This tiny ancient fish only lives between two waterfalls in one river. It’s now under threat

Brisbane, Australia (CNN) — Along a nine-mile stretch of river between two waterfalls in the Australian rainforest lives a tiny fish that’s been there for millions of years, hiding between rocks along the riverbank by day and emerging at night to feed.

This is the only place in the world you’ll find the planet’s sole tropical cod –– a remnant of times past that is believed to have split from its closest southern relatives about 25-30 million years ago.

For all that time, the tiny cod swam undetected and undescribed by modern science until 1993 when two researchers –– Mark Kennard and Brad Pusey –– stumbled upon the fish in the Bloomfield River, along the northern reaches of the Daintree Rainforest, which is recognized by UNESCO as the world’s oldest rainforest and is brimming with rich and unique biodiversity.

“It’s a beautiful little fish,” said Kennard, now deputy director of the Australian Rivers Institute at Griffith University, who still works with Pusey, a senior research fellow at the same university, more than 30 years on.

Back then, Kennard and Pusey named the fish the Bloomfield River Cod, with the scientific name Guyu wujalwujalensis, after the Aboriginal name for fish and the Wujal Wujal community, the land’s traditional owners. It’s also known as the tropical nightfish.

But this ancient species that grows to just 10 centimeters is now under threat from introduced predators and violent storms produced by man-made global warming.

Cyclone Jasper, one of Australia’s most destructive tropical cyclones, tore through the cod’s habitat between two waterfalls on the Bloomfield two years ago –– felling trees, flooding the river and washing more introduced predators into its sanctuary.

This October, Kennard and Pusey returned to the river to survey the damage and count the cod in the hope of having it formally listed as endangered under Australia’s biodiversity laws, which would offer it more protection.

“If we lost this, we’d lose a representative of a really complex and long period of evolution,” said Pusey. “It would be a tragedy… certainly a personal tragedy.”

Getting to the Bloomfield River takes at least four hours along an inland road from Cairns, where tourists board boats to visit the Great Barrier Reef.

The longer, wilder and more scenic route winds around the coast, on unsealed roads through “croc country” where Australia’s giant saltwater crocodiles dominate the briny waterways and smaller freshwater crocs occupy rivers and streams.

At this time of year, summer dials up the heat and humidity, steam rises from roads after sudden downpours, and the smell of sweat mixed with sunscreen and insect repellent lingers in the air.

Kennard and Pusey were on a research trip to survey Queensland’s Wet Tropics when they found the fish in the 1990s.

They initially had no idea what it was. But on a return trip a few years later they found enough information to confirm it was a new species.

“When you find new species here, they’re almost always so minor a difference, and most of the time it’s not until people do the genetics, and they go, ‘oh, that is a new species.’ But this was clearly new, and it is quite exciting to find that,” Pusey said.

The stretch of river that’s home to the Bloomfield Cod is inaccessible to most visitors – but not remote enough to guarantee the cod’s safety.

Their biggest predatory threat is the Tully Grunter, a larger native Australian fish up to 35 centimeters long, that scientists believe was introduced to the river by recreational fishers wanting a decent catch.

The grunter is now competing with the Bloomfield Cod for food and potentially eating its eggs and young.

“There’s been an explosion in their population in the last five to 10 years, and that’s why we are so concerned. They’ve spread throughout most of that upper catchment area where the cod population is,” said Kennard.

In parts of the river where there are no Tully Grunters, cod could be seen from the riverbank, swimming openly, seemingly without fear, he said. But in places inhabited by grunters, researchers diving with snorkels found the cod hiding behind rocks.

Along with the grunters, researchers have found eel-tailed catfish, native to rivers further south, and guppies that come from Central America, suggesting pets were freed and have since started breeding.

But it’s the impact of Cyclone Jasper they were most interested in assessing on their most recent trip.

“You can still see the scarring in the landscape,” said Kennard. “They say it’s one of the most intense weather events ever recorded in Australian history.”

Cyclone Jasper had weakened by the time it made landfall in December 2023, but it stalled over Cape York Peninsula for several days, dumping rain on already saturated land.

Around 975 mm of rain fell on some places in one day – the highest ever 24-hour total reliably measured in Australia.

So much rain fell, it turned the Bloomfield River into a raging torrent.

“Normally what happens in forested rivers like this, the trees fall into the river and provide habitat for fish and other animals that live in the river,” said Kennard.

“But a lot of that has been stripped out and flushed downstream.”

Scientists feared some little fish had been flushed out, too – downstream into the path of other predatory fish, like Barramundi and Mangrove Jack, or into a salty estuary where they might not survive.

A lot has changed in the three decades since Kennard and Pusey first discovered the fish.

Back then, they were a team of two in a vehicle, without even a mobile phone to communicate their whereabouts. “We’re probably lucky not to get lost or die or get bogged,” said Kennard.

Now, drones map the stream and the vegetation from above, and environmental DNA samples indicate the presence of creatures in the water, without having to catch them.

Funding from the National Environmental Science Program has broadened the team to include scientists from James Cook University and natural resource management group Terrain NRM, who know this rugged landscape better than most.

Most significantly, scientists have been working closely with the Jabalbina Yalanji Indigenous Rangers, who manage the land and are learning more about the cod.

Bobby Kulka, an Eastern Kuku Yalanji Traditional Owner, said stories passed down through his ancestors didn’t include mention of the mysterious fish.

“It’s good to know they’re here. I’ve never known they were here before. I’ve never heard any stories about them,” he said.

Local Aboriginal people are being trained to help monitor the fish so that when the scientists leave, their work will help fill in the blanks about the Bloomfield Cod.

So little is known about how the fish reproduces that no one has been able to successfully breed them in captivity.

One option to save the species may be to start a captive breeding program in case they become extinct in the wild.

Another potential solution is to try to establish other populations in nearby catchments, where introduced species aren’t a threat.

But finding those protected catchments could be difficult, given the dense forest that surrounds the river and the spread of invasive species.

Introducing populations at other sites is fraught with risk, too. Would they upset the natural balance? Or create unintended consequences for other species that live there?

Clearing the river of introduced species is not an option, Kennard said.

“You could potentially remove an isolated population. But when they’re in a big river like this, and it’s all connected, it’s impossible,” said Kennard.

The Bloomfield Cod is listed as “vulnerable” on the International Union for Conservation of Nature’s Red List of endangered species, but that was before Cyclone Jasper and before researchers discovered non-native fish within the catchment.

Right now, the emphasis is on gathering enough data to nominate the fish for legal protection.

A listing would raise its profile and create new opportunities to save one of Australia’s rarest fish that Kennard warns could become extinct within 10 to 20 years.

“I think it’s at real risk of becoming extinct,” he said. “That’s not going to happen on my watch.”

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