Are Piranhas Dangerous to Humans? Myths vs Real Bites

How often do piranhas bite people?

Millions of people live, fish, wash and swim in rivers with piranhas, and most never get bitten. When bites do happen, they tend to come in clusters at a particular lake or beach over a few weeks, then stop.

Several outbreaks have been well reported in South America:

• In the first half of 2007, about 190 bites were reported at an artificial lake near Palmas, Brazil, formed by damming the Tocantins River. All were single bites to the feet.
• In 2009, a series of bites in the Tietê River in São Paulo state caused minor injuries to 15 people.
• In 2011, around 100 bathers at José de Freitas in Piauí, Brazil, were treated for bites to their toes or heels.
• On 25 December 2013, more than 70 bathers in the Paraná River at Rosario, Argentina, were bitten on the hands or feet by piranhas known locally as palometas.

These numbers sound high, but the injuries were mostly small and the outbreaks happened in places with large crowds of swimmers. A review of bites in southeastern Brazil found that the typical case is one bite per person, generally linked to a fish defending its eggs or young.

Why do piranhas bite?

Piranhas do not see people as food. Most bites come from a few predictable situations.

• Guarding nests. Many piranhas breed in shallow, weedy water and guard their nests. A foot that comes too close may be bitten once and released, much as an angry bird might peck an intruder.
• Crowded, low water. In the dry season, rivers shrink and piranhas become packed into lakes and pools with little food. Hungry, crowded fish are more likely to bite.
• Commotion and food. Splashing, struggling fish on lines, bait and scraps from cleaning fish attract piranhas. This is one reason children, who splash more, are bitten more often than adults.
• Reservoirs. Dams create calm, shallow lakes that suit some piranhas, especially smaller Serrasalmus species, and bring swimmers and fish together at the same beaches.

Can piranhas kill a human?

Fatal piranha attacks on healthy swimmers are extremely rare. Researchers who reviewed the evidence in Brazil found very few documented cases of people being eaten by piranhas, and in several of them the person had died of other causes, such as drowning, before the fish fed.

A handful of deaths have been blamed on piranhas in news reports, mostly involving people who could not swim, were drunk or had fallen into the water from a boat or dock. Even in these cases it is often unclear whether piranhas caused the death or fed on the body afterward. Compared with drowning, boating accidents or disease-carrying insects, piranhas are a very small risk, and they do not appear among the deadliest animals to humans.

The Roosevelt story and the feeding-frenzy myth

Much of the piranha’s fearsome image dates from 1914, when former US president Theodore Roosevelt published Through the Brazilian Wilderness. During his expedition he heard and repeated dramatic stories, including one of a cow stripped to the bone within minutes, and he called piranhas the most ferocious fish in the world. According to later accounts, the cow episode was staged: local fishers had fenced off part of a river and left the fish without food for days before driving the animal in.

Horror films such as Piranha (1978) and Piranha 3D (2010) later turned the fish into swarming man-eaters. Real behavior is very different. Studies of the red-bellied piranha in the Amazon found that its shoals form mainly for safety from predators such as caimans, river dolphins and birds, not for hunting together. Piranhas are often nervous, cautious fish that scatter when disturbed.

Group feeding frenzies do happen, but mostly on dead animals, on fish trapped in nets or on animals already weak or injured. That scavenging habit, closer to that of a vulture than a wolf, is the real basis of the legends. See What do piranhas eat? for the full menu.

Which piranhas bite most often?

The red-bellied piranha has the biggest reputation, partly because it is the most widespread and best known species. Its bite is strong and sharp, and it can take a neat piece of flesh. The black piranha (Serrasalmus rhombeus) has the strongest bite measured in any bony fish, more than 320 newtons.

Many bite outbreaks in Brazilian reservoirs, though, have involved smaller Serrasalmus piranhas, such as the speckled piranha, rather than the red-bellied piranha. These fish thrive in the still water behind dams, nest in the shallows and defend their young boldly. In Argentina and Paraguay, bites at river beaches are often blamed on palometas, a local name used for several piranha species.

How to stay safe in piranha rivers

The practical rules are simple and mostly about avoiding the situations above:

• Follow local advice and warning signs, and use protected swimming areas or barriers where they exist.
• Avoid swimming near fishing activity, docks where fish are cleaned or places where food scraps are thrown into the water.
• Stay out of shallow, weedy water during the breeding season, when piranhas guard nests.
• Be extra careful in crowded dry-season pools and lakes.
• Do not enter the water with an open wound, and avoid heavy splashing.

Any serious bite, especially heavy bleeding, needs professional care. Seek emergency medical help immediately, using the local emergency number; in the US that is 911, in the EU 112 and in the UK 999.