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Sonar on the high seas

By Jessie Perelman, Safina Center Launchpad (“junior”) Fellow

What animal species live in the open oceans around the world? At what depth do they live? How many animals are there? Scientists use many tools to try to answer these questions. We can tow large nets and see what species of fishes, squids, and crustaceans are caught, send robotic vehicles into the ocean to record videos and images of the creatures living there, and use sonar systems on ships to ‘listen’ for the echoes of animals deep in the waters below. These methods have all provided valuable information about open-ocean habitats across the globe, and they all point to a similar fact: there are A LOT of animals swimming around in the ocean’s midwaters, and they play a significant role in supporting these ecosystems.

As discovered by shipboard sonar systems 80 years ago, small animals form massive aggregations (called ‘scattering layers’) at midwater depths in the ocean during the day and migrate to the surface at night to feed. However, not all animals migrate, and the depths of these layers can change quite a bit across space and time. Studying the movement patterns and depths of scattering layers helps researchers understand how different open-ocean ecosystems function as a whole, and how important these animals are to marine food webs and transporting carbon from the sea surface, where they eat, to the deep ocean, where they defecate and die. But, at the moment it’s not entirely clear how the behavior of midwater populations might change in response to extensive human activities on the high seas….

Small animals, mostly fishes, squids, and crustaceans, form massive aggregations called ‘scattering layers’ at midwater depths in the ocean during the day and migrate to the surface at night to feed. The layers can be detected by shipboard sonar systems as they scatter sound waves, which is how they got their name. Image: Pearson Prentice Hall

One of these activities is deep-sea manganese nodule mining, which could begin as early as 2022 in the Clarion-Clipperton Fracture Zone (CCZ)—a 2-million square mile area in the eastern Central Pacific Ocean. Mine tailings and sediment released into midwaters will generate sediment plumes that could extend up to 125 miles away from each mining site, affecting midwater animals (possibly for years) as particles slowly settle back to the seafloor.

The CCZ is a large ocean region in the international waters between Hawai`i and Mexico. It spans the width of the continental U.S. and is the target of deep-sea mining operations by many countries. Mining regulations and claim area designations are governed by the International Seabed Authority (ISA). Image: Wedding, et al.

So, monitoring these future impacts requires a strong understanding of the natural conditions across the region’s midwaters. This is where fisheries sonar systems come in handy. In an effort to begin studying these habitats, I was recently awarded a research grant (valued at $1 million in remote research time) by the company Saildrone to collect information about scattering layer communities and their depths and behaviors in the western CCZ before mining occurs.

This diagram shows the setup of a standard seafloor mining operation and highlights the midwater sediment plumes expected from mine tailing discharges. Image: Lee, et al.

What is a Saildrone? It is an unmanned surface vehicle that looks something like a large orange surfboard with a hard carbon-fiber sail, which is powered by wind and solar energy and equipped with a variety of oceanographic sensors. The drones are remotely operated through satellite communication and can spend up to 12 months at sea.

We launched Saildrone 1043 on November 1st, 2019, from Honolulu Harbor, sending it on its way to the western end of the Clarion-Clipperton Zone (CCZ) to collect information about scattering layer depths and behaviors in the western CCZ before mining occurs. Photos: Jessie Perelman

Unfortunately, there is currently no system in place for water column monitoring in this remote area of the Central Pacific, for instance, to assess large-scale changes in scattering layers as indicators of environmental distress. So this project, called ‘Building Midwater Baselines,’ uses the Saildrone’s fisheries echo sounder (sonar) to build a record of scattering layers, these aggregations of ocean animals, in the western CCZ alongside oceanographic conditions in real-time.

Unmanned surface vehicle could revolutionize adaptive environmental management for mining operations, and real-time monitoring within mining areas would provide a convenient solution to this enormous task.

At the end of the day, I hope this project and its outcomes will highlight just how effective ocean-observing tools like unmanned surface vehicles can be for deep-sea mining operations, as well as the value of protecting midwater ecosystems from mining’s potentially harmful influences.

One Comment on “Sonar on the high seas

  1. Mahalo, Jessie. Your work is vital and this piece informative. I look forward to learning more.

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