Thousands of bumblebee catfish climb a waterfall in never-before-seen footage

Previously unseen footage has captured thousands of catfish climbing a waterfall in southern Brazil – the first detailed observation of the species' migration.

The bumblebee catfish (Rhyacoglanis paranesis) gets its name from its black spots on its orange body and its tiny size, measuring just 89.2 mm (3.5 in) long. These little jumping fish have been filmed scrambling over slippery rocks 1 to 4 metres (3.3 to 13.1 ft) below a waterfall on the Aquidauana River.

Scientists know next to nothing about the behavior of this rare fish species. But in 2024, members of the environmental military police of the Brazilian state of Mato Grosso do Sul noticed the phenomenon and called in a team of scientists to document it. The results of the study were published on August 8 in the Journal of Fish Biology.

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The genus Rhyacoglanis includes nine species of small catfish found in the Amazon, Orinoco, and La Plata river basins. “[This genus] is rare in nature and even rarer to observe,” Manoela Marinho, a biologist at the Federal University of Mato Grosso do Sul in Brazil and the first author of the new study, told Live Science in an email. “This is why the large aggregation we found is so remarkable, as there was no previous indication that this behavior was part of the natural history of this genus.”

In the new paper, the scientists report that the bumblebee catfish only rose towards the end of the day, finding shade under rocks on hot days. But around 6 p.m. local time, as the sun was setting, thousands of the catfish began to rise.

Starting from small pools connected to the river, the catfish jumped onto steep and sheer cliffs. On flat rocks there were so many of them that they climbed on top of each other. In their frantic climbing, some catfish even climbed onto the scientists' plastic bucket.

The catfish rose by holding its fins straight and using side-to-side bobbing and tail flicking to propel itself forward. The scientists also suggest that the fish may have developed a suction mechanism to attach itself to rocks as it rose, creating a small cavity between its body and the rock — a strategy shared by some other fast-moving river fish, such as brook loaches and Andean climbing catfish.

The research team doesn't know why the catfish were climbing the falls, but suspects they were migrating upstream to breed. The climbing group included males and females, most of which were adults. The behavior also began in November, the beginning of the rainy season, after a particularly dry winter.