🦞 What happens when you remove 20,000 invasive crayfish from a single pond?
A murky, lifeless pool of brown water transforms into a crystal-clear haven bursting with aquatic plants, dragonfly larvae, and even endangered species— all in just 12 months. Here's the story of one conservationist's battle against Japan's most stubborn invasive species, and what it tells us about the surprising resilience of nature.
A Pond on the Brink
Across Japan, countless ponds, rice paddies, and wetlands have been quietly devastated by a single culprit: the American crayfish (Procambarus clarkii). Originally brought to Japan from Louisiana in 1927 as feed for farmed bullfrogs, these hardy crustaceans escaped into the wild and have since colonized waterways in all 47 prefectures.
The damage they cause is severe. American crayfish are voracious omnivores that devour aquatic plants, chew through root systems, and prey on insects, tadpoles, and small fish. Once they gain a foothold in a pond, they systematically strip away vegetation. Without plants to filter the water and provide habitat, water quality collapses. The pond turns brown. Native species vanish. What conservationists describe as a "black hole of nutrient cycling" takes hold—the crayfish consume everything coming into the ecosystem while giving almost nothing back.
This was the state of a small rural pond that caught the attention of Komiya, a YouTuber and biodiversity conservation activist who documents his work on his channel "Komiya's Reflections on Biodiversity" (@ariake538). The pond was clouded with sediment, virtually devoid of visible life, and thoroughly overrun with American crayfish.
The Threshold Strategy
Previous efforts to control American crayfish in Japan have often relied on a technique called ike-boshi (literally "pond drying")—draining ponds completely to remove unwanted species. But crayfish are notorious survivors. They burrow into the mud and wait out the dry period, re-emerging when water returns. Traditional trapping methods also tend to fall short, because if you don't catch more crayfish than the population can reproduce, the numbers simply bounce back.
Komiya and his team adopted a different approach based on the concept of a "threshold" (ikichi). The idea is straightforward but demanding: you must capture crayfish at a rate that exceeds their reproductive capacity. Fall below that threshold, and the population recovers. Exceed it consistently, and the population starts to collapse.
To hit that target, the team deployed large numbers of traps and maintained a relentless schedule. Over the course of one year, they removed approximately 20,000 American crayfish from a single pond.
A Dramatic Transformation
The results were astonishing—even to Komiya himself.
Within the first month, large adult crayfish had largely disappeared. By spring, native amphibians began returning. Larvae of the San'in salamander (Hynobius setoi) and tadpoles were spotted in the now-clearing water. By summer, aquatic plants were spreading rapidly, and water insects such as diving beetles and water scorpions followed.
One year after the removal effort began, the pond was unrecognizable. Water that had been opaque brown was now clear enough to see the bottom. The surface was covered with lush green vegetation, including water chestnut (Trapa japonica) and stonewort algae (Chara species) that acted as natural water filters.
Even more remarkably, endangered plant species began reappearing. Surveyors confirmed the presence of Ceratopteris gaudichaudii (a small aquatic fern classified as Near Threatened) and Limnophila sessiliflora (an aquatic herb that several prefectures list as endangered). These species had likely persisted as dormant seeds in the pond's sediment—a natural "seed bank" that sprang to life once the crayfish were no longer destroying every shoot.
From "Black Hole" to Living Ecosystem
Komiya explained the transformation in ecological terms. When crayfish dominated the pond, it was a dead-end in the nutrient cycle—a place where organic matter entered but never left. The crayfish consumed fallen leaves, visiting amphibians, and virtually every living thing.
With the crayfish suppressed, the cycle reversed. Aquatic plants now convert sunlight and nutrients into biomass. Insects and tadpoles feed and grow in the pond, then leave as adults—dragonflies take to the air, frogs head into surrounding forests. Those frogs become food for snakes, birds, and mammals. The pond shifted from being a place of pure consumption to a place of biological production, feeding the wider landscape.
The Fight Isn't Over
Despite the dramatic recovery, Komiya cautions that the job isn't done. Small juvenile crayfish are still being found in the pond, and he estimates that complete eradication will take at least three years of sustained effort.
There's also the constant threat of reintroduction. The original infestation likely began when just a few crayfish were released—perhaps dumped pets or escaped specimens. From that small start, the population grew slowly before exploding. Even a handful of survivors could restart the cycle.
"There is no breakthrough method," Komiya says of the road ahead. "We just have to keep doing the work, steadily and consistently."
Surviving Climate Challenges
The first year also brought unexpected tests. The summer of 2025 saw severe drought conditions across much of Japan, and many ponds and streams dried up entirely. Komiya's project pond nearly lost all its water too—but because the team had previously dredged and deepened parts of the pond bed, pockets of water remained. Many organisms survived in these refuges.
"This year made it clear that building ecosystems resilient to climate change is essential," Komiya reflected.
Japan's Legal Framework for Invasive Species
Japan has been strengthening its legal response to the crayfish problem. In June 2023, the American crayfish was officially designated as a "Conditionally Specified Invasive Alien Species" under Japan's Invasive Alien Species Act. This is a unique regulatory category—the crayfish couldn't simply be banned outright because millions of Japanese households keep them as pets, and a blanket prohibition would likely lead to mass releases into the wild, worsening the problem.
Under the current rules, people can continue keeping pet crayfish without any permit. However, releasing them into rivers, ponds, or any natural waterway is now illegal and subject to penalties. Selling, distributing, and importing crayfish are also prohibited. The regulation reflects a pragmatic balance: acknowledging reality while closing the door on further spread.
