At the end of the rainy season and the beginning of the dry season, thousands of small fish, only about 5 centimeters long, began to climb a nearly vertical rock wall in the Democratic Republic of the Congo. This rock wall is soaked in waterfall mist all year round. These small fish kept climbing, resting, and climbing again, and finally succeeded in conquering the 15-meter-high waterfall.
While the ability of the small-eyed Parakneria thysi to climb waterfalls has been observed before, this is the first time the behavior has been fully documented through photography and videography. "The biggest surprise for me was seeing them accomplish this feat for the first time," said lead author Pacifici Kivele Mutambara from the University of Lubumbashi. Study co-author Emmanuel Frevin explained to the media that there are often genetic differences between fish populations upstream and downstream of the Cascades, even if they are of the same species, but surprisingly, there were "no genetic differences" between the upstream and downstream populations here.
The research team observed the fish migration during 2018, 2019 and 2020 and recorded the first visual evidence of this feat. The researchers found that the fish take advantage of an area around the waterfall that is constantly drenched in water mist - the so-called splash zone - which is the perfect place to climb, allowing the fish to breathe and stay moist while avoiding being swept down by the waterfall's powerful current. The fish clings to the rock wall by relying on its pectoral and pelvic fins, which are covered with tiny single-cell hook-like protrusions (unculi). When these hook-like structures are pressed against wet rocks, the fish can "grasp" the slippery surface and then propel itself vertically upward through a sideways wave motion similar to swimming.
However, progress on the climb was extremely slow. A fish requires 30 to 60 seconds of active movement to climb a certain distance, taking eight to nine breaks of up to 30 minutes each along the way. It would take a total of about nine hours to successfully complete the climb in such dangerous conditions. Fravin observed that this phenomenon seems to be a non-reproductive partial migration behavior. Only smaller fish will choose this most difficult path to migrate upstream, while larger fish (most likely to be in the active reproductive period) avoid joining the ranks of their peers because their size adds a lot of difficulty. This is essentially a risk management strategy.
Because schools of fish separate in this way, the team speculates that genetic changes within the population are likely due to the adventurous, small-eyed Paracnei fish being able to find a larger mate pool on the other side of the waterfall. A 2022 study showed that morphological changes triggered by waterfall climbing actually created new species in another species of Parakneria, an evolutionary behavior that is difficult to observe in many populations and even rarer in underwater species. Researchers said that the rainy season will form a lake or pond-like environment downstream, where fish gather and crowd, and the bottom of the waterfall is more likely to encounter predators, so capable fish are not interested in "taking a chance." Reduced competition for food may also be driving this ambitious migratory mission, Frevin added. The research has been published in the journal Scientific Reports.