As winter begins in South Africa, the dry season ends, and rains return to the Western Cape province. Parched washes become streams again. Wildflowers bloom. And around August, Clanwilliam sandfish rush upstream to spawn en masse.

Or rather, that’s what they used to do.

These torpedo-shaped, silvery fish—which get their name from a town in the area—were once were so numerous in this southwestern corner of the country that their reproductive pilgrimages roiled the tributaries of the Doring, a major waterway that rises in the interior before flowing into the mountains of the Cape.

“There were so many sandfish they would make a wave,” recalls Sarah Fransman, who long has lived near the confluence of the Doring and a tributary called the Biedouw.