Under the cover of darkness, thousands of tiny bumblebee catfish were caught orange finned by police, wriggling their way up a waterfall in Brazil.
The boulders of the Aquidauana River waterfall are up to 4 meters high, yet somehow these catfish (Rhyacoglanis paranensis), which are less than 4 centimeters (1.6 inches) long, manage to scale them like mini Olympic rock climbers.
Researchers from the Federal University of Mato Grosso do Sul observed the black-striped orange fish for 20 hours after the police alerted them to this mass migration event, occurring right near their department, in 2024.
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"On more flattened and horizontal rocks, aggregation was so massive that specimens were seen above each other, climbing the large cluster of fish," zoologist Manoela Marinho and colleagues write in their paper.
"The migratory movements of smaller (fish) species remain largely unknown, mainly because these events occur briefly, under specific environmental conditions, and are challenging to detect in highly diverse ecosystems."
They noted the fish spread their fins wide as an anchor, then wriggled to push themselves upwards.
Marinho and team suspect the fish are also aided by a suction mechanism. The water bumblebees were so keen to get upwards, that they even scaled the bucket the researchers were using near the waterfall.
Sampling suggests the climbers were mostly mature adults, so they're likely swimming upstream to breed just as salmon are famous for. The individuals more closely examined had empty stomachs, supporting this theory, as other fish migrating in freshwater often don't eat during their arduous journeys either.
"These findings highlight the importance of field observations in understanding small migratory fishes' ecological role and conservation needs, particularly in the context of potential threats posed by habitat fragmentation and river damming," Marinho and team write.
This research was published in the Journal of Fish Biology.