWA

Studies like this, and the use of tools like fisheries acoustics to remotely study deep-sea environments, will likely be very influential as mining industries continue to evolve. This is one of several projects forming my doctoral research, which aims to evaluate the possible impacts that deep-sea mining activities will have on DSLs and mid-water habitats where suspended sediment plumes may become persistent features.

Over the next several years, I’ll continue developing these and other methods to study organisms in the deep ocean, to understand the potential consequences that mining will have on their behavior, and to highlight the significant ecosystem services provided by animals in mid-water ecosystems.

A calm day on Wellington Harbor just outside the NIWA regional office. Photo: Jessie Perelman

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