Humans are experts at helping species move from their native habitat into new territory. Sometimes, the new habitat suits the invader so well that the results are catastrophic for local species. This is true on land as well as in the water. Ecosystems around the world have been dramatically altered as fish species are shifted around, whether for commercial fishing stock or the aquarium trade. When the fish are let loose or escape, a cascade of changes often follows, sometimes causing significant deforestation, an upheaval in the economy, or the decline of species that live above water.

These species are some of the most hearty and adaptable, and therefore the most invasive on the planet. Most of these species are so destructive that they are listed on the Global Invasive Species Database of 100 of the World's Worst Invasive Alien Species.

Here are 10 fish species that are wreaking havoc around the globe.

Walking catfish

The walking catfish is a freshwater, airbreathing catfish from Southeast Asia.The walking catfish is a freshwater, airbreathing catfish from Southeast Asia. (Photo: Wibowo Djatmiko/Wikipedia)

The walking catfish is an extraordinary species. Native to Southeast Asia, it is able to "walk" on dry land using its fins and tail to wriggle its way from one pond to another. It has a hearty appetite and a varied menu, ranging from smaller fish to mollusks to aquatic weeds and other plant matter. The adaptations that allow it to survive in its native habitat have also helped it thrive in ponds and lakes where it has been introduced.

The species was introduced in Florida in the 1960s. It has also been spotted in California, Nevada, Connecticut, Massachusetts and Georgia. Though the walking catfish isn't a significant threat to many native species since wading birds prey on them and keep their numbers in check, the walking catfish is a troublemaker among fish stock ponds. Because it can "walk" its way from pond to pond, it finds its way into feed stock ponds and feasts on the fish being raised there. Fish farmers have be forced to put up fencing around their ponds to keep the fish from literally walking right in. Over the years, walking catfish have caused millions of dollars in damage in Florida alone.

Common carp

common carp caught by fishermanThe common carp is a contradictory fish, with wild native populations considered vulnerable to extinction and yet introduced populations are destructive. (Photo: Fabien Monteil/Shutterstock)

This enormous freshwater fish is considered vulnerable to extinction in the wild, and yet it's also listed as one of the worst invasive species in the world. This contradiction is human-caused: It has been domesticated as a fish stock and has, either accidentally or illegally, been introduced to ponds and lakes on almost every continent in the world. The damage carp do to their new homes is extensive.

Carp feed by rooting through bottom sediments, destroying submerged vegetation that feeds other species, including other fish and also duck species. They also eat the eggs of other fish, causing native fish populations to plummet.The plant matter they do eat isn't fully digested, which means it rots after excretion and promotes algae growth.

The species is so widespread and yet so destructive that ingenious ways of getting rid of them have been devised, including netting them by the millions and turning them into fertilizer, deliberately exposing them to a deadly koi herpes virus, and genetically modifying them so that they only produce male offspring.

Mosquito fish

mosquitofish eat a variety of prey,Despite their name, mosquitofish eat a variety of prey, and can consume between 42-167 percent of their own body weight in a single day. (Photo: topimages/Shutterstock)

You don't have to be big to be a big troublemaker. Mosquitofish are both celebrated and reviled. The fish is known for eating large quantities of mosquito larvae and therefore reducing mosquito populations and the prevalence of diseases that mosquitoes spread. In Sochi, Russia, the fish is credited with eradicating malaria and is commemorated with a monument. The species has been introduced all over the world as a way to battle mosquitoes; however, the results are not usually so positive as what happened in Sochi.

Mosquitofish don't only feed on mosquito larvae. In fact, they die if that's the only food available to them. They feed on a variety of small insects and insect larvae as well as zooplankton. They eat so much that they out-compete other native species. They're also mean little buggers, with a propensity for injuring or killing other small fish. In Australia, they pose a threat to native fish and frog species and are considered a serious pest — and to add insult to injury, they have made no impact on mosquito populations nor have they helped reduce mosquito-borne illnesses. In fact, in many areas where they have been introduced, they are less effective at mosquito control than native species, which they tend to wipe out. In these cases, mosquitofish benefit mosquitoes by reducing predation by the other species that eat mosquito larvae.

