The Madeira River, the largest tributary of the Amazon, is the river with the greatest diversity of fish species in the world, according to a recent study by the Federal University of Rondônia. During two years of research, almost 800 different species were identified, of which 40 were previously unknown.

“We were surprised by the fact that despite the large quantity of species, few of them are endemic,” Carolina Doria, the coordinator of the university’s Ichthyology and Fisheries Laboratory, told Tierramérica.

Other major Brazilian rivers, such as the Xingú, Tapajós and Tocantins, are home to around 500 species each, but have never been the focus of a research study like this one, Doria added.

The study was carried out as part of an environmental assessment regarding the Santo Antônio hydroelectric dam and power plant, which is being built on a stretch of the Madeira River.