They lurk beneath the water, unseen until snagged by a fisherman or researcher. Some of these fishy intruders have traveled entire continents to their new habitats, relocated by accident in a manner that just would not have been possible until human beings began crisscrossing the globe at will. A number of these fish, on the other hand, were purposefully introduced to provide food and sport, or even to fight disease. Others are escaped pets, chancing their way in the great blue yonder.
Yet, whatever their origins, invasive fish now often wreak havoc on the ecosystems they have occupied, displacing native aquatic life in such a way that has consequences both above and below the water. From the Nile perch (pictured here), to the northern snakehead, to the mosquitofish, here is a list of 12 of the most destructive invasive fish species out there today.
12. Nile Perch
Growing up to six feet long and weighing as much as 440 pounds, the Nile perch is a real-life ‘monster of the deep’. And it’s on the International Union for Conservation of Nature’s (IUCN) top 100 worst invasive species list for good reason. Nile perch introduced to Lake Victoria in Africa in the 1950s have either annihilated or seriously threatened hundreds of the lake’s indigenous fish species.
Big fishing operators have actually profited from the Nile perch invasion of Lake Victoria, but local fishermen have suffered with the depletion of their traditional fish supplies. In fact, the damage to the native fishing trade has been enough to force many of the fishermen in the area into becoming economic refugees. This is just one example of how introducing a species from outside can have far-reaching repercussions, not just for the ecology, but for the people who depend on it as well.
11. Common Carp
This is another fish on the IUCN’s 100 most invasive species list: the common carp. In the wild, in its native waters of Europe and Asia, the common carp is considered at risk of extinction. There’s not much likelihood of it disappearing altogether, though, as this fish has been domesticated and brought in to new environments across the globe.
It’s not just what the carp feed on that causes problems; it’s the way they feed. Common carp scavenge for food around the roots of aquatic plants, muddying the water and making it generally murky and unclean. This has a negative effect on the plants as well as the native fish and waterfowl that depend on them.
The behavior just described and the fact that the carp release phosphorous into the water encourages excessive quantities of algae to grow. And because of the serious damage the fish cause, countries like the US have spent large sums of money trying to destroy invasive carp populations, mostly without much success.
10. Northern Snakehead
If there were a Public Enemy No. 1 in the world of invasive fish, the northern snakehead might very well fit the bill. Not only are they fierce carnivores with few, if any, predators, but – freakishly – they can also live out of water for several days, enabling them to spread further around any invaded territory. Kept as exotic pets, these fish – which are native to China – will easily eat five goldfish a day, and they can do untold damage if released into the wild.
Scientists in America are concerned that northern snakeheads could prey on local fish, introduce disease, reduce food resources for native species and generally disrupt the ecosystem. It’s no wonder the discovery of a few of these predators in a pond in Maryland created national headlines in 2002 – especially when you bear in mind reports that females can release as many as 150,000 eggs in just two years.
9. Brown Trout
A number of the fish on this list, including the brown trout, were introduced to their non-native environments on purpose. In the case of the brown trout, originally a European species, it was because of their popularity as a source of food and as a target for fly-fishing. Even today, the trout are said to contribute $300 million to the New Zealand economy as sport fish. Unfortunately, however, this comes at a cost to the native wildlife.
Brown trout can have a bearing on the populations of the small fish, insects and mollusks on which they feed, affecting biodiversity. They also compete with native species for food, and perhaps even mate with some of those species, threatening their genetic integrity. As brown trout are able to survive in warmer waters than other indigenous trout species, it has been suggested that their populations may grow even further as a result of climate change. This is good news for the brown trout but not so good for the species they are displacing.
8. Largemouth Bass
Largemouth bass were also introduced to non-native environments due to their popularity with recreational fishermen. These fish are aggressive apex predators and will take anything they can find – including other fish, small birds, amphibians, and even baby alligators, according to some sources. These bass have appetites to match their mouths and eat day and night.
Largemouth bass can rapidly populate a lake or pond, and each individual lives for around 16 years. They are widespread throughout the US but have also invaded the Canadian province of New Brunswick, where they allegedly not only feed on the native fish but also bring in sea lice.
7. Walking Catfish
Now here’s a fish that really does resemble an alien invader. The walking catfish is an incredible, albeit creepy looking fish with the ability to ‘walk’, using its fins, and actually breathe on dry land.
Walking catfish are a particular problem in the US state of Florida. They are believed to have been introduced from Thailand in the 1960s thanks to the aquarium trade. And the threat from these omnivorous fish is not just to native wildlife. They are also reported to have squirmed their way into fish farms, where they have proceeded to prey on stocks. This has led to a ban on owning walking catfish in the state.
