The population of this predator is much larger than we anticipated
Along the banks of the Tagus, where lamprey and eel have fed communities for generations, an uninvited predator has quietly multiplied into a force that now reshapes the river's living order. A coordinated operation at Belver Dam removed 2.3 tons of invasive catfish — triple what experts had anticipated — only to reveal that the catch barely disturbed a population researchers now estimate at over one million adults. What began as a measured intervention has become a reckoning with how swiftly nature fills the spaces we leave unguarded, and how much may already be lost before we learn to count what is missing.
- A team of seventy people — fishermen, rangers, firefighters, and officials — pulled 254 catfish from the Tagus expecting 700 kilograms and surfaced 2.3 tons, a result that tripled every projection.
- Researchers now estimate 300 catfish per square kilometer in the Belver reservoir alone, with the total adult population across three reservoirs potentially surpassing one million fish.
- These are not passive inhabitants: the catfish consume European eels, lamprey, shad, and barbel — the very species that anchor both the river's ecology and the livelihoods of traditional fishing communities along its banks.
- Despite the scale of the removal, scientists acknowledge the operation touched only a fraction of the actual population, leaving the river's million-strong predator class largely intact and still feeding.
- The Life Predator project continues its work, but the Belver operation has reframed the challenge — from containment to confronting a biological invasion that has already metastasized across the system.
At Belver Dam, near the village of Ortiga, seventy people gathered to pull invasive catfish from the Tagus. They had come prepared for roughly 700 kilograms. What they removed was 2.3 tons — 254 fish, some capable of growing to two and a half meters and over a hundred kilograms — a result that exceeded every projection by a factor of three.
The catfish is a creature built for dominance. A single female produces tens of thousands of eggs annually, and the species has spent decades colonizing European river systems with quiet efficiency. In the Tagus, it preys on the fish that matter most: European eels, lamprey, shad, and barbel — species tied not only to the river's ecological balance but to the food traditions and economic survival of the communities living along its banks.
Filipe Ribeiro, coordinator of the national Life Predator project and researcher at the University of Lisbon, delivered the operation's most sobering finding: approximately 300 catfish per square kilometer in the Belver reservoir alone. Projected across the three major reservoirs in the system, the total adult population may exceed one million fish — a number that reframes the scale of what river managers are actually facing.
The operation was a genuine mobilization, the kind that signals a problem has grown too large to defer. But it also made plain that removing 2.3 tons from a river holding a million predators is, in the end, a beginning rather than a solution. The Tagus still holds its catfish. And the native species — and the people who depend on them — are still waiting for an answer equal to the problem.
The Tagus River holds a predator that has grown far larger than anyone realized. At Belver Dam, near the village of Ortiga in Mação, a team of seventy people—fishermen, wildlife officers, forest service technicians, national guard, firefighters, and local officials—spent days pulling catfish from the water. They came expecting to haul out about 700 kilograms. What they actually removed was 2.3 tons: 254 individual fish, each one a testament to how thoroughly an invasive species can colonize a river system when left unchecked.
The catfish itself is a creature of extremes. Some grow to two and a half meters long and weigh more than a hundred kilograms. A single female can produce tens of thousands of eggs in a year, which explains how a species introduced to European waters decades ago has come to dominate stretches of the Tagus with such speed and totality. These are not passive residents. They are voracious hunters that feed on the fish that matter most to the river's ecology and to the people who live along its banks: European eels, shad, lampreys, and barbel. These are the species woven into the region's fishing traditions, its food culture, its sense of place.
Filipe Ribeiro, a researcher at the University of Lisbon's Faculty of Sciences and coordinator of the national Life Predator project, stood at the dam on a Monday and spoke words that carried the weight of an unwelcome discovery. The team had exceeded even its most optimistic projections by a factor of three. "The population of this predator is much larger in the Tagus than we anticipated," he said. The data they collected during the operation painted a stark picture: roughly 300 catfish per square kilometer in the Belver reservoir alone. Extrapolate that across the three major reservoirs in the system, and the numbers become almost incomprehensible. Ribeiro estimated the total adult population could exceed one million fish.
One million. The figure sits in the mind like a weight. It means that every native fish species in these waters faces predation on a scale that no ecosystem management plan had fully accounted for. It means that the traditional fisheries that have sustained riverside communities for generations are under siege not from overharvesting or pollution alone, but from a biological invasion that has metastasized while authorities were still calibrating their response.
The operation itself was a show of coordinated effort—the kind of mobilization that happens when a problem becomes impossible to ignore. But it also revealed something else: that even when you remove 2.3 tons of invasive predators in a single operation, you are barely scratching the surface of what is actually there. The fish pulled from the water that week represented a fraction of a fraction. The river still holds its million. And they are still hungry.
Notable Quotes
We expected to remove about 700 kilograms. We removed more than three times what was anticipated.— Filipe Ribeiro, University of Lisbon researcher and Life Predator project coordinator
The population of this predator is much larger in the Tagus than we anticipated.— Filipe Ribeiro
The Hearth Conversation Another angle on the story
Why does it matter that the catfish population is so much larger than expected? Isn't removing 2.3 tons still a win?
It matters because it changes what you're actually fighting. If you thought the problem was manageable and it turns out to be ten times worse, your entire strategy has to shift. You can't remove a million fish one operation at a time.
So what happens to the native fish species—the eels, the shad—while this is going on?
They're being consumed at a rate that the ecosystem can't sustain. These aren't just fish. They're part of the food chain, the culture, the economy of the communities along the river. A fisherman's livelihood depends on species that are now prey.
Is there a way to control the catfish population without removing them one by one?
That's the question everyone is asking. Removal operations like this one are necessary but they're also a symptom of how late we are to the problem. The catfish got here, established itself, and reproduced for years before anyone fully grasped the scale.
What does Ribeiro think comes next?
The research continues. They're studying the population density, the breeding patterns, the impact on native species. But the honest answer is that this is a long-term problem. You don't solve an invasion of a million predators in one summer.