CAPE TOWN — Pictographs of the golden gills of Clanwilliam sandfish, endemic only to the Olifants-Doring River basin, stretch across rocky walls of sandstone caves in South Africa’s Cederberg Mountains — a range north of this coastal city that straddles two biodiversity hotspots, the Cape Floristic Region and Mars-like Karoo.
Annual migrations of the glistening fish with the Dickensian name have long defined local life, marking spring’s arrival and sustaining subsistence lifeways. But the scenes painted millennia ago by Indigenous San communities are a far cry from the dry basin’s outlook in 2025.
The Clanwilliam sandfish, named for a nearby town christened after the Irish Earl of Clanwilliam, is today South Africa’s most threatened migratory freshwater fish species and a representative fauna of an urgent global crisis. Where tens of thousands of sandfish once migrated through dozens of rivers, just several hundred survive now in the Doring River and a handful of its tributaries, including the Biedouw River.
The decline of the sandfish is a microcosm of a global challenge. Nowhere on Earth is the loss of biodiversity more rapid than in freshwater ecosystems, and no waters are more at risk to warming — under all emissions scenarios — than those in dry, low-flow basins such as the Olifants-Doring watershed. One in four freshwater species — ravaged worldwide by drought, pollution, water overuse, and invasive species — are threatened with extinction.
“It is often said that freshwater species are ‘out of sight, out of mind,’” said Catherine Sayer, the freshwater biodiversity lead at the International Union for Conservation of Nature. “If you’re not aware of something then it is impossible to place any value on it, hence why the realm has not been given priority.”
Healthy Rivers Are Essential
For migratory fish like the Clanwilliam sandfish, the outlook is even more dire. Between 1970 and 2020, the populations of nearly 300 global freshwater fish species have declined by 81 percent. Those with at least one known local threat nearly disappeared completely, losing an average of 96 percent of their population.
The health of a fish is only as good as the health of the river system it calls home. Likewise, the quality of a river depends upon thriving native species.
Pointing to this crucial symbiosis, researchers in South Africa, one of the 30 driest countries in the world, have called for the protection of freshwater fish to guide the conservation of scarce water resources. These efforts are especially relevant to the watersheds of South Africa’s interior, a region warming twice as fast as the rest of the world, and where agriculture accounts for more than 60 percent of all water usage.
To Follow a Fish, Follow the Water
To understand the plight of the Clanwilliam sandfish is to understand the water cycle that, at its most pristine, pulses with tenuous timing throughout the Olifants-Doring River basin.
The Doring — which means “thorn” in Afrikaans — originates high in the dry Cederberg Mountains and flows east through a rain shadow speckled with prickly flora.
Dry brush and bramble dot the desert Karoo, marked by temperature extremes and sporadic precipitation. So scarce are summer rains that the Doring ceases to flow for months at a time, forcing adult sandfish to concentrate in a few deep, ephemeral pools that hug the river’s shores.
“They’re pretty packed in during the summer,” said Jeremy Shelton, a freshwater conservation biologist with the Freshwater Research Centre (FRC) in Cape Town. “It’s hot and dry, regularly over 40 degrees Celsius (104°F). In these extreme environments, the fish wait out the summer. I don’t think it’s very pleasant for them.”
Reprieve arrives in June through August — winter in the southern hemisphere — when cold fronts roll across the region and flood the basin, reconnecting the Doring with its tributaries. As recently as the 20th century, sandfish journeyed upstream from the Doring by the thousands, their golden scales likely sustaining communities who lived along any of the myriad tributaries the sandfish once called home.
Elders recall that there were once so many it felt like they could walk across them to get to the other side of the river.
“Sandfish have relied on pulses of seasonal floods to migrate, likely traveling overnight,” said Cecilia Cerrilla, a freshwater fish ecologist at the FRC and recent PhD graduate from the University of Cape Town. “The Biedouw has become a critical habitat, because it’s the only river that we know for sure where these annual spawning migrations still happen.”
After the high winter flows subside and the water warms during springtime, sandfish migrate 10 miles into tributaries to spawn and lay eggs in shallow riffles before quickly returning to the mainstem Doring, where the cycle begins again.
The new eggs, which hatch in the Biedouw roughly 10 days after they are laid, are proverbial fish in a barrel. Invasive bluegill and bass — introduced to the basin for sportfishing since the 1930s, and now pervasive dwellers — quickly eat the defenseless fingerling and fry, which like their parents in the Doring huddle closely together in sparse pools.
Historically, these feeding frenzies have been the leading cause of sandfish declines, Cerrilla said. But today, researchers have shifted their efforts to prioritize a more existential threat in a warming Biedouw. “Their pools are drying up,” Cerrilla said. “The lower 20 kilometers dry up completely each summer. There’s no water.”
To Save a Fish, Use ‘Bucket Science’
By 2030, South Africa is poised to face a 17 percent water shortage without drastic reallocation; 98 percent of its water resources are already committed, largely to agriculture. In the Biedouw Valley, the spread of thirsty, non-native mesquite, poplar, black wattle, and blue gum trees — and the expansion of agriculture — have further strained the fresh water supply.
Over the past decade — and against the backdrop of several Day Zero scenarios that menaced the Western Cape and forced water usage restrictions in Cape Town — Shelton has led a small group of scientists, local land owners, Indigenous stewards, and young students to devise a fleeting chance for sandfish salvation.
“Water is a scarce resource here. It’s a dry place. We’ve over-allocated our water in our rivers in quite a hectic way,” Shelton said. “A lot of water is being taken and used to grow food for livestock.”
One of the first land-owners Shelton invited to participate in his project, called Saving Sandfish, was Gus Bradley, a native South African who returned with his family from the United Kingdom in 2019 to the Biedouw Valley. The land on which their new farm stood was in the midst of a seven-year-long drought.
