Chile’s prized fish finds a surprising home in the deep sea

Tucked inside a bush of giant tubeworms 1,427 feet below the Pacific, dozens of red cusk-eels peered out like sentries.

A remotely operated vehicle off central Chile captured the scene, documenting for the first time that this commercially prized eel species uses a methane seep as habitat.

It’s a discovery that links a deep-sea ecosystem once considered remote and mysterious to a fish deeply woven into Chilean cuisine and culture.

Chile’s prized eel goes deep

Red cusk-eels (Genypterus chilensis) aren’t true eels, but their long, undulating bodies and rich flesh have made them a staple on coastal menus and in fish markets. They were even celebrated by the poet Pablo Neruda.

The species ranges from northern Peru to southern Chile, hugging the seafloor to depths of about 1,150 feet (350 meters).

In 2022, Chilean boats landed roughly 2,000 tons of “congrio colorado,” though researchers warn the fishery is under pressure and often harvests juveniles before they reach maturity.

So, finding 46 to 48 adults tucked into a single tubeworm bush on a methane seep – counted across two dives on back-to-back days – came as a surprise.

“Methane seeps are important places for deep-sea biodiversity,” said Lisa Levin, professor emerita at Scripps Institution of Oceanography and co-author of the study describing the discovery.

“Our findings show these seeps are not just home to communities of obscure creatures that are cut off from the rest of the deep sea. They are also important for commercially fished species and may be much more connected to the rest of the ocean than one might expect.”

Exploring deep-sea methane seeps

The October 2024 expedition aboard Schmidt Ocean Institute’s R/V Falkor (too) set out to map and explore methane seeps along central and southern Chile.

Using shipboard sonar and geological expertise, the team flagged seafloor mounds likely to be active seeps. Then they sent down the ROV SuBastian for close-up reconnaissance.

About 11 miles (18 kilometers) off El Quisco, south of Valparaíso, SuBastian glided over a mound and found a thicket of tubeworms so dense it looked like a deep-sea hedgerow.

These worms thrive at seeps thanks to symbiotic bacteria that turn methane and sulfide into energy – a process called chemosynthesis – building life in places the sun never reaches.

“At first we were amazed by the size of the tubeworm bush,” Levin said. “Eventually we noticed all these fish with their heads poking out from inside the bush.”

The cusk-eels backed into the thicket like drivers reversing into garage bays, their heads angled into the flow. Video showed spider crabs weaving through the tubes and, at times, approaching the fish.

The behavior looked a lot like parasite cleaning, in which one species grooms another – yet another hint that seeps host complex interactions, not just chemical curiosities.

Why a specific methane seep?

Seeps act as biological oases, concentrating food and shelter in otherwise sparse deep-sea landscapes. They can also function as safe harbors. Thickets of tubeworms and mussels offer structure that deters predators and dampens currents.

For the cusk-eels, several theories fit the footage: refuge from predators, foraging in a food-rich patch, a rendezvous for reproduction, or a cleaning station to rid themselves of parasites.

What’s striking is how specific the aggregation seems to be. Eleven nearby mounds turned up only three cusk-eels in total.

The “bush mound” was the clear outlier. The surrounding seabed was littered with ghost gear – lost nets and lines – that suggest local fishers already know this area draws fish.

The mystery of one mound

Scientists have found thousands of methane seeps along continental margins worldwide. They’re chemical engines – leaking methane and hydrogen sulfide – that fuel microbes at the base of compact, thriving food webs.

In recent years, scientists have found that seeps aren’t just chemical curiosities. They’re also home and hunting grounds for species of direct commercial value – thornyhead rockfish, Chilean seabass (Patagonian toothfish), and deep-sea snow crabs. The red cusk-eel now joins that list.

That matters for management. If seeps concentrate fish that are already under pressure – especially if they’re gathering to spawn – indiscriminate fishing or gear loss could cause outsize damage.

“The presence of this important fishery on a methane seep ecosystem where it faces pressure from fishing and pollution forces us to think about measures to protect and conserve these habitats,” said Eulogio Soto of Valparaíso University, the expedition’s chief scientist and a co-author on the paper.

“Our discovery took place over the course of just two days, so we don’t know what life exists or what is happening at other times of the year. We must go back.”

Insights into the deep sea

The study highlights how quickly our view of seeps is changing. Scientists now recognize methane seeps as widespread, diverse, and ecologically connected, rather than rare curiosities.

They funnel energy into the deep sea, offer shelter and structure, and – crucially – appear to intersect with fishery species.

That overlap raises practical questions. Should some seep mounds be off-limits to fishing? Can gear restrictions or seasonal closures protect aggregations without undermining livelihoods? Might cleaning-station behavior make fish more vulnerable to capture?

The discovery also expands the frame for monitoring. If fishers already know certain mounds, catch records and local knowledge could help scientists map which seeps attract commercial species and when.

Conversely, new seep surveys can tip managers to hotspots before fishing pressure focuses there.

Protecting Chile’s prized eel

Almost every expedition to these habitats turns up something unexpected. This one is no exception: crabs may be grooming a market fish nestled in a chemical garden.

The team’s short window means they captured only a snapshot. Are there seasonal pulses? Do the eels return to the same thickets year after year? Are these sites spawning grounds, feeding stations, refuges – or all three?

“Almost every time we visit these ecosystems, we find something new,” Levin said. “There is so much more for us to learn, and we need to keep exploring and studying them.”

For Chile’s “congrio” and the people who depend on it, that knowledge could translate quickly into action. It means protecting a deep-sea hedge that, improbably, shelters a fish with a place on the nation’s plates – and in its poetry.

The study is published in the journal Ecology.

Image credit: Schmidt Ocean Institute

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