10 of the world's most invasive fish


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The effects of non-native fish on an ecosystem can be far-reaching. Nathan Hill lists his top ten worst offenders - and some of them might surprise you!

We humans have a penchant for taking luggage with us around the world, and in the process have brought terrible ecological woes trailing behind us. Sometimes our intentions are good: to provide food, or eradicate an illness. Sometimes we have done it just for the sport.

Unlike some other animals, invasive fish lack the 'cute' factor.

Some invasive creatures we take for granted, even ascribe higher importance to them than the native species they devastate. Just look at the domestic cat. These things are everywhere in the modern age, scoffing down rodents, mustelids and birds like there’s no tomorrow, but should a native snake in, say, South America turn the tables and munch one back, then everyone starts bawling like the skies are falling down.

The big problem with many invasive fish is that unless you’re directly involved with the aquatic world, it’s hard to see the effects. Anglers notice gradual alterations to their catches, and eco systems start to wither and die, seemingly from some invisible source.

As responsible aquarists, we’re actually pretty good when it comes to keeping fish where they belong. For the greater proportion of us, our fish are tropical, and aside a few peak days in summer, much of what we could be stupid enough to slip into a pond or river would be dead in next to no time. Though that’s to say nothing of the impact of any pathogens those fish might be carrying.

Some fish are more obviously destructive than others. It stands to reason that the problem becomes more visible in regions where human population abounds. A stray fish species, rampaging through some obscure wilderness and gobbling up everything in its path like some organic hoover, is less likely to appear on our radars. But they’re still out there, doing their thing.

So who’s on my 'all time worst' list? I’ve avoided invertebrates, plants and even algae for now, though these are undeniably a massive problem. Chinese mitten crabs are tearing apart the Thames, and our native Gammarus are being beasted by Eurasian amphipods, but that’s a tale for another time.

For now, my eyes are on the following…

Northern snakehead

I reported on Northern snakeheads a while back, explaining about how bad journalism in America was making a hash of dealing with the problem.

Northern snakeheads are hardy, temperature tolerant, air breathing, monster fanged, land-walking nightmares. On the plus side, we’re so aware of the problem that we’ve banned them all over the place. You can’t legally keep one in the UK, and you can’t legally keep one in America. But that said, they’re still crawling out of ponds, and dashing themselves onto angler’s lures at every opportunity.

It would appear that the snakeheads themselves haven’t worked out that they’re banned yet.

Volitans lionfish (picture by Willy Volk, Creative Commons)

Oh, Pterois, you venomous, garish-coloured eating machine, you.

When I started this blog, I was hoping to get us fishkeepers totally off of the hook, absconding us of responsibility for any of these problems, but if an angler stood tapping their foot in front of me, a long finned volitans hanging from the end of his line, I’d have to look sheepishly at my shoes and mutter an apology.

Yup, it’s likely that we did this one. Lionfish somehow found their way from their home aquaria in North America (most probably) and into the coastal seas, where they’ve proceeded to get fat on the native species and spawn like it was going out of fashion.

If they weren’t so venomous, they’d probably fall into that ‘cute’ category I mentioned earlier, and people would want to dive down, feed the things, and embrace them, oblivious to their black-hole appetites, and the fast-diminishing fish stocks in the immediate vicinities.

But they are venomous, and swimmers don’t want to risk a sting, so they’ve got to go. In fact, fishermen have been given a green light to catch as many as they want, through pretty much any means necessary. The locals want them gone.

Ah, what’s the worst that could happen?

Gambusia affinis

A perfect example of how good intentions can go astray, the mosquito fish were introduced all around the world to deal with – you guessed it – 'skeeters'.

In fairness to the fish, it did exactly what it says on the tin. They were introduced into those regions where you were more likely to have mosquito borne malaria than not, and they pigged out on the larvae, reducing the spread of disease quite considerably. They were so good at it, that one Russian town even built a Mosquito fish monument, to commemorate the way these little tykes saved them from almost certain demise.

But there was a price to pay. As good as Gambusia are at eradicating mosquito larvae, they’re equally good at wiping out other native species, often through aggressive outcompeting for food, and sometimes just through aggressiveness for aggressiveness’s sake.

Common goldfish

I’ll tread carefully here, because I know how divisive a point the goldfish thing can be.

Seen a goldfish in a pond recently? As in a natural pond, as opposed to a man-made one? I have. Loads and loads of the things. Everywhere, like a shiny golden pox in our waterways.

Now, I’m sure that the occasional goldfish introduction is accidental. Some eggs carried on the foot of a bird from a domestic pond to a wilderness one, that kind of thing. But I’m also painfully aware of how often I’ve overheard the line ‘oh, I’ll just drop it in the local pond’.

