Presence of arapaima in São Paulo rivers becomes more frequent and worries scientists

Natural from the Amazon, the arapaima (Arapaima gigas) is one of the largest freshwater fish on the planet, reaching over 3 meters in length and weighing around 200 kilograms. In recent years, fishermen have recorded the presence of this “giant” with increasing frequency in the Rio Grande, a body of water belonging to the upper Paraná River basin that bathes the states of São Paulo and Minas Gerais.

The introduction of a non-native species that feeds mainly on other aquatic animals aroused the concern of researchers from Unesp (Universidade Estadual Paulista) about the impacts on ecological relationships and on the local fish population, stimulating the creation of research projects that investigate the consequences of the presence of this Amazonian predator in waters of the Southeast region.

The researchers report that, so far, the arapaima has only been found in a stretch of the Rio Grande. More precisely, between the dams of the Marimbondo hydroelectric plant and the Água Vermelha hydroelectric plant, a segment of approximately 120 kilometers in which the Grande River divides the territories of Minas Gerais and São Paulo.

Lilian Casatti, a professor at Unesp’s Institute of Biosciences, Letters and Exact Sciences, in São José do Rio Preto, explains that the two dams, built in the 1970s to produce electricity, cause a decrease in the river’s original current. This forms an ecosystem very similar to the natural habitat of the arapaima in the Amazon, where it is usually found in lakes of water formed by the river.

Large fish such as arapaima are highly valued by producers in aquaculture and are often produced outside their original watershed. The escape of these animals from production tanks is the main cause of introduction of non-native species into rivers. Accidental escapes, such as the one that occurred in the Rio Grande, have already resulted in the introduction of arapaima in aquatic systems in the North, Northeast and Southeast regions of Brazil.

Invasive species?

Casatti explains that, once the presence of the arapaima in the waters of the Rio Grande has already been established, the key at this point is to measure the real impact of the species on the new habitat. “Today, in the Rio Grande, the arapaima is classified as a non-native fish. In order for it to be considered an invasive species, we need to research and prove that its presence causes damage to some species or ecological process”, he says. “We are talking about a stretch of the Rio Grande that has already been significantly altered, where there are dams, silting processes, loss of habitat, pesticide residues and the presence of several non-native species”, he argues.

The teacher makes a point of stressing that the fact that the arapaima is apparently feeding on species that are also not native should not be seen as a license for its introduction to the site. “When a non-native fish is introduced, it does not come alone. It comes with a pool of parasites that are present in this organism and can also cause impacts that must be measured. That is why it is important to develop research on the subject”, he points out.

Lidiane Franceschini, a researcher at the Unesp campus in Ilha Solteira, is responsible for a project that will investigate the effects of the arrival of the arapaima to the Rio Grande. “Knowledge about the effects of the pirarucu invasion in the receiving basin is still incipient, information on biological aspects of the species and factors that may influence the success of its establishment in the new area are essential to support measures of local management of the species”, explains the researcher.

A postdoctoral project, which will have the collaboration of researchers from UEL (State University of Londrina), UFPR (Federal University of Paraná) and University of Valencia, in Spain, was approved and will receive funding from Fapesp.

Franceschini explains that the arapaima is a predatory species with a generalist or omnivorous carnivore profile, which usually occupies the top of the food chain. In the absence of natural predators or competing species, as may be the case with the Rio Grande, arapaima can cause local extinction of fish and invertebrate species and compete for environmental resources with other species, in addition to introducing new parasites into native species. “The presence of pirarucu can cause a decrease in species that are important for regional fisheries,” he says.

amazon hake

The situation would not be unprecedented. Since 2020, doctoral student Aymar Orlandi Neto has been evaluating the impacts caused by the introduction of Amazonian hake, also known as river croaker (plagiarism squamosissimus), in the Jaguari River reservoir. From the project, also supported by Fapesp, it was already possible to verify the success in the adaptation, becoming the most abundant predator and to which the reduction of the richness and diversity of native species in the place is attributed.

Franceschini explains that similar impacts have already been recorded in the scientific literature regarding the introduction of peacock bass (cichla ocellaris).

Once introduced into the aquatic environment, reversing this situation is almost impossible, experts point out. “Currently, the main measure to contain these introduced species is the release of professional sport and artisanal fishing throughout the year, an insufficient measure to contain such biological invasions”, laments the researcher.

Faced with the difficulty of reversing the introduction, Cassatti calls attention to the need for authorities, residents and fishermen to make an effort so that the arapaima does not escape again to other water bodies in the region. Close to this stretch of the Rio Grande, explains the teacher, is the Turvo River, one of the few in the state of São Paulo that still does not have dams along its course to retain water. “The little that still exists of native fish in this basin is surviving due to the absence of these dams and the existence of marginal lakes in the Turvo River that serve as true nurseries for the native species”, she highlights.

The professor also defends that aquaculturists cultivate only species of fish native to the region where the breeding ponds are located, in order to avoid escapes and new unwanted introductions of species. “Brazil is one of the most diverse countries on the planet and we are home to more than 5,000 species of fish. There is no need to create species that are not native to the basin in which production is located”, she highlights.

*With information from the Journal of Unesp