In 1995, a Tasmanian fisherman found a dead carp. What followed was a four-decade operation to remove nearly 50,000 European fish

In January 1995, a fly fisherman walking the shore of Lake Crescent noticed something odd lying in the shallows. It was a dead carp. Nothing about that moment looked historic. But it set off one of the longest and most methodical wildlife removal operations the world has seen.

Tasmania's Inland Fisheries Service has confirmed that European carp are functionally eradicated from both Lake Crescent and Lake Sorell, two connected freshwater lakes on the state's Central Plateau. Getting there meant pulling 49,301 carp out of the water over almost three decades. Not a typo. Nearly fifty thousand fish, one net at a time.

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Fisheries scientists elsewhere have watched this project for years, mostly because conventional wisdom said it couldn't be done. Eradicating an invasive fish from a small, enclosed pond is one thing. Doing it in open lakes spanning dozens of square kilometres is another matter entirely.

How a Few Illegal Fish Became a National Threat

Nobody knows exactly who released the first carp into these waters. The leading theory points to anglers using live carp as bait, a practice that's both illegal and, as Tasmania learned the hard way, spectacularly destructive.

Carp aren't picky eaters. They root around in soft lakebed sediment searching for food, and in doing so they tear up aquatic plants, wreck native fish breeding grounds, and kick enormous clouds of silt into the water. The lakes started turning murky. Sunlight stopped reaching the plants below. And that put an entire ecosystem at risk, including the golden galaxias, a small native fish that exists nowhere else on Earth, and wetlands protected under an international conservation treaty.

Officials moved fast once they understood the scale of the problem. Public access to the lakes was shut down. Screens went up to stop carp escaping into the Clyde River system. A dedicated Carp Management Program was born, and it would run for the next 28 years.

No Poison, No Draining, Just Patience and Radio Signals

Here's the thing about carp: you usually can't just poison your way out of a problem this size. The lakes were too large, and dumping chemicals like rotenone into the water risked killing off exactly the native species the whole operation was meant to protect. Draining the lakes wasn't realistic either, these are massive basins, not backyard ponds.

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So the team improvised. They caught male carp, surgically implanted them with small radio transmitters, and released them back into the lake. It sounds almost cruel until you realise the payoff: come spring spawning season, these tagged fish, nicknamed "Judas fish", would instinctively swim toward the nearest group of carp, betraying their location.

Crews tracked the signals to shallow marsh areas where carp gathered to breed, then closed in with gill nets and seine nets before any eggs could be laid. In high-water years, they strung kilometres of mesh across marsh entrances to physically block adult carp from reaching breeding grounds at all. Electrofishing boats handled the stragglers hiding in thick vegetation, stunning them just long enough to scoop them out.

Lake Crescent fell first. Its last carp was caught in 2007, and the lake was declared clean by 2009. That left the bigger, messier target: Lake Sorell.

A Genetic Quirk Sealed the Final Chapter

Lake Sorell held out for over a decade longer, and clearing it required a level of biological detective work that borders on obsessive. Researchers leaned on something called "jelly gonad," a naturally occurring condition that leaves some male carp completely infertile. Through constant genetic testing, they tracked exactly how many fish in the population could still actually reproduce.

The last known fertile male was pulled from Lake Sorell during the 2018–19 season. After that, surveys turned up nothing, no evidence of successful breeding since 2013, even in years when conditions were perfect for spawning. A lone female carp showed up in a net in November 2022. Nothing since.

By March 2025, the evidence was overwhelming. A massive survey effort, 66 fyke nets and backpack electrofishing gear deployed across the lake's marshes, racking up more than 6,000 fyke net hours, turned up exactly zero carp. That was the confirmation officials needed to make the eradication call official.

The Lakes Are Already Bouncing Back

What happens when you remove nearly 50,000 bottom-feeding fish from a lake system? Quite a lot, it turns out. Water that used to be clouded with stirred-up silt has cleared dramatically. Sunlight reaches the lakebed again. Native plants have started returning to shallow bays, and with them, the insects that native fish depend on.

Wild brown trout, the fish that made Lake Sorell famous among anglers in the first place, are thriving again. The lake has reopened to recreational fishing, and early reports are encouraging: anglers are landing healthy trout in the two-to-three-kilogram range, a sign the food chain is genuinely recovering rather than just hanging on.

Conservation experts are now holding up Tasmania's carp campaign as proof that large-scale invasive species removal isn't a fantasy, even in open water systems long assumed to be beyond saving. It just takes decades of unglamorous, repetitive work, tagging fish, chasing radio signals, hauling nets, testing genetics, and the discipline to keep doing it long after the novelty wears off.

Authorities aren't declaring total victory just yet. Monitoring continues, and strict biosecurity rules remain in place to make sure nobody's bait bucket undoes 28 years of work.