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A villager holds up a pacu fish that was caught in the Beni River.

A villager holds up a pacu fish that was caught in Bolivia.

Photograph by Joel Sartore, National Geographic

Ker Than

for National Geographic

Published August 13, 2013

Danish male swimmers, you can go back into the water now.

The recent capture of a fish species off the coast of Denmark that is rumored to sometimes mistake human male genitalia for nuts sparked fears of "testicle-eating fish" invading Scandinavian waters—and even led experts to caution men to keep their swimsuits well tied.

But the scientists say that their warning wasn't intended to be taken seriously, and that it's highly unlikely that swimmers will encounter a pacu, as the fish is called.

"All we said last week (with a smile) was that male swimmers should keep their pants on in case there are more pacus out there in our cold Baltic waters," Peter Rask Møller, a fish expert at Denmark's University of Copenhagen, said in an email.

"Its teeth and powerful bite can for sure be dangerous, but to meet one here and [have it bite you] is highly unlikely, of course."

Lars Skou Olsen, curator of Copenhagen's Blue Planet Aquarium, said that the pacu's predilection for private parts is overblown and probably not even true.

"I think it's just a rumor," Olsen said. "There's no need for swimmers to worry at all. They will be lucky if they see [a pacu]."

The pacu that was found is the only one that has ever been caught in the wild in Scandinavian waters. It was captured on August 4 in the strait of Oresund, which separates Denmark and Sweden, by fisherman Einar Lindgreen, who spotted the red-bellied, big-toothed fish among the eels and perch in his net.

From the creature's size—it was around 8 inches (20 centimeters) long—Olsen thinks it was young, only about a year old.

We asked Olsen, whose aquarium features pacus, to talk more about this attention-grabbing fish.

What does a pacu look like?

Pacus look like piranhas when they're not very old. This individual that was found was young, and it had a red belly and a silver back.

What part of the world does the pacu come from?

They come originally from South America, where they live in the rivers of the Amazonian area.

No one ever thought that a pacu could live in the wild in Denmark, first of all because the water temperature is too low. And no one ever in their minds imagined that it could live in the sea. It's a freshwater fish. It lives in the Amazon River where there's no salt at all. That this fish thrived in the seas is a mystery.

Did the fish that was caught appear sickly?

No, it looked like it was doing really well. It was alive when they caught it, and it was still kicking. But I think in a month or so, it would have died due to the low temperature that will come very soon.

How long do pacus live?

In our aquarium, we have had some pacu fish live for nearly 20 years. They grow very big when they're old, around 20 kilos (44 pounds).

You've said that you don't think the pacu that was caught came from your aquarium because it's too small, and because the facility has fish filters in place to prevent escapes. Where do you think the pacu came from?

My theory is that during the holiday, someone who had the fish in an aquarium at home set it free in the ocean.

Is the sale of pacus legal in Denmark?

Yes, they're sold legally. Only poison fish are banned in Denmark.

Do pacus make good pets?

Yeah, they're excellent pets. The only problem is they grow too big for people's home aquariums. Many people mistake the fish for piranhas, and many pet stores sell them as piranhas.

Often we have people calling us saying they have some piranhas that have grown too big for their tanks. And when you ask them how big the fish are, you know they can't be piranhas.

What do pacus eat?

They eat lots of things, but mainly vegetables, which they love. In the wild, they eat nuts falling down into the water. They have very powerful jaws that they use to crack the nuts. That's no problem for them. They also eat fish. In our aquarium, they eat fish as well as greens.

How many pacus do you have in your aquarium? And how big are they?

We have six to eight pacus. When we buy them, they're 5 to 10 centimeters (2 to 4 inches) long, and it takes them five to six years before they're around 20 kilos (44 pounds) and about a meter (3 feet) long.

They're very powerful. We moved from our old aquarium earlier this year, and we moved some big pacus, and when you get them in the net, they will drag you around in the tank until you get them up.

Are piranhas and pacus related?

Yes, they're related. In fact, when the pacu are young, they hide between schools of piranhas in the wild for protection [from their predators]. They're secure among the sharp-toothed piranhas. That's why they have the same colors as piranhas. When they grow older, they run away from the piranha schools and live on their own.

Do you think there could be more pacus out in the ocean?

Yes. Normally, when you buy pacu fish, you buy eight to ten. One pacu is a lonely pacu, so people typically buy more than one. So my guess is there could be more out there.

But you don't think swimmers need to worry about them?

No. They fear humans and will try to escape.

Follow Ker Than on Twitter.

13 comments
Richmond Acosta
Richmond Acosta

"Are piranhas and pacus related?"

"Yes, they're related."

I remember, about ten years ago, my teacher killed her Pacu fish after seeing a news about piranhas. She was like, "I thought they were pacus but then I saw the news and they were in fact piranhas."

Sophie Yang
Sophie Yang

Ha, very interesting article, I have thought the fish is piranhas when I see the picture because they are alike in fact. I have to say the name of this article is much attractive so that we guys are curious to glance over.

Brigitte Meier
Brigitte Meier

EVer since when are fish eating nuts? Where are there any nuts growing in the sea? 

Jeff Toback
Jeff Toback

Well, when I read they sometimes mistake human male genitalia for nuts, I almost fell out of my chair laughing.

Wilson Zorn
Wilson Zorn

Thanks for an article that has a great, balanced headline and material:  the rest of the media is still either now going the other extreme and using headlines calling the phenomenon a "myth" or still going with the other extreme of "testicle-eating fish are out to get you."

Richmond Acosta
Richmond Acosta

@Brigitte Meier Pacu fish are actually known to help spread seeds of trees in the amazon during the annual flooding when the water reach out inland where the trees grow.

Shea Sandy
Shea Sandy

@Brigitte Meier They eat the nuts that fall from the nut trees that grow along the Amazon River where they are from- in SOUTH AMERICA. Did you read the article? They are not salt water fish that is why everyone is surprised this fish survived in the salty sea as long as it did. Someone had it as a pet and released it into the sea because it grew too big for their aquarium. Rather cruel to do to a fresh water fish. It was meant as a joke to tell men to watch out for their "nuts" but no one got it and everyone freaked out as usual. Just read the article again. Peace.

Ker Than
Ker Than

@Brigitte Meier The Pacu's native habitat is freshwater rivers, and they eat nuts that fall into the water.

miss write
miss write

@Theodore Christian Residilla  I do not think sucking up to the Pacu will save your jewels in the water...they will still think they are yummy. bwhahaha!

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