https://phys.org/news/2018-06-safely-unknown-deep-dwelling-fishes-ocean.html

Shepherd is part of a deep-diving research team that explores the twilight zone, a mysterious coral habitat stretching across a narrow band of ocean 200—500 feet beneath the surface. At this depth, sunlight is scarce between the light-filled shallows and the deep dark sea. As part of its Hope for Reefs initiative, the Academy team is exploring this unknown frontier with the help of high-tech equipment like closed-circuit rebreathers, which require extensive training and allow scientists to extend their research time underwater. Most fishes in the twilight zone have never before been seen by humans—many are also new to science. The long journey to bring new species to the surface is a delicate, carefully orchestrated process for both divers and their aquatic finds.

"A rapid ascent can rupture a fish's swim bladder," says co-author Dr. Luiz Rocha, the Academy's Curator of Fishes and Hope for Reefs co-leader. "Using an adjustable pressure control valve, we ensure that the pressure inside the chamber is similar to the depth where the fishes were collected. Over the course of two to three days, we bring them to the surrounding surface pressure in a highly controlled manner."