In recent years, new residents began to be seen in the tributaries of the Paraná and Tietê rivers, in the interior of São Paulo. These are freshwater rays, more specifically species of the genus potamotrygon.
With their disk shape and body covered with black and yellow polka dots that resemble a leopard print, these cartilaginous fish, despite not being exotic, are considered invasive — that is, they did not originally occur there. This presence has sparked an alert in experts.
Rays (or stingrays, the two are used equally for these fish that, together with sharks, form elasmobranchs) have more than 600 species in the world, of which about 30 are freshwater and endemic to South America.
Despite its well-demarcated presence in the Amazon region and the Pantanal, its southern distribution was limited below the Paraguai river basin, common only in the Rio Prata basin in Argentina.
With the formation of dams on the Paraná River, especially for the construction of the Itaipu Power Plant, leading to the transposition of the natural barrier of Sete Quedas, these animals managed to climb the river and are now spreading over almost the entire territory of the interior of the states of São Paulo and Paraná.
In about 30 years, the fish have already advanced 140 kilometers above the Tietê River, an extremely fast speed for animals that are about 1 meter long and usually stay on the banks of rivers, without traveling long distances.
The problem, according to experts, is that rays are venomous animals and their presence in new places has led to an increase in accidents.
“The first time I learned of the presence of stingrays in Paraná was 20 years ago, at the point of Paranapanema [afluente do rio Paraná], near the municipality of Teodoro Sampaio, in São Paulo. In that time, they rose at an unbelievable speed, and now there are data on their presence in the lower Tietê River, in cities like Três Lagoas and Itapura and, more recently, Araçatuba and Buritama”, explains the professor at the Faculty of Medicine of the University State of São Paulo (Unesp) of Botucatu, Vidal Haddad Júnior.
Although they are considered not very aggressive, accidents with stingrays arise when they are stepped on. At that moment, to defend themselves, they use their tails as if it were a whip. They also have a venom-filled stinger below their tail and usually hit mainly feet and legs.
In her doctoral research at Unesp, nurse Isleide Moreira documented the invasion of these animals in the interior of the state of São Paulo, arriving in areas known as “prainhas”, much sought after by local populations as an option for leisure and tourism. The longest record she found in her analysis was in Buritama (531 km from the capital).
According to biologist Patricia Charvet, from the Federal University of Ceará, stingrays from the Potamotrygonidae family (of which the species found in Tietê are part) are already recognized in the Amazon region as one of the main causes of removal due to health reasons for fishermen.
“It is important to emphasize that the sting is a defense mechanism for these animals. They generally live buried in a sandy environment and not too deep, and it is in situations like this that the accident can occur, or when handling after fishing”, he says. .
Charvet points out that riverside communities in the Amazon and also in Argentina already have an “adapted” behavior for stingrays: locals tend to enter rivers by “dragging” their feet on the bottom, without lifting them, because at the slightest contact, stingrays tend to flee.
As it is a type of accident that is not usually registered, there is a lack of official data in Brazil on injuries caused by stingrays. According to research by the State University of Amazonas, the National Institute for Research in the Amazon and the Health Foundation of the State of Amazonas, published in 2018, the incidence of stingray accidents in that region is 1.7 per 100,000 people. .
According to Haddad, the rapid expansion of stingrays in Tietê is also related to their biology. “They are viviparous fish, that is, they give birth to already formed young. As they have no natural predators, they spread very quickly. I managed to collect more than 50 animals in the same morning”, he says.
The ichthyologist (specialist in the study of fish) and PhD in zoology from USP Thiago Loboda has already carried out several studies with rays and explains that human action in the Tietê and Paraná rivers facilitated a natural colonization of the animal.
To reduce the occurrence of accidents, the biologist gives some tips. “The moment to be careful is when entering the river or getting off a boat, because that’s when they can sting. Entering running, splashing water everywhere, is not a good thing, because they will be scared”, he explains.
One of the factors that prove how well these species have adapted to the “new environments” is that genetic studies of some populations have shown that they are stable, he adds.
The biologist and coordinator of the Tubarões e Rayas de Noronha project, Bianca Rangel, points out that there is a lot of concern about accidents that can occur at sea with elasmobranchs, but little concern about cases in fresh water.
“Accidents with stingrays occur daily in all rivers, especially in the Amazon region. [por ser a maior área de distribuição das espécies]but people don’t care, they treat it as if it were a common accident”, he says.
She says that the fact that the animals are there should now be seen as something natural and should not be controlled. “It’s that old environmentalist fight, when the projects are made there is no long-term assessment of the effects they can generate [na natureza]. Now that it’s done, we have no way of controlling, but informing about the way of life of these animals,” he says.
According to Haddad, environmental education and awareness programs are already being carried out with fishermen from Pereira Barreto, on the lower Tietê. “For fishermen, it’s an animal that they didn’t know before, and it can seem strange. The care to raise awareness, show how to react, what should be done, this is a way to reduce accidents”, he says.