Despite the risk they pose, not everyone is aware. For instance, several counties in California provide mosquitofish to residents who have standing pools of water on their property in an effort to reduce the spread of West Nile virus.

Nile perch

The Nile perch s one of the largest freshwater fishThe Nile perch is one of the largest freshwater fish, reaching up to 6 feet and up to 440 pounds. (Photo: smudger888/Wikipedia)

The introduction of this fish, native to Ethiopia, had a devastating and cascading impact on Lake Victoria when it was introduced in 1962. The biodiversity of the lake utterly collapsed, and the fishing economy of the area changed dramatically, helping some new businesses make million in exports while causing many local fishermen to fall into poverty. It even caused an increase in severe deforestation. But let's start with the impact on the lake itself.

The Nile perch eats pretty much anything, from crustaceans and mollusks to insects and other fish, and even will eat its own species. A single female can produce as many as 16 million eggs at once, so it doesn't take much time to take over an area. Columbia University writes, "As the fish matures its appetite increases. This ravenous creature now searches for larger fish than the usual minute supply. This ability to prey on different size fish enables it to dominate many habitats and have a catastrophic effect on the many species it encounters as it moves from area to area in search of food. The introduction of the Nile perch into Lake Victoria has had a catastrophic effect on the ecosystem. Hundreds of fish native to the lake have become extinct, by the 1980’s, 300 of these fish were nonexistent."

Imagine having a lake rich with hundreds of different species of fish, and within a couple decades, the majority of them are gone and only the Nile perch dominates. The effect is overwhelming and extends to the shore. The fish is fattier than the native fish species, so rather than drying it in the sun, fishermen have to smoke their catch. This requires a good deal of firewood, resulting in the widespread loss of surrounding forest that was already under pressure — and the likely the decline of species reliant on the forest. The cascading disastrous effects have placed it firmly on the list of one of the 100 worst invasive species in the world.

Brown trout

The brown trout is a freshwater fish, but can also adapt to salt water.The brown trout is a freshwater fish, but it can adapt to salt water. (Photo: IDAK/Shutterstock)

This trout species may be a favorite among fishermen, but it's not necessarily a favorite among other fish. Brown trout are originally native to Europe, North Africa and western Asia but today can be found all over the world. It was part of an aquaculture trend that started in the mid-1800s in Europe and has been moved around the world ever since as a popular fish for farming and fishing. However, its impact on native fish species can be problematic.

Not only does the brown trout compete — and usually win — against native trout species such as brook trout and golden trout, but it also out-competes other fish species. Where it doesn't out-compete other trout species, there's evidence that it breeds with them. This has conservationists worried about the genetic make-up of native species. Conservation measures, including restricting the introduction or stocking of brown trout, are important steps in battling this invasive species, and in some cases, it's working.

Rainbow trout

rainbow troutRainbow trout increase the prevalence of whirling disease, which is caused by a parasite, and threatens recreational fishing. (Photo: pu_kibun/Shutterstock)

The trauma from trout doesn't end with the brown trout. The rainbow trout is also a problem in areas where it has been introduced. Rainbow trout is native to the western United States but like its brown counterpart, is can now be found all over the world. It's similar to brown trout in that it's an adaptable predator that can out-compete many other species, driving some to the edges of extinction, including the Californian golden trout and humpback chub. They can easily populate streams and cause a shift in invertebrate populations, a shift that has an impact on every other species that feeds on invertebrates. Like brown trout, rainbow trout can hybridize with other trout species, causing rare trout species to become even more so.

Another significant issue with rainbow trout on a global scale is that they are a significant carrier of a parasite that causes whirling disease in both farmed salmon and trout populations as well as wild populations. Once a problem only among rainbow trout, it has spread to other fish species.

The rainbow trout problem isn't likely to slow down anytime soon. They are a favorite of anglers and the U.S. stocks streams with them every year.

Largemouth bass

largemouth bass caught by fisherman

Where largemouth bass have been introduced, they have caused the decline and even extinction of native species. (Photo: Matt Jeppson/Shutterstock)

Another favorite of anglers, the largemouth bass has made its way around the world because of the excitement of catching them. They tend to put up a good fight on the line, and that's because they're tough fish — tough enough to take on and beat native species. And it's not just fish species at risk; largemouth bass will eat small birds and amphibians.

