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Bringing Fish Up from the Deep

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Article by Lauren Farrar

Deep below the ocean’s surface lies a mysterious region known as the “twilight zone.” Located 200 to 500 feet beneath the surface, this region receives scarce amounts of light, mimicking twilight—the time of day just after sunset. Some areas of the twilight zone are vast ocean space, but some are home to incredible coral reefs.

Scientists have many unanswered questions about this region, in part because it is so hard to reach. Diving to these depths requires specialized training and gear, and takes hours to safely ascend. Bart Shepherd and Luiz Rocha at the California Academy of Sciences are among a group of scientists that get to explore these depths.

“More people have been to the surface of the moon than have been to these reefs,” says Bart Shepherd, director of the Steinhart Aquarium at the California Academy of Sciences.


“Everywhere we go, about half of the fish are not known. They don’t have a scientific name. The other half, we didn’t know that they went that deep. So, these are all new records, either new records of depth extensions or range extensions, or new species,” says Luiz Rocha, curator of ichthyology at the California Academy of Sciences.

The scientists are studying the twilight zone to learn about the biodiversity of this region and the greater role it plays in the health of the ocean. “You know, one of the great questions that we still have is ‘What are the connections between the shallow reefs and these twilight zone reefs?’ And, that’s really why we are studying it. That’s really why we continue to go and look there,” explains Shepherd.

As part of their research, Shepherd and Rocha wanted to collect live fish of newly discovered species in order to study their behavior and to display in the Academy’s aquarium for the public to see. “I think in order for people to really understand and want to conserve and protect life in these depths, they really have to have a direct connection with it, and they need to be able to see things that came from there,” adds Shepherd.

However, the researchers ran into a problem. They knew if they tried to bring fish up from that depth, the swim bladder -- an organ that helps fish maintain their buoyancy -- would expand, crushing other vital organs inside the fish, often causing the fish to die.

Figuring out a solution to this problem was a team effort. Matt Wandell, a biologist at the Steinhart Aquarium, was a key player in designing the device that would allow fish to be brought up safely. “I had started to hear about the idea of the California Academy of Sciences going down and exploring deep under the ocean with divers, and possibly collecting fish down there. And since my job is to make sure that those fish come back healthy, I thought about different ways that we could make sure that they could handle that pressure change,” explains Wandell.

The team engineered a portable device in which they could collect fish during their dives. The device, known as a portable decompression chamber, was designed to maintain the pressure of the twilight zone while the fish are transported to the surface. Once at sea level, the pressure inside the chamber is slowly reduced, often over two to three days. This gives the fish enough time to safely adjust to living at lower pressure.

With this portable decompression chamber, researchers at the California Academy of Sciences have been successful in collecting fish from the twilight zone. Species from their recent expedition to the Philippines are now on display at the Steinhart Aquarium.

This video is part of our Engineering Is: Bringing Fish Up from the Deep e-book. The e-book explores the science and engineering principles behind the California Academy of Sciences’ portable decompression chamber, and includes videos, interactives and media making opportunities. You can find our other e-books at kqed.org/ebooks.

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