Quirks & Quarks

Engineering a decompression chamber to retrieve fish from the 'twilight zone'

Deep-water twilight zone reefs host many unknown species that can now be collected safely
Bart Shepherd collecting deep-water reef fish. (California Academy of Sciences)

A team of marine biologists has developed a kind of decompression chamber for fish, so they can capture rare living fish from deep water reefs and bring them to the surface without harm.

Bart Shepherd, of the Steinhart Aquarium in San Francisco and his colleagues have been investigating largely unknown "mesophotic" reefs, in deep water 60 to 150 meters below the surface.

These reefs are in what researchers call the "twilight zone" where surface light barely reaches.

"In many cases we're the first set of human eyes on these ecosystems," says Shepherd, "we're seeing just the most unbelievably beautiful fish in invertebrates many of which are unknown to science."  

A newly discovered species of reef fish from the genus Tosonoides that rode the decompression chamber to the surface. Its vivid colours would be invisible in the twilight zone. (Luiz Rocha © California Academy of Sciences)

Working and diving at these depths is risky and difficult, and requires specialized deep-water scuba rebreathing gear. Capturing fish on these reefs for scientific study and display also presents a challenge. Bringing fish from the high pressures of these deep-water reefs to the surface can be harmful or fatal to them.

Deep water fish feel the pressure

According to Shepherd, the problem is the gas-filled swim bladder that the fish use to regulate their buoyancy. "As you bring fish up to the surface this balloon inside their body starts to inflate and get bigger and bigger and bigger and it presses up against the other organs and it causes all kinds of stress and trauma to the fish."

The solution they developed was something similar to the decompression chamber human divers use to recover from spending extended time at depth. Shepherd and his team capture fish on the reefs, and put them into acrylic collection jars. They then deposit these collection jars in their "pressure canister."  

The decompression chamber with fittings that allow oxygenated and filtered water to circulate and keep the collected fish healthy (California Academy of Sciences)

The final step is to add a bubble of air to the pressure canister and seal it up.  "Much like the fish's swim bladder [the air bubble] will expand and hold the pressure constant inside there so we can move that decompression chamber from depth to the surface as quickly as we want to and it will maintain this sort of virtual depth."

Once the pressure canister is sent up to biologists in the dive boat, they quickly connect it to systems that will filter and oxygenate the water, and maintain it at the same temperature and pressure as the fish were used to in the deeps below. 

Bringing the depths to the surface

The next step is to bring the fish to surface pressure. This happens at a makeshift site ashore — near wherever the team is exploring.

"Usually we take over a hotel room and we pull all the furniture out of it and we build a temporary aquarium with various barrels and buckets and boxes of water," says Shepherd. The collected fish stay in their decompression chamber, and the pressure is slowly dropped over a period of two or three days. "Once this is all over, you've got these fish that are now swimming around happily"

Bart Shepherd with the collection chamber and the pressure-tight decompression chamber (California Academy of Sciences)

What's striking to Shepherd is that these deep water reef fish are at least as colourful and spectacular as shallow water reef dwellers — often vivid pinks, reds and oranges.

This is a little surprising as these wavelengths are filtered out by the deep water and are basically invisible where the fish live, but the colours still serve a purpose.

"When you're a fish that's down deep and you're red or orange or pink you appear grey and you sort of disappear," says Shepherd. "If you want to camouflage yourself, be a colour that doesn't exist, right?"

The fish aren't just colourful and attractive, they're of considerable scientific value as well. Many are species new to science. Perhaps more importantly, says Shepherd, they can provide important ecological baselines for a deep water ecosystem that's basically unknown.

"We know quite a bit about what climate change and what pollution and plastic trash and fishing is doing to shallow water reefs, but we know very little about what that's doing to deep water reefs."

They may also provide information useful for recovery and rehabilitation of shallow water reefs. If these fish turn out to be genetically related to shallow water fish, Shepherd thinks that may be a sign of a potential safe haven for reef fish.

"Can these deep reefs act as a refuge? Can species move down there and and find a safe haven in these deep reefs? Can they repopulate nearby shallow water reefs? So that's one of the questions that we're also working on answering."