6. Smallmouth Bass
Don’t let the name fool you; there’s nothing small about the mouth of this fish. In fact, the North American smallmouth bass is a predator that will eat any other fish it can comfortably swallow. And as you can see from this picture, that means it can gulp down some pretty sizable prey. Even small mammals and snakes aren’t safe! Once this bass has its prey, there’s little chance of escape, as its mouth is lined with tiny gripping teeth that work like Velcro.
Because of their voracious appetites and fierce predatory instincts, smallmouth bass are able to supplant native fish; they even eat the prickly three-spined stickleback. Factor in their adaptability to new environments and the ability of the female smallmouth bass to lay up to 21,100 eggs at a time, and the result is bad news for native wildlife – notably in parts of British Columbia, where they have been slowly spreading since their arrival in 1901.
Sure, they look small and harmless – a bit like an anchovy – and you may wonder what possible danger a tiny fish like this could be to an ecosystem. Well, quite a lot, as it turns out.
As their name suggests, mosquitofish eat mosquito larvae; an adult female can hundreds of them in a single day. Yet unfortunately, this larvae-eating behavior only just outweighs the destructive behavior of these fish.
Introduced to many water systems in the mistaken belief that they would deal with the mosquito population better than the local fish, the mosquitofish has thrived, going on to become what is regarded as the most widespread freshwater fish.
To give credit where it’s due, mosquitofish have helped eradicate malaria in several regions, including South America and southern Russia. However, their appetite, aggressive behavior and the fact that they compete with other species have made them a threat in many places.
4. Common Rudd
With its flame-colored tail and fins and silvery green body, the common rudd is quite nice to look at, but it’s not just a pretty fish – it’s an ecological nightmare. In New Zealand, where they were illegally introduced in 1967, common rudd are known as the “possums of the waterway.” Although native to Europe, they are now also widespread in the US, where it is feared they may interbreed with the native golden shiner, altering the shiner’s original genetic makeup.
The problem with the common rudd is that they like to eat native water plants – and a lot of them! So many, in fact, that they deplete food sources for indigenous aquatic wildlife as well as threatening the plant species themselves. And with females able to produce around 50,000 eggs per kilogram of body weight, we’re betting they’re hard to get rid of once they’ve invaded a particular environment.
One of the qualities that seems to be common to all invasive fish is adaptability. It’s this trait that allows them to reproduce and thrive in non-native environments, often at the expense of the indigenous species of the area. The common tilapia – native to Africa and the Levant – is no exception.
Like the common carp, tilapia muddy up the waters they inhabit, decreasing the amount of light available and making them unsuitable for aquatic plants and other animals. They also eat small fish and invertebrates, competing with native fish for food. What’s more, they’ve had quite an impact on the ecosystems of several countries, including Australia, where they were introduced in the 1970s for weed and mosquito control but are now considered a major pest in local water systems.
2. Giant Snakehead
The giant snakehead is one mean fish. It can grow up to six feet in length and weigh as much as 66 pounds. And it is by far the most aggressive fish on this list. In their native Southeast Asia, there are even tales of these fearsome fish launching unprovoked attacks on people! And considering the size they grow to and the sharp teeth with which they’re armed, that's not something to take lightly. The giant snakehead is even considered more dangerous than piranhas by some pet owners.
In Canada and the United States, the giant snakehead could threaten not only the local ecology but the economy as well, with fears that this top-level predator might have an adverse effect on the $30 billion US fishing industry. Although there have been attempts to eradicate this fish from American waters – several specimens having been caught, thought to have been released as pets – the worry is that they will continue to spread.
Lionfish are rare in their native waters of the Pacific Ocean. However, their population has exploded in Caribbean waters, where they are devastating the numbers of native fish found around the coral reefs there. Researchers have found that a single lionfish can eat up to 79% of other fish in a given area in just five weeks!
Armored with venomous spines, lionfish are dangerous not only to other aquatic life, but to divers and fishermen as well. Even sharks keep their distance! So far, there have been attempts to cull this beautiful but invasive species, but scientists believe this will do little other than control their numbers.
It is not the fault of the fish on this list that they are invaders of other habitats. They have been moved from their natural environments, where they likely lived in harmony with their native ecology. This highlights the fact that even the most insignificant-seeming act, like releasing a pet fish into the wild, can have devastating effects on local ecosystems. And sadly, the negative repercussions of these badly thought-out actions will be with us for many years to come.