For the best part, a released goldfish is heron food. Now I think of it, I see more gold things going down a Heron’s hatch than silver or drab.

For the worst part, though, goldfish are throbbing disease grenades, waiting to cause a catastrophe on a national scale. As a vector for illness, there are all sorts of things that could obliterate native stocks. SVC, for example, and maybe even KHV. Remember, diseases mutate, which is why the popular press starts hooting like a Howler monkey every time Avian or Swine flu pops up in the world.

It’s a no brainer. Stop putting goldfish into local ponds and rivers. It’s illegal, you’ll get fined until your life savings implode if you get caught, and one day you might be the one introducing 'case zero' into a stream and wiping out English life as we know it. Think '28 Days Later' in a pond. Not so tempting now, huh?

Walking catfish

This one is kind of a shame to me, because I’ve got a real soft spot for Clarias cats. I’m under no illusion that they feel the same way about me. I’m sure that if I was only 30cm/12" tall and swimming in a stream with one, it’d have no qualms about taking my legs off and sucking out my delicious organs.

And that’s really the problem with Clarias. They’ll pretty much eat anything, and they’re just too damn dense to be afraid of any other fish. I’ve seen big cichlids that were scared of these things.

Clarias are out there because they make for a good food fish, especially in regions where protein is scarce. They grow fast, and provide chow for many people who would likely otherwise starve to death.

Alas, when they escape, they escape far and wide. Even dry land doesn’t stop them. Try to confine Clarias to a pool, and they’ll eat everything in it, beat a few fish up, and then just clamber out and go find somewhere else to harass.

I still love them though.

Brown trout (picture by Eric Engbretson for U.S. Fish and Wildlife Service)

I have to concede, I take my hat off to a fish that manages to conquer a whole new hemisphere, and that’s exactly what Brown trout have done.

What started out as a European, North African and West Asian fish has gone global. You and I might be most familiar with seeing a Brown trout nipping at a fly in some lovely, middle-England backwater on a sunny day.

But now it’s a familiar sight in places like Argentina, Ecudaor, Chile, Bolivia, Colombia, India, Sri Lanka, Nepal, Papua New Guinea, Madagascar, Tanzania and Malawi. Even turns out they quite like it down under, relishing the cooler parts of Australia. Oh, and New Zealand. They got so comfy in this last location that not so long back somebody caught what could be the world record Brown trout from New Zealand waters.

So, what’s the damage? Well, trout like to munch on molluscs, crustaceans and small fish – those essential factors in a thriving eco system.

To digress, it’s worth noting that what we should be thinking about right now is the potential of the GM trout hybrids. I’m not going to get into the whole GM debate here (but for what it’s worth, I think it has a viable place in the future of food production, just in case anyone wants to pick a fight some time) but the fact is that these GM versions grow ludicrously fast, not just outcompeting their own, unmodified species, but everything else in the waterways too. If a few of those start getting out into the mix, we have big problems.

Be afraid. Be very afraid.

Nile perch (picture by smudger888, Creative Commons)

Not so contentious for humans, maybe, but a terrible move for local populations of fish, the Nile Perch has proven to be disastrous across East African lakes, especially in Lake Victoria.

Now, it’s a bit shaky to just condemn this introduction outright, if we’re taking an anthropocentric view of things. Since the perch went in, exports and economy, as well as local nutrition, all increased. Though it may be argued that the proliferation of shanty town with limited amenities, and smoking stations to preserve the fish don’t exactly represent a huge leap forward in the quality of life for most.

It’s just that all of this came at the cost of a large number of cichlid species that had, up until that point, been happily living in the same lake. Ill equipped against the ferocious appetite of the perch, they stood little chance, being used to just shooing off rival, similarly sized cichlids.

If you’re not familiar, Nile perch are huge. We’re talking about a 200cm/80" long fish, which can weigh a good 200kg or so. A 15cm/6" Haplochromis doesn’t really have much hope when faced up against something like that.

Bear in mind that Nile perch eggs hatch in just 20 hours, and that a female can produce up to nine million in one go, and you can see how quick the problem can escalate.

On the other hand, the Nile perch is being so vigorously fished, and its own numbers so impacted, that in a roundabout way, the fish is now getting close to being an endangered species. Fishing communities are vanishing, and the lake is slowly moving back to a state that it was in before some British Colonel dropped the things in there in the 1950’s. Just minus all the fish the perch has wiped out in the meantime.

Basically, we cocked that one right up.

