In November 2024, members of Brazil’s environmental military police and scientists observed a “massive aggregation” of small catfish making their way up waterfalls in Brazil. These first-of-their-kind observations of the rare bumblebee catfish likely migrating upstream to spawn were published in a recent paper.

The researchers’ video footage shows thousands of bumblebee catfish (Rhyacoglanis paranensis), a small, colorful fish with blotches on its body and fins, aggregating at small pools at the base of waterfalls on the Aquidauana River in Mato Grosso do Sul state, then climbing up vertical rock faces and inching their way up. In more horizontal parts of the waterfall, the aggregations were so huge that individuals were seen climbing over each other.

Rhyacoglanis species are considered rare and scientists don’t know much about their biology and behavior. It doesn’t help that the catfish live in rapidly flowing bodies of waters, making the latest observations of their aggregations and waterfall-climbing valuable.

 Scientists have previously documented a few freshwater fish species from tropical South and Central America climbing waterfalls, but this is the first time that a member of the Pseudopimelodidae family, which includes bumblebee and dwarf marbled catfishes, has been observed doing so.

The researchers aren’t entirely sure why the bumblebee catfish were climbing the waterfalls in November. However, based on the presence of fully mature males and females within the aggregations, and the timing of the migration at the start of the rain season, the researchers say the fish were most likely heading upstream to spawn.

 “It happened right after a long and severe drought in the region, and the sudden rise in water levels seems to have triggered the species to spawn, bringing together a massive concentration of fish that had never been seen before,” Manoela Marinho, study lead author from the Federal University of Mato Grosso do Sul, told LiveScience.

While the details of how the bumblebee catfish climbs waterfalls is still unknown, the researchers observed that it keeps its paired fins wide open, likely using them as anchors as it pushes its body forward. The authors speculate that the fish might be using its belly to create a small suction effect to stick to the vertical rocks.

Among the shoals of bumblebee catfish, the researchers also observed a few other small fish species like Characidium zebra, Hypostomus khimaera and Ancistrus spp. climbing the waterfalls for the first time.

All of these field observations provide valuable insights into the migratory behavior of smaller fish species in Brazilian rivers that remain poorly understood, the researchers write.  The findings also spotlight the potential impacts of river damming on the movement patterns of small fish species, the authors add, since dams change the natural flow of water, potentially disrupting the fish’s preferred habitats

Banner image of Rhyacoglanis paranensis migrating upstream in Mato Grosso do Sul, Brazil, courtesy of Marinho et al., 2025.