When a scuba diver comes up from the deep too quickly, the rapid decrease in pressure can give them a case of "the bends".

Key points: Fish get sick from rapid decompression just like divers get 'the bends'

Fish get sick from rapid decompression just like divers get 'the bends' This can happen when they are wrenched from deep waters to shallow by a fish hook, or by going through a dam

This can happen when they are wrenched from deep waters to shallow by a fish hook, or by going through a dam The impacts could affect fish populations but fish-friendly solutions can be implemented

Bubbles of gas build up in their body, causing pain and other potentially fatal effects.

The same thing can happen when a fish is wrenched from the deep, said Brett Pflugrath of the Pacific Northwest National Laboratory in Washington, who studies decompression sickness — or barotrauma — in fish.

Rapid decompression can also cause a fish's swim bladder — a pocket of air inside a fish that is used to control their buoyancy — to blow up and cause problems.

And according to Dr Pflugrath, when fish get their version of "the bends", it's a lot worse than in humans.

"It's like the bends on steroids," he said. "You tend to see more extreme injuries from it."

Rapid decompression can happen when a fish is wrenched up from the deep on a fishing hook or is tossed out into the shallow side of a river dam and can be especially troublesome for species that are already under threat.

Rapid decompression can lead to bubbles ending up in strange places, such as in this Murray cod's body cavity wall ( Supplied: Brett Pflugrath )

In fish, barotrauma is governed by two laws of physics.

According to Henry's Law, as pressure decreases, so does the amount of gas that can be dissolved in the body. If decompression happens too fast, bubbles form — in the most inconvenient places.

"If those bubbles get large enough they can disrupt the flow of blood or cause other issues," said Dr Pflugrath, who has a background in environmental science and engineering.

Then there's Boyle's Law, that governs the expansion of the swim bladder as a fish rises to shallower depths and pressure decreases.

"When pressure decreases by half, the swim bladder will double in size," said Dr Pflugrath.

Fish with barotrauma are often swollen and have their eyes popping out ( Supplied: Chris Lowe )

A fish with the bends can have physical symptoms that you can see — such as its stomach hanging out of its mouth, bulging eyes and it may even look dead.

"Often the swim bladder ruptures, releasing gas into the abdomen," said fish biologist David Fairclough of the Western Australian department of primary industries.

"And then there can be internal injuries that you don't see."

In some fish barotrauma involves pressure on their stomach that causes it to protrude from their mouth ( Supplied: Chris Lowe )

Many fish must be released after being caught, because they are below the permitted size limit — young breeding-age fish are needed to sustain fish populations.

Yet, many of these are at risk of dying from barotrauma, said Dr Fairclough, and this could be hindering efforts to sustain some populations.

He said in the case of the West Australian dhufish, for example, recreational fishers were currently releasing 70 per cent of their catch.

"We estimate that in the order of 50 per cent of those may die from barotrauma," said Dr Fairclough.

Meanwhile, Dr Pflugrath has been discovering that Australian native fish can also get the bends when they go through irrigation and hydro power weirs and dams that hold back deep water and release fish at depth into shallow water downstream.

At weirs like Yarrawonga water is banked up or let through for irrigation and hydro power by a set of gates that open and close ( Supplied: Brett Pflugrath )

"The greater the ratio is from the deeper depth compared to where they come out, the more likely fish are going to be injured or die," said Dr Pflugrath, who presented some of his findings at a recent conference in Albury.

"The bad part is that fish go from extremely high pressure to extremely low pressure in a fraction of a second," he said.

For example, Dr Pflugrath has found the vulnerable sliver perch is at risk of getting the bends as it passes through the Yarrawonga Weir power station on the River Murray.

"Analysis estimates 24 to 66 per cent of silver perch passing through the turbines at Yarrawonga power station would be injured.

"And 6 to 12 per cent would sustain injuries likely to lead to mortality."

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For his research, Dr Pflugrath put a plastic device called a "sensor fish" through the weir to measure the pressure changes fish would be exposed to.

He then exposed fish to the same conditions in the lab to see what impact it had.

Dr Pflugrath found some species, like silver perch, are more sensitive to barotrauma than others, such as the Murray cod.

Fish-friendly solutions

According to Lee Baumgartner, a fish ecologist from Charles Sturt University, Dr Pflugrath's research has revealed big pressure changes can occur even at small weirs.

"Unfortunately, when a lot of infrastructure was built, we had no idea this would be a problem," he said.

The good news is that there are engineering solutions that could minimise or eliminate these impacts.

Dr Pflugrath's research in the US has already been used in the design of fish-friendly hydro turbines, Dr Baumgartner said.

Researchers at Yarrawonga Weir retrieving data from a 'sensor fish' after it has gone through the weir ( Supplied: Brett Pflugrath )

He said other options included using a screen to lift fish up to the surface of the upstream side of the weir, or using gates that tilt forward rather than lift up, giving a chance for fish to adjust to shallower depths before they go through the weir.

"The solutions are so straightforward when you think of it," Dr Baumgartner said.

Fish-friendly hydro turbine designs also minimise the possibility of fish being struck by blades, or being injured by turbulence, added Dr Pflugrath.

This Australian bass is showing blood in its eye from blood vessels ruptured due to barotrauma ( Supplied: Brett Pflugrath )

And what about reducing barotrauma to fish caused by fishing?

Research over a decade ago by marine scientist Chris Lowe at California State University, Long Beach found that if a fish with barotrauma were quickly sent back to the depths they came from, they would likely recover.

To this end, all manner of "descending devices" have been developed to help get fish back down to safety.

A removable weight hooked on to a fish's lip can be used to help quickly lower them down to safety ( Supplied: WA DPIRD )

For example, if someone has to throw back in an undersized fish they've just caught, they can attach a weight to its lip, using a special attachment that can be easily released.

But Dr Fairclough from the Western Australian government said it's tricky making sure this happens.

Barotrauma isn't always visible, so the weight would need to be applied to all undersized fish captured and then released, he said.

Even if fishing boats are required to have 'descending devices' on board, it is difficult to enforce their use, said Dr Fairclough.

"If the release rate is very high and we know that a particular species dies when it's released then it's probably better restricting the number of fish being caught and kept rather than having a size limit."