Just 10 miles from the confluence of the Doring and Biedouw rivers, Bradley looked often onto the watershed and felt for the newly hatched fry, whose ephemeral pools were disappearing by the day. At home, he confronted worries of his own: drinking water collected from several mountain springs was “barely enough to get through summer months,” he said.
“When I look at the photos of the farm then versus today, the place is unrecognizable,” said Bradley, who with his wife, Lauren, owns Enjo Nature Farm, located on the banks of the Biedouw. “Those first two or three years in the drought were a challenge. There were no pools in the river, and during those dry years presumably very few fish made it.”
With a background in zoology, Bradley “jumped at the chance to be involved in local conservation,” he said. One critical task was wading hip-deep into the Biedouw in November and December, shortly after the sandfish eggs hatch, to gather as many fry as possible in buckets and transport them to irrigation ponds and reservoirs on nearby farms. Protected from invasive fish and river drying, the sandfish would grow for anywhere between one and five years into adults too large to be eaten by bluegill and bass. If all went well, the mature fish would be introduced into the Biedouw and spawn.
Bradley and his wife agreed to use their irrigation ponds to raise immature sandfish. With their two young boys, they were among the first volunteers conducting what the project calls “bucket science.”
Within months of settling into their new home, several thousand juvenile sandfish were swimming in the farm’s ponds. Half a dozen other farmers soon joined, seeing the effort came at no cost to them.
“It’s not a big sacrifice,” Bradley said. “I think once the local farmers got over their conservative views of these sort of hippie eco-warriors wanting to use their dams, the project really took off.”
To Save a River, Save a Fish
The spring of 2021 marked the first time that researchers returned sandfish from the farms to the river. Each specimen was outfitted with a tiny location-tracking tag to monitor their movements and test the ultimate goal: ensuring the fish, after months outside the river, would travel back to the Biedouw to spawn in springtime.
“It was the first time we were going to see whether our work — a huge outpouring of money and human effort — would bear fruit,” Cerrilla said. “We had to wait a full year to see if they would come back.”
For four anxious days in September 2022, working alone in the Cederberg wilderness, Cerrilla looked out onto the silent Biedouw, its waters defined by lack.
No signs of life flickered gold within its turbid springtime surge. No hopeful pings, indicative of passing schools, lit up the fish-tracking radar set back across the channel. Each morning she relayed dejected status reports to her colleagues in Cape Town, 150 miles south. The static crackle of her calls, carrying news of no news, seemingly further sealed the bleak fate of the cherished local swimmer.
As the sun rose on a fifth morning in the valley, hope running dry as the surrounding shrubland, Cerrilla returned to the river as dawn reflected in the Biedouw’s rippling flows. She found herself questioning her vision — her eyes perhaps playing sleepy tricks — when she saw the radar had registered six golden fish migrating beneath the surface.
Tears streamed down Cerrilla’s face as she delivered the optimistic message, years in the making, back to Cape Town: “I hope you’re sitting down…”
“It was just the most exciting moment, because these fish that we’d rescued two years before, and released one year before, had survived a year in the wild and actually managed to come back up the Biedouw,” Cerrilla said. Over the next several days, dozens more sandfish swam through.
To date, 45,000 fish have been placed from the river into farmers’ sanctuary ponds, and nearly 6,000 have been returned to the Biedouw. At least 18 percent of these — some 480 adults — have successfully migrated back to the Biedouw, nearly tripling the size of the 2020 spawning run, the first ever surveyed.
The healthy returns buy wild sandfish precious time to recuperate numbers, but the team also acknowledges that the bucket method is a temporary, shoestring approach. The project’s long-term success runs through water reform and the uneasy reality that, eventually, their efforts are likely to place them at odds with the very partners they’ve relied upon.
“There’s going to come a time in the future where we are going to have some more difficult conversations with some of the landowners,” Shelton said. “Up until now, it’s been ‘Okay, we’d like to use some of your farm’s reservoirs as a sanctuary for sandfish.’ But I think it’ll be a different story once we have to start having some harder conversations about water use.”
The team confronted this water-scarce reality head-on this past field season. For the first time since 2022, the number of sandfish returning to spawn in the Biedouw didn’t increase from the year before. Following an already dry winter, very little rain fell in August and September. No high flows filled the tributary, severely limiting the migration window. Just 23 adults made the journey, the lowest number so far.
“It really paints a very vivid picture of the hugely important role that water plays in the species’ life cycle, and underscores why water is such an important future focus of our project,” Cerrilla said.
Golden Scales Mark a Golden Age
Stories of healthy sandfish runs live on the tongues of elders and Indigenous stewards of the valley. Their oral histories of plentiful populations, brimming in valley waters as recently as the 1980s, testify to how quickly the species’ population has plummeted.
“If you walked the river, there were schools and schools,” said Sarah Fransman, 77, who has lived in the basin since she was a little girl, in a 2020 interview conducted in Afrikaans with FRC researchers. “You see little pools and then you see them move their tails. It’s the most beautiful thing, as you see them coming up the stream and making the water look like gold.”
One of the major global challenges to preserving freshwater ecosystems, Sayer says, is the lack of perceived “cute and cuddly” species that take up residence within their waters. Fortunately, in the Olifants-Doring basin, where the health of sandfish reflected quality waters and fulfilling meals, the aspiration of rivers flowing golden again has proved a worthy motivator for collective action.
“When you have a charismatic species, it’s allowed us to have conversations with different groups of people and rally up support in a way I just don’t think we ever would have if it was just the river,” Shelton said. “The power of a flagship species, especially a migratory one, can’t be underestimated.”