Their big appetite and position at the top of the food chain mean that other native fish species are driven to extinction. They are responsible for the decline of native frogs in California as well as the California tiger salamander, the Chiricahua leopard frog in Arizona, and a wide variety of fish species across the world.

Mozambique tilapia

the Mozambique tilapia is a robust and adaptable fishA robust and adaptable fish, the Mozambique tilapia can become a significant problem when introduced to new habitats. (Photo: Greg Hume/Wikipedia)

Another member of the World's 100 Worst is the Mozambique tilapia. This fish is a winner for aquaculture, but when released on purpose into new habitats, or when it escapes from fish farms, the robust, hearty and adaptable fish tends to take over. The Mozambique tilapia is omnivorous and can eat everything from plants to small fish. It can thrive in a range of water temperatures from colder than 50 degrees to hotter than 100 degrees Fahrenheit. It also reproduces like nobody's business. Females are very protective of their young and can have several broods in a season.

In the U.S., introduction of this species is responsible for the decline of the desert pupfish in the Salton Sea, which is now an endangered species, and Hawaii's striped mullet. Because the species is found in the waters of Dade County, Florida, there's concern that the species will soon establish itself in the Everglades, which would have terrible consequences for native wildlife.

In a weird twist, Mozambique tilapia are threatened in their own home range by the introduction of the Nile tilapia.

Northern snakehead

The snakehead fish The snakehead fish can survive on land for up to four days and can migrate up to a quarter mile over land. (Photo: Wie146/Shutterstock)

Let's move away from familiar trout, bass and tilapia into a slightly more unnerving category of fish. National Geographic did a piece on this species and dubbed it Fishzilla. The snakehead can breath air and can live out of water for up to four days, provided it stays wet. And if that isn't amazing enough, it can travel across land, wiggling as far as a quarter mile when looking for a new pond to inhabit. And yes, its head is shaped much like that of a snake, leading to its common name.

Originating from Asia and Africa, they are tough, hardy fish at the top of the food chain and they lack natural predators in introduced locations. There are four species of snakehead in the United States, but the Northern snakehead has established breeding populations in the wild. Because they're at the top of the food chain and will eat anything from fish and crustaceans to small mammals and birds, they can cause significant disruptions in any ecosystem they enter. Native species often lose out to this new predator. During breeding season, they get extra aggressive while protecting their nests, even biting humans who have gotten too close to a nest.

The damage they've done is extensive. Since 2002, it has been illegal to possess a live snakehead in the United States.


common lionfish in oceanAfter being introduced to the Atlantic and Caribbean Sea, this voracious predator has become one of the most invasive species on the planet. (Photo: Alexander Vasenin/Wikipedia)

The lionfish is one of the most worrying tales of an invasive fish taking over a new habitat. There are nine species of lionfish, all native to the waters of the Indo-Pacific. However, after being released from aquariums either accidentally or on purpose, two of those species have established themselves in the Atlantic off the East Coast of the United States and throughout the Caribbean. They have moved from Florida up as far north as Delaware, as far south as Brazil, and as far east as Barbados. They are considered one of the most aggressively invasive species in the world.

Lionfish are known for two things: long fins equipped with venomous spikes, and an insatiable appetite. The combination makes it a top level predator, with few predators and the ability to eat pretty much anything that will fit in its mouth. They could cause significant declines in the biodiversity of the already fragile reef systems they inhabit. They also threaten commercially important species of fish including snapper, grouper and sea bass. According to the National Oceanic and Atmospheric Administration, "Unfortunately, NOAA researchers have concluded that invasive lionfish populations will continue to grow and cannot be eliminated using conventional methods. Marine invaders are nearly impossible to eradicate once established."

Possible solutions to at least trimming their numbers include putting lionfish on the menu at restaurants, as well as training sharks to eat lionfish. Sharks can eat lionfish without suffering the effects of the venom, so for the last few years, researchers have been trying to train local shark populations, which haven't historically fed on lionfish, to consider them a new prey source. Whether or not it will make an impact in the long term is up for debate, but that it's considered a serious option illustrates how dire the situation is.

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Jaymi Heimbuch ( @jaymiheimbuch ) focuses on wildlife conservation and animal news from her home base in San Francisco.