- A group of scientists paddled the length of Zambia’s Kafue River to document ecological pressures, including invasive species, habitat changes and human encroachment.
- Australian red-clawed crayfish (Cherax quadricarinatus) have infested the river, outcompeting native species, disrupting fisheries and altering fishing practices. The crayfish invasion spans nearly the entire 1,600-kilometer (995-mile) river, traced to an original introduction in 2001.
- Overgrazing and invasive plants like the giant sensitive bush are transforming some riparian zones, threatening biodiversity, including endemic species like the Kafue lechwe (Kobus leche kafuensis).
- Researchers with The Wilderness Project’s Great Spine of Africa project are using standardized field methods to monitor river health and the spread of invasive species to inform future conservation efforts.
KAFUE, Zambia — “Three cattle on the left, 20 elephants on the right,” shouts the expedition leader from the bow seat of the first of five canoes snaking along a stretch of the Kafue River in central Zambia.
The cattle are grazing near one of the many small farms and settlements that dot the river’s southern bank. On the northern bank lies the Lunga-Luswishi Game Management Area (GMA). Originally designated as buffer zones between populated areas and Kafue National Park, at least half of the area within the nine GMAs has now been settled and converted to farmland and homesteads, according to park officials.
At the front of the lead canoe sits expedition leader and biologist Mike Ross, binoculars around his neck. He calls out observations, which are recorded by fellow scientists Lauren Searle and Katongo Kampamba, who briefly lower their paddles to log the data while their respective boat captains, brothers Vincent and Earnest Ifunga, continue to steer and propel the second and third canoes downstream.
The team records fishing camps, nets and traps along the banks; farming activity and people traveling on foot or bicycle; boats, water pumps, buildings, invasive plants — and wildlife: mammals, reptiles and birds.
Some birds signal a healthy ecosystem. Within the park, grey crowned cranes (Balearica regulorum) dance on a grassy island, wind ruffling their brown wing feathers and yellow crests. Giant kingfishers (Megaceryle maxima) flash beneath overhanging branches. White-fronted bee-eaters (Merops bullockoides) rise in flocks above steep riverbanks, where in summer they’ll tunnel their nests.
Others, like the cloud of white egrets bursting from a herd of cattle inside the GMA, signal something else.
“Cattle cause overgrazing that can open up areas that will allow invasive plants to come in,” says Searle, a conservation ecologist.
One such invader is the giant sensitive plant (Mimosa pigra), a South American shrub whose tiny leaflets fold at a touch. With its seeds floating down the river, or its furry pods clinging to the hides of wild animals or cattle, the plant is spreading along large sections of the river.
The plant’s invasion is affecting both people and wildlife, says Kampamba, a master’s student in biology at Zambia’s Copperbelt University and the expedition’s research assistant. In the Kafue Flats, a vast wetland downstream of the GMA and outside the park, the shrub is choking the habitat of the Kafue lechwe (Kobus leche kafuensis), an antelope found only in Zambia.
“It’s reached a point where it’s outcompeting some native plants and even forcing out certain forms of biodiversity that benefited from the flats,” he says.
“Ideally [the lechwe] belong in the Kafue Flats, and the Mimosa pigra has invaded their home.” The team’s work is part of an initiative called the Great Spine of Africa, led by researchers, storytellers and boat captains from The Wilderness Project (TWP), to document the condition of major African rivers and their basins.
The giant sensitive bush isn’t the only invader here. Another, just as damaging, is found right in the river itself.
After six and a half hours of paddling downstream through the Lunga-Luswishi GMA, the team stops to camp in Mushingashi Wildlife Conservancy, adjacent to the national park. As the sun sets, it casts golden light across a nearby wetland, where great egrets (Ardea alba) stalk along the water’s edge and elephants (Loxodonta africana) graze. Ross’ second-in-command, Kyle Gordon, heads out alone to lay a trap baited with dog biscuits. By morning, it’s teeming with crayfish, their red claws waving.
These are Cherax quadricarinatus, Australian red-clawed crayfish, also known as yabbies, with speckled, bluish-green shells.
Experts say the species was first introduced to the river in 2001, the result of lax biosecurity at an aquaculture farm in the Kafue Flats. A separate introduction occurred in 2014 in the Barotse Floodplain, western Zambia, when road workers allowed them to accidentally escape into the wild.
Now, they’re a serious pest — outcompeting native crabs and eating fish eggs.
“They eat everything they see,” Searle says.
They’ve also changed how people fish, with ripple effects on the river’s ecology, Ross explains.
“Instead of [fishers] being able to set nets, leave them overnight and collect them in the morning, the yabby comes and eats all of your fish. So, then you’re having to fish with a dragnet, and that is potentially catching smaller and smaller fish.”
Set nets can selectively target certain fish sizes, and their use in the Kafue is generally sustainable. Dragnets, known locally as sefa-sefa nets, are made from shade cloth lined with mosquito nets that allow nothing to escape.
The crayfish invasion is extensive.
In 2024, Ross and a team from TWP paddled the entire 1,600 kilometers (995 miles) of the Kafue, from its source near Yowela, close to the Democratic Republic of Congo, to its confluence with the Zambezi River below Lake Kariba on the Zimbabwean border.
They found crayfish at 20 sites along nearly the entire length.
“There are no native crayfish in continental Africa — so any crayfish invasion poses potential for serious risk in terms of ecological disruption, as the native ecology is not adapted to them,” says Josie South, an aquatic invasion ecologist at Leeds University, who was not involved in the expedition.
“The Zambezi floodplains are very similar to the Australian systems where they evolved, so the crayfish populations have boomed enormously,” she adds. Combined with the use of the sefa-sefa dragnets,it’s a double whammy for the river.
“The crayfish continue to expand and the native fish continue to decrease.”
As the winter sun warms the Mushingashi campsite, Searle inspects each of the 34 crayfish caught in the trap. She checks their undersides to determine sex, looking for the placement of reproductive openings known as gonopores. Thirteen are female. “It helps us to understand the strength of the invasion front,” she says. More females mean more eggs — and faster spread.
Using calipers, she measures the crayfish, careful to avoid their snapping tails. Later, the animals are euthanized, and one leg from each crayfish is removed and preserved for DNA analysis. These will likely be analyzed by technicians at the South African Institute for Aquatic Biodiversity.
“They can compare DNA amongst populations to see how closely related different populations are,” Searle says.
The findings will be useful to researchers like South, who says that TWP’s standardized trapping method provides consistent updates on the spread and demographics of the crayfish. “We can combine this data with water quality and hydrology to try and predict when and where they will invade next and to understand the population connectivity.”
For Kampamba, after five weeks of sampling fish, testing water and studying macroinvertebrates — first in the Lunga, a major tributary, and now in the Kafue — what stands out is the fragility of Zambia’s rivers.
“It just shows the importance of understanding the river system, knowing what helps it and what destroys it,” he says.
“The Kafue is the most important river for supplying drinking water for most Zambians, but that doesn’t mean that others are less important. We need to take care of all of them.”
Mongabay joined an expedition of The Wilderness Project traveling the length of the Kafue River, part of TWP’s wider project to gather data in Africa’s major river basins. Read Ryan Truscott’s other articles from this journey here.
Banner image: The Wilderness Project team makes its way down the center of the river, repeating a continuous monitoring survey first done last year. As the researchers travel down the river, expedition leader Mike Ross in the lead canoe calls out what he sees, and this is recorded by researchers Lauren Searle and Katongo Kampamba in the second and third canoes. Image by Ryan Truscott for Mongabay.
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