Japan's Ministry of the Environment has also published detailed manuals and educational materials on crayfish control, recognizing that community-based removal efforts like Komiya's are essential to the national strategy.
Inspiration for Other Communities
Komiya's documentation of the project has had ripple effects beyond his own pond. By sharing detailed data on how many crayfish need to be removed before populations start declining, he has given other communities a roadmap and, crucially, hope.
"By showing in video how many crayfish you need to catch before the population starts to decline, we demonstrated that this is not an endless battle," Komiya says. "We've received messages from people saying they felt it was possible and started their own removal efforts."
His broader goal now extends beyond a single pond. He is working to remove invasive species from surrounding ponds as well, aiming for regional ecosystem recovery and eventual certification under Japan's "Nature Symbiosis Sites" (officially, the Ministry of the Environment's "30by30" initiative, which aims to conserve 30% of land and sea by 2030).
What Does Your Country Do About Invasive Species?
Japan's American crayfish story is both a cautionary tale and a story of hope. A species introduced nearly a century ago for an entirely different purpose has caused widespread ecological damage—but determined, strategic action by citizen-conservationists is proving that recovery is possible.
Invasive species are a global challenge. From zebra mussels in North America's Great Lakes to cane toads in Australia, every country faces its own version of this battle. What invasive species are causing problems where you live? Has your community found effective ways to fight back? We'd love to hear your stories.
References
Reactions in Japan
Watching Komiya's videos, I can't believe the initial brown pond and the current green pond are the same place. Nature really can recover this much...
I loved crayfish fishing as a kid, but I had no idea they caused this much damage to ecosystems. Mixed feelings.
20,000 is beyond imagination. The sheer effort of doing this steadily over a year is humbling. Volunteer power is incredible.
The threshold concept is rational, but doing this across every pond in Japan is realistically impossible. Not enough people or money.
Humans are the ones who brought them here in the first place. The crayfish are victims too, not villains. Removal is necessary, but I don't want to lose that perspective.
Endangered plants reviving from the seed bank gave me chills. Seeds can sleep in the soil for decades—that's amazing.
The pond near my house is full of crayfish too. When I asked the local government, they just said 'no budget.' The government should be supporting activities like this much more.
The 'conditional' invasive species designation is half-baked. Pet shops can't sell them anymore, but the wild populations are left alone. We need bolder measures.
My son's elementary school has them observe crayfish, but I wish they'd tie it into lessons about invasive species. Don't just stop at 'they're cute.'
Having this kind of painstaking work visualized with real data is so valuable. Videos resonate with ordinary people more than research papers.
The fact that wildlife survived the drought because of prior preparation is the same logic as disaster prevention. Being prepared matters.
Honestly, 3 years to eradication seems optimistic. There's reinvasion risk from surrounding areas. I think we need to be prepared for a decade-long commitment.
Pond restoration matters, but I worry the restored pond might get trashed if it becomes an Instagram-worthy spot. Maybe the location should stay private.
The part about water chestnuts overgrowing and causing unhealthy recovery is fascinating. Removal alone doesn't create the ideal ecosystem. Balance is tricky.
The name 'American crayfish' almost makes you resent America lol but we humans brought them here, so it's our own fault.
I'm from the American South. We consume huge amounts of crawfish as a regional dish. Why not combine food use with removal in Japan? Cajun cooking—highly recommended.
In Sweden, crayfish parties (kräftskiva) are an annual tradition in August. But we also have signal crayfish threatening native species here. The problem structure is very similar.
As someone in Australia struggling with cane toads, this gives me hope. But success in one pond is a different story from a national-scale effort.
I can't believe endangered species returned in just one year. Nature's resilience is truly amazing. In India, invasive fish are destroying river ecosystems, and we should look at Japan's approach.
In Brazil, giant African snails cause huge agricultural damage. Japan's example showing citizen-led removal can achieve real results is a model Brazil should study.
American crayfish are driving our native white-clawed crayfish to the brink of extinction in the UK too. We have regulations but they're not very effective. Japan's conditional designation system is interesting.
I'm an ecologist in Germany. The threshold approach is theoretically sound, but scaling it up is challenging in terms of effort-to-outcome ratio. This Japanese case has academic value as a success model for small enclosed water bodies.
In Ontario, Canada, zebra mussels clog pipes and cause millions in damages. Invasive species are a serious problem everywhere. Prevention is always cheapest, but we're always playing catch-up.
American crayfish are widespread in China too, but we consume over 2.5 million tons per year as food. It's become a massive food industry, so the idea of removal doesn't even exist. The contrast with Japan is fascinating.
I live in Hawaii. We have a history of invasive species like mongoose and brown tree snakes devastating bird populations. Sharing conservation work via YouTube is effective for both awareness and fundraising.
In Poland, signal crayfish are displacing European crayfish. They carry crayfish plague, which is especially problematic. It seems Japan faces the same issue.
New Zealand is one of the strictest countries on ecosystem protection. But complete invasive species elimination is extremely difficult outside islands. Japan's pond-level success is relevant for scale.
Italy's Louisiana crayfish problem is also quite serious. They burrow into levees in rice-growing areas and cause agricultural damage. I wish the culture of citizen-level action like Japan's would spread more.
In Nigeria, invasive water hyacinth blocks rivers and devastates fishing. The form of the invasive species differs, but the pattern of ecosystem destruction is strikingly similar.