Tilapia (picture by Panellet, Creative Commons)

Specifically, I’ve got it in for Mozambique Tilapia. You’ll know these as Oreochromis mossambicus, or Mossies. Sometimes they call them Mozambique mouthbrooders. They’re robust, easily spawned tank wreckers with a headful of attitude.

On the bigger scale of things, they’re food fish, juts like Clarias cats and Nile perch, and they’ve been slung around all over the place to provide fishy protein to starving people (or less hungry fishing corporations at the least).

As usual, they’ve done their thing of getting out of pens, and now dominate a large range. They’re implicated in eating loads of Lepomis sunfish, American pupfish, and even Mullet.

Like Nile perch, they breed like they’re on amphetamines, tolerate pretty much any water condition they meet – including seawater in some cases – and eat everything in sight. These are the industrial skips of the eating world. If they could drum up the extra stomach acid, they’d probably start eating rocks, too, the ravenous little sods.

Common Rudd (picture by George Chernilevsky, Creative Commons)

Who’d have thought it? A common fish we can find in UK ponds and lakes turns out to be one of our most lethal exports.

Actually, we can’t take the blame there. Common Rudd are all over the place in Europe and Asia, but they’ve managed to find themselves a home in Ireland, Madagascar, Spain, Canada, Norway, Tunisia, New Zealand Morocco, and even the old USofA.

From the point of view of being a bug, or a fish, Rudd aren’t going to bother you too much. They’re herbivore leaning and like their greens. And therein lies the problem. They get into a waterway, and like some tubby caterpillar on a juicy lettuce leaf, they munch, and munch, and munch their little vegetarian hearts out until there’s not really anything left for anyone else.

But there’s an extra whammy. Rudd don’t have a taste for everything. They’re quite finicky at times, even, and what’s being noted is that they’ve having an effect on which species of aquatic plants get to thrive, and which don’t. And seeing as plants are at the very base of the food chain, it’s an uphill spiral from there.

Common plec

That big suckermouth in your tank. He’s no problem, right? Think again, those lovable glass kissers are causing issues in tropical regions around the globe. It’s gotten that bad, there are even websites dedicated to their control.

I’m leaving the term ‘Plec’ open, as there are a few. Admittedly, most of those that are an issue are of the Pterygoplichthys lineage, but the occasional other beast turns up.

They’ve even started bothering Florida Manatees, nibbling all over the flanks like some freshwater Remoras. If there was ever a line that didn’t need crossing, it was bothering the Manatees.

But how can Plec be an issue, surely? They’re not meat munchers, and spend most of their time hankering down with a nice, algae festooned rock.

Well, they’re tough, for one. I’m not sure I’m the only aquarist who’s ever had a punch up with a plec and lost. If I can’t beat it in a straight fight, what chance has a small characin or cichlid?

But they’re also prolific nest churners. I’m sure they don’t mean it, but they blunder from place to place, sort of hoofing everything in their way to one side, and whether that’s a handful of pondweed, or a nest of ultra rare Gobies, they just don’t really care. One or two knocking about might not be an issue, but when you start getting hundreds or thousands in a pond, something’s got to give.

Agreed, this is a problem for more tropical regions, but you know what? One even turned up dead not so long ago in a UK canal. That’s right, some halfwit thought it would be a good idea to release one into some local waterway.

*slow clap*

The end of the world?

Of course, there are some folks out there that simply fall back on some archaic, simplified form of Darwinism when greeted by the reality of invasive fish. 'It’s just survival of the fittest,' they’ll explain, oblivious to the notion that introducing a well endowed super-predator to a region rife with passive, ill equipped herbivores undermines millions of years of finely tuned evolution in a single stroke.

There are arguments for invasives bringing new life and diversity to some regions, but these are few, and usually confined to the smallest of areas. For the best part, they are trouble.

The fact remains that this world is a lot more finely balanced than we can hope to comprehend any time soon. Just replacing one link in a long food chain can have disastrous effects.

Imagine if I were to genetically engineer a super, bee-eating sparrow and unleash it up the UK tomorrow. Immune to stings, and popping out 20 chicks at a time, if these swines were to make short thrift of our bee population, then we could say goodbye to flowers, crops, and ultimately everything that depends on them. Life as we know it would change on every fundamental level.

Just because what is happening below the waterline is less obvious to us, we should be very aware of the ramifications to us if we mess things up down there.

Ultimately, we don’t have another planet to race off to if we screw this one up. We have an experimental study group of one, here, and if it goes awry, then it’s pretty much game over.

Keep those fish in their tanks, everyone. I don’t want to start having to memorise a whole new list of 'native' species when I go to the local ponds, if you don’t mind.

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