Rockfish Under Pressure

There are more than 100 species of rockfish, and many prefer life down deep. They can be found at 300 feet or more, and some live at depths to 1,600 feet.

But just as human divers have problems when they come up from the deep, rockfish that are caught and brought to the surface can suffer pressure-related ailments, such as over-inflated swim bladders or bulging eyes (exophthalmia).

Fortunately, the Monterey Bay Aquarium and its partners have developed innovative ways to ensure the health of rockfish destined for our exhibits, where they can serve as ambassadors and promote awareness of the conservation issues that rockfishes face in the wild.

Pumped Up for the Ride Home  

While some rockfish are rare, others are abundant. The California Department of Fish and Game has established catch limits based on depth. It’s also established guidelines for returning rockfish that are accidentally caught at depth.

In order to be able to display these beautiful fish, Aquarium collecting staff has developed a series of chambers that “recompress” the captured fish and gradually allow them to acclimate to the shallower depths of our exhibits.

One type of recompressor, made of plastic pipe, uses two chambers that allow newly collected fish to be added as they are caught. “As soon as they’re brought on board, they go into a small chamber,” says Associate Curator of Collecting Joe Welsh. “We lock that down, and in 10 seconds we inflate it to 70 pounds per square inch using a small water pump.” After that, he opens a valve that allows the fish to go into a larger pressurized chamber that can hold up to a dozen fish. The “hyperbaric” device is carried straight from the boat into the Aquarium and connected to a pressurized recirculation system. The fish are gradually depressurized over a period of three to five days before being transferred to a holding tank, and then onto exhibit. The whole process is done under a special permit from the California Department of Fish and Game.

Since Joe started using the process in 2007, he estimates he’s successfully transferred more than 50 rockfish into our exhibits, and numerous others have been transferred to other institutions. We’ve successfully recovered rockfish out of traps from as deep as 600 feet (mostly yelloweye, cowcod, and geenspotted).

“They all recover from the treatment quite well,” says Joe.

How Our Exhibits Benefit

The recompression system “opens up a whole list of species we couldn’t collect before,” says Joe, including fish that live as deep as 300 feet on the continental shelf that extends far offshore. “This shelf habitat is one of the largest in Monterey Bay, and now we can represent it in our exhibits.”

Rockfish species that have been acclimated and displayed include greenspotted (see photos), bocaccio, canary, chilipepper, cowcod, halfbanded, rosy, starry, yelloweye, pygmy, cowcod and widow. At the Aquarium, you can see some of these rockfish in the giant Pacific octopus exhibit, the spot prawn gallery, the brittle star gallery or the Monterey Bay Habitats exhibit.

The recompression process is also being experimented with as a form of therapy for fish that have swim bladder or eye infection problems. “We’re also trying other species, and are using the system to provide fish for research,” says Joe.

But the greatest benefit has been the ability to show more of these colorful and beautiful species. “We’re opening up new habitats and locales for our visitors,” says Joe. “These are fish you otherwise wouldn’t be able to see. It allows us to show a fuller community.”

Joe helped construct the complicated array of pumps, pipes and fittings, and says that “building the train set was half the fun of the whole project.

“How many institutions have the opportunity to do what we do?”

Learn more about rockfish


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About me

The Monterey Bay Aquarium, perched on the edge of a world-famous coastline, is your window to the wonders of the ocean. It’s located on historic Cannery Row in Monterey and is open daily except Christmas Day.

For more information about our animals and exhibits, and to view our live web cams, please visit

Hours of operation vary by season. Daily schedules and tickets are available on our website or by calling
(831) 648-4800.