Bringing Fish From Ocean Floor: Carefully Released Rockfish Can Survive Barotrauma

Oregon State University researcher Alena Pribyl is completing one of the first studies to look at the long-term effects on rockfish of barotrauma, a series of physiological changes caused by the expansion of gas in the fishes’ swim bladders as a result of lower water pressure at the surface. Bulging eyes, tight gill membranes and an everted esophagus are among the symptoms.

Pribyl’s research suggests that the fish can, indeed, survive as long as 31 days – at least, in captivity – despite experiencing the noticeable effects of barotrauma.

“What happens when a rockfish is brought up from depth is that the pressure change causes the gas within the swim bladder to expand,” said Pribyl, a doctoral candidate in fisheries and wildlife at OSU. “The enlarged swim bladder often displaces the fish’s internal organs and eventually can push the esophagus out of the fish’s mouth. When a fish is recompressed, the excess gas within the fish contracts and most external barotraumas symptoms disappear.”

The Oregon Department of Fish and Wildlife recommends recompressing fish by using an inverted barbless weighted hook to lower the fish, or a weighted cage with a trap door to protect the fish from predation on its way down.

Past studies at the University of California-Davis have shown that bottom fish that have experienced barotrauma can survive the experience for at least two days, but scientists were unsure what happened beyond that. Pribyl’s study was the first to examine the impacts of recompression on fish at the cellular, blood and gene expression levels, as well as on the whole fish, as long as a month later.

In her study, Pribyl put 30 black rockfish in specially designed pressure chambers at OSU’s Hatfield Marine Science Center in Newport. Designed by Polly Rankin of the Oregon Department of Fish and Wildlife, the chambers can simulate the water pressure fish experience at different depths.

Pribyl let the fish acclimate to a simulated depth of 35 meters and then “brought the fish to the surface” – lowering the water pressure to surface levels – in 90 seconds, which is about the time it might take for fishermen to reel in their catch. She then looked at 10 of the fish after three days, another 10 fish after 15 days, and the final 10 fish after 31 days, comparing them to another group of control fish.

Her study found that 80 percent of the fish brought rapidly to the surface had ruptured swim bladders. After 31 days, 20 to 50 percent of those swim bladders remained unhealed. Yet there were no mortalities, and after 31 days, 80 percent of the fish had resumed feeding.

“In biological studies, feeding is an important sign of recovery,” Pribyl said. “It doesn’t guarantee survival, but it greatly improves the animal’s chances. However, while this may be true in captive fish whether or not fish that have ruptured swim bladders could effectively forage in the wild remains to be determined.”

Pribyl said fish could overcome a ruptured swim bladder – if ocean conditions are favorable. They can still maneuver up and down in the water column, but would have to expend more energy to do that. “If conditions were good and there was a lot of food, it might not be a problem,” she pointed out. “If it was a lean year and there were many predators, it could be a different story.”

Her study also found no long-term cellular damage from barotrauma in several internal organs, or changes in blood plasma enzymes indicative of tissue injury, which she termed “remarkable.” And preliminary data suggest that the fish produced higher levels of certain genes related to the immune system at the third day, but these genes were no longer expressed by the 15th day, which she said could indicate that their immune system cranked into higher gear to deal with the barotrauma and later returned to normal levels.

“The bottom line is that these black rockfish, if carefully handled, have the potential to survive at least 30 days after being caught and properly released,” Pribyl said. “Fishermen who have caught a limit of one type of fish, or who accidentally hook a canary or yelloweye, can help increase the chance of that fish surviving by using a weighted hook or cage to recompress the fish.

“Just don’t puncture the esophagus,” she added. “When fishermen accidentally catch a species of fish that is protected, they may try to help the fish recover from the pressure change by puncturing the ‘swim bladder’ and allowing the expanded gas to escape. Unfortunately, it isn’t the swim bladder protruding from the fish’s mouth, but the esophagus.”

Her studies are funded by Coastside Fishing Club in San Francisco, the Oregon Department of Fish and Wildlife, the National Oceanic and Atmospheric Administration Saltonstall-Kennedy Grant, OSU and other sources.

Tips on Recompressing Fish:

• Do NOT puncture the internal organs protruding from the fish’s mouth; 
• Use a weighted barbless hook, cage with a trap door, or a milk crate to lower fish to the bottom;
• Handle the fish on deck as little as possible, and lower it quickly into the water;
• If using a weighted barbless hook, consider having a dedicated fishing pole to speed things along;
• If you are catching protected species of fish, move to a different area.