By Paul Rincon
BBC News Online science staff
Recent outbreaks of piranha attacks on bathers in south-east Brazil may have been caused by the damming of rivers.
They may only bite once, but piranhas can cause serious injury
The link may help to explain other unusual examples of piranha attacks in the country's rivers.
Dams slow the flow of rivers, and may cause an increase in piranha numbers because the fish favour gentle stretches of water for breeding.
Details of the outbreaks appear in the scientific journal Wilderness and Environmental Medicine.
One outbreak occurred in the town of Santa Cruz of Conceicao, whose main river is the Rio Mogi Guacu.
Dammed portions of the stream are popular with tourists and locals who go there to bathe and swim at weekends.
Piranhas belonging to the species Serrasalmus spilopleura - also known as the speckled piranha - had dwelled in the river and its tributaries in small numbers for many years.
No injuries had previously been recorded with bathers or swimmers, according to the authors of the latest report.
The piranhas spawn in submerged vegetation like water hyacinth
But four years ago, injuries due to piranha bites began to be recorded in the town. They reached a peak in the late summer of 2002. Over five weekends in 2002, 38 piranha attacks were recorded.
The rise in attacks has occurred since a dam was built on the river.
Two more outbreaks were recorded at the towns of Itapui and Iacanga, close to dams on the river Tiete in southeastern Brazil.
Over 50 attacks were recorded in total over two weeks at the sites. Neither of the towns previously reported a high frequency of injuries from piranhas.
"It's a direct consequence of damming. When you dam a river, you create ideal conditions for the piranha population to rise," Professor Ivan Sazima, a zoologist at the Universidade Estadual de Campinas in Sao Paulo, Brazil, told BBC News Online.
Professor Sazima says damming rivers may cause as much as a ten-fold increase in piranha numbers.
Bathers often ignore signs like this, which warn of piranhas in the water
Piranhas lay their larvae in submerged or floating waterweeds such as water hyacinth, which collects in slow-moving rivers.
When rivers flood, much of this vegetation is swept away and this probably controlled piranha populations in the past.
The vegetation offers protection for these nests of piranha larvae, and parents often "brood" over, or guard, them.
"Single bites are caused mainly by the people walking and wading in the waters nearby a piranha nest," says Professor Sazima.
The fish usually bite their victims once, ripping a chunk out of the person and leaving a round, crater-shaped wound with accompanying loss of tissue and bleeding.
One of the people bitten during the outbreak at Santa Cruz of Conceicao had to have their toe amputated. But there are even worse tales of aggression by the fish.
The roots of water hyacinth offer protection for juvenile piranhas
Over the years, numerous stories of people being attacked and eaten by ferocious schools of piranhas have surfaced.
The authors of this paper claim there is little scientific evidence to support such behaviour.
They say at least three of the people supposedly killed by schools of piranhas actually died from heart failure or drowning and were only feasted on by the creatures after they expired.
The damming of rivers is becoming increasingly common in south-east Brazil due to a greater need for flood protection in this heavily populated region of the country.