Seventeen years ago, Auguste Chocha Manda, a researcher at the Université de Lubumbashi in the Democratic Republic of Congo, traveled to the Luvilombo waterfall in the south of the country where he saw something remarkable.
Thousands of tiny fish — a species called shellear (Parakneria thysi) — were climbing up the 50-foot rockface behind the waterfall.
"If you would ask a regular person, do you think fish can climb falls, most of them will tell you: you are crazy," says Emmanuel Vreven, an ichthyologist at the Royal Museum for Central Africa in Belgium. "Well, it exists, it is out there."
The behavior has been documented in fish in other parts of the world, but Vreven says never in Africa. Manda had filmed the phenomenon 17 years ago but he ended up losing the footage. So beyond his anecdotal observation, there was no hard evidence. And Pacifique Kiwele Mutambala, then a Master's student and now a PhD student at the Université de Lubumbashi, was determined to go get some.
In a new paper published in the journal Scientific Reports, Mutambala, Vreven and their colleagues (including Manda, who's credited posthumously) describe their observations of the shellear in detail, explaining the unique traits that enable the fish — which are about the size of a fat french fry — to climb up a rock face.
"It really reinforced to me just how cool fish are, right?," says Steven Cooke, a fish ecologist at Carleton University in Ottawa who wasn't involved in the research.
"The scale is really impressive," adds Cooke. "That would be like a salmon trying to make it over Niagara Falls or climb the CN Tower."
Chasing waterfalls
To see whether the shellears were actually climbing, Mutambala spent a few rainy seasons at the raucous Luvilombo Falls in search of the fish.
"I try to go close to the falls and observe very clearly what fishes can do," he says. And sure enough, he soon saw thousands of them shimmying up the vertical rock surface, seemingly defying gravity.
"Ah, the first time I was very excited," says Mutambala.
Not all the fish made the climb — only the ones that were a couple inches long or less. Above that, Vreven believes "they become too heavy and so the animal cannot bring its own weight to the top of the falls."
In addition, the fish that were scaling the falls didn't do so in the center where the water flow was strongest. Rather, says Vreven, "the fish are climbing in the splash zone, so they are moving upwards at the sides of the falls, but not in the full current."
Even so, Mutambala became drenched while filming the fish. "I was totally wet," he says with a laugh.
How fish climb
One of the big questions the researchers had was how the shellears manage to climb. Back in the lab, they reviewed the fish's vertical movements in the video footage and ran CT scans to examine their anatomy and work out how they made their way up the rock face.
They saw that the fish support themselves with their rear pelvic fins. And their front pectoral fins have an array of tiny hooks that function kind of like Velcro, which they use to grip the rock. The fish also have a hefty arch of bone called the pectoral girdle that supports the musculature needed to make the climb.
And, says Vreven, "you see also the lateral undulations of the fish very fast. It's as if they are swimming vertically," wriggling their way gradually upwards. The movement is called a power burst.
"When they arrive at a flat surface," he says, "they will pause for a longer time. When they recover the energy, they can begin another step of the climbing. Most of the time is in fact resting."
Sometimes the shellears cling to an overhang, upside down. Some of the fish fall down and have to begin again. The entire ascent takes close to ten hours. "It's an enormous effort," says Vreven.
A vertical migration
Mutambala explains that studying the shellears isn't purely a quest of curiosity — the findings have implications for biodiversity and conservation of the region.
The scientists think that the shellears scale the falls as part of a migration upstream. If this is the case, then cutting off the water supply to this waterfall — to fill a dam or for irrigation, which happens — could harm the fish. "Of course, if there is no water," adds Vreven, "there are no fish."
The scale of the shellear migration may pale in comparison to something like that of the wildebeest, but Cooke explains that it's just as important.
"Migratory fish are several times more at risk of endangerment or extinction than fish that don't migrate," he says, meaning it's important to protect the habitat across the entire range of the species, waterfalls and all.
As for why the fish climb the waterfall, more research is required. Maybe there's better food up there or less predation. Either way, it keeps the upstream and downstream populations of shellears connected and the researchers say it's the first time that the behavior has been formally documented on the African continent.
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