The Maloti minnow, a fish out of water

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The Maloti minnow, a small endemic fish, has almost completely disappeared from the radar in certain areas of Lesotho. The likely cause of the population crash could be the opening of the inter-basin transfer (IBT) in 2006, connecting the Katse Dam to the Mohale Dam as part of the Lesotho Highlands Water Project.

A study 10 years after the construction in the Mohale catchment area revealed the Mohale population of the Maloti minnow (Pseudobarbus quathlambae) had crashed. The study netted only five minnows in its former stronghold, the Jorodane and Bokong Rivers. No minnows where caught in the Senqunyane River.

“Dams are essentialfor water security in southern Africa. However, when we construct and operatethem, it’s really important to be considerate of the species and ecosystemsthat they affect, and put measures in place to ensure minimal ecological impact,”says Jeremy Shelton, a researcher at the Freshwater Research Centre in South Africa.

maloti minnow sketch
Sketch by Graham Kearney

The University of Lesotho warned that the IBT would have implications for Lesotho’s only endemic fish; they advised that barriers, like mesh nets, metal grids or man-made weirs should be implemented to prevent the dispersal of fish. Unfortunately this advice was not followed.

The IBT also allowed smallmouth yellowfish (Labeobarbus aeneus), Orange River mudfish (Labeo capensis), rock catfish (Austroglanis sclateri) and exotic trout species to migrate upstream. The Maloti minnow is now a really small fish in a big pond, with other larger, more aggressive fish.

The Maloti minnow evolved in isolation without predatory or competitive fish species. Natural barriers like waterfalls would have prevented the smallmouth yellowfish from migrating upstream. “My intuition is that that the yellowfish will prey on small minnows, and compete with adults for food and habitat. The mudfish may also compete with the minnows, and could also change the stream habitat by suspending benthic sediments while feeding,” says Shelton.

 Senqunyane River
The upper Senqunyane River upstream of Mohale Dam – a former stronghold for the‘Mohale Maloti minnows. Image by Barry Clark

There are two distinct populations of Maloti minnow. While the eastern linage is still abundant, “the Mohale lineage is nearly extinct with probably less than 100 adults left in the wild”, says Shelton. “Through careful planning, small areas of suitable minnow habitat can be re-seeded with minnows,” he adds, referring to a success story in the Cederberg where introduced, predatory smallmouth bass were removed to reclaim habitat for endemic freshwater fishes in the Rondegat River.

Written by Georgina Lockwood 

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