For more than 50 years, people have been telling strange stories about tiny fish called shellfish.Parachneria tissi) Climb the waterfall. Researchers have now announced that they have filmed this behavior for the first time. Their discovery is scientific report.
Pacific Kiwele Mutambara, researcher at the University of Australia Lubumbashi, Emmanuel Vreven, an ichthyologist at Belgium’s Royal Museum of Central Africa, and colleagues recorded thousands of small fish climbing Lubilombo Falls in Upemba National Park in the Democratic Republic of the Congo in 2018 and 2020.
“The ability of shellfish to climb waterfalls has been anecdotally known for more than 50 years,” the study authors wrote. “This behavior was documented here for the first time with supporting film and photographic evidence.”
Fish chased waterfalls at the end of the rainy season (April-May). During years of high rainfall, they used their pectoral fins, supported by their pelvic fins, to pull up the vertical walls of the spray zone.
Underneath the fish’s fins, there are “tiny, single-celled, hook-like projections” that help the fish grip walls. “They propel themselves vertically by lateral movements of the posterior part of their bodies, just as they do when swimming,” the authors write. I mean, they wriggled their butts off.
The shellfish, which is less than 5 centimeters long, does not move quickly and it took about 10 hours to climb the 15 meter high waterfall.
As you can imagine, this little fish took many breaks during this difficult journey. I would rest for less than a minute during certain sections of the climb, and sometimes I would take longer breaks when I held on to a vertical rock wall and reached a horizontal ledge. This is usually a 2-15 minute pause.
“As a result, large numbers of fish will congregate on those ledges before entering the next climbing phase,” the authors write. “This seems to indicate that these upward movements require more energy. As observed, the specimens require longer rest periods to recover from their efforts.”
On average, it takes a fish about 9 hours and 45 minutes to climb the waterfall. This includes nine 15-minute climbs, 30-minute short breaks, and 1-hour breaks.
Despite taking many breaks, the climb is dangerous. “In fact, some specimens may fall suddenly, for example when suddenly hit by a stream of water,” the authors write. “Such falls are also more likely if the fish moves upside down trying to avoid an overhanging cliff.”
So why do the animals take this risk? The authors suggest that the fish may be trying to return to their upstream habitat after being swept downstream by floods, or they may be trying to avoid competition for food or predators.
Curiously, this climbing behavior has been previously observed in other fish species, such as catfish (Lycogranis paranensis), but it is not well documented.
The Lubilombo River is affected by human threats, including illegal fishing using mosquito nets (mesh small enough to easily catch small fish) and diversion of water flow to irrigate crops during the dry season. Although this happens only occasionally, it “undermines the continuity of the river’s ecosystem” and is a significant threat, the authors write.
The scientists hope their study highlights the importance of protecting this ecosystem, stating that “more focused attention to these waterfalls and identifying them as natural monuments and/or ecosystems of national interest may make this possible.”
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