Many freshwater fish species in the southwestern Cape are found nowhere else in the world, says Emeritus Professor Paul Skelton of the South African Institute for Aquatic Biodiversity.
The award-winning ichthyologist and author of the new edition of Freshwater Fishes of Southern Africa: A Complete Guide, Skelton is calling for greater protection of rivers and the “out of sight, out of mind” species which depend on them.
“When you go to Kruger National Park and cross the Sabie River, do you ever hear people talking about the fish?” he asked his online audience in a public talk on Wednesday.
By 2024, 315 freshwater fish species had been identified in Southern Africa, up from 222 species in 1993, said Skelton. Showing a bar graph of species diversity, he pointed out the cyrinidae group as the “biggest skyscraper” with about 102 species.
When freshwater rivers change course this affects the evolution of fish.
— Ichthyologist Prof Paul Skelton
Fishes are the largest group of vertebrates (animals with a backbone and a skeleton including humans) in Africa and their lineages date from the break-up of Gondwana to much more recent times, a few million years ago, said Skelton. He has studied freshwater fish in Southern Africa for more than four decades.
The first scientific records of freshwater fish in SA, by English naturalist William Burchell, date back to the early 19th century. In 1861, the French consul Count Castelnau described a fish called Galaxias zebratus — named for the tiny galaxy of stars and zebra-like stripes on its scales — collected from the stream that runs through and below Kirstenbosch.
Castelnau recorded fish in pools on the Rondebosch Common and in the Liesbeek River in Cape Town said Skelton, raising concern about pollution today in this canalised river.
“Rivers are a living medium with living organisms,” he said, concerned that they are treated, or rather maltreated, just as a resource. “When freshwater rivers change course this affects the evolution of fish.”
Just less than half the fish in the world (43%) live in freshwater rivers and lakes, a disproportionately high number given that freshwater makes up only about one to two percent of all the surface water on earth. Oceans account for about 98%.
Freshwater rivers and lakes also have an abundance of fish species — roughly 45% of all 35,000 to 40,000 recorded fish species — which evolved from three main evolutionary lineages, one branch of which is sharks and rays.
During his research career, Skelton has explored how certain species — such as the redfins (rooivlerkies) which he has studied — reached the southwestern Cape, given that the Cape Fold mountains throw up a barrier to the north.
“Barnard’s Gate” in the Cape Folds region is a significant passage to the distribution of freshwater fish into the Cape said Skelton. This is why the southwestern Cape, with its isolated habitats and rivers, has a high concentration of endemic species.
“The Orange River to the north might have been linked to the Olifants River,” said Skelton, who used to camp along the banks of this river in the Cederberg as a young researcher.
As a veteran researcher with less acute hearing underwater, he was surprised when one of his colleagues Steven Lowe asked about redfins communicating underwater. Lowe discovered that the redfins appear “to talk to each other” and recorded the first sound production by minnows in Africa, Skelton said, noting some freshwater fish use electricity to communicate.
Southern Africa has a lower diversity of freshwater fish species than further north on the continent, from where they migrated south, he said. “One of the richest areas is the Congo basin with about 800 to 900 species and I would not be surprised if they push beyond 1,000 species,” he said.
On how the Galaxiidae zebratus may have ended up in the south, Skelton said: “Their lineage is only about 25-million years old so they must’ve arrived at the southern tip of Africa through the ocean,” he said.
The oldest lineages of freshwater fish go back to about 60-million years ago, “just after the dinosaurs went extinct”, Skelton said.
In the modern world, human population growth is one of the biggest threats to freshwater fish species, given how development affects rivers and landscapes, said Skelton. He hopes that people will learn more about the freshwater fish he cherishes so they then care enough to conserve them.
Skelton said he “fell into” ichthyology after he walked into the Albany Museum in Makhanda one day and met his first mentor, Rex Jubb. Jubb curated the “first major collection of freshwater fishes at the Albany Museum”, the museum states.
“If you had said I would study fish, I would have sneezed,” said Skelton. “But Jubb had fascinating stories about freshwater fish that I could not resist. The day I walked into there, I knew this was my career, where I wanted to be.
“Ichthyologists are crazy,” he said, “all they want to talk about are fishes.”
• Freshwater Fishes of Southern Africa: A Complete Guide written by Paul Skelton and published by Struik Nature will be in stores from mid-October at R470
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