The teenager and his father, from Rustenburg, North West, were fishing at the dam on Wednesday when three men armed with knives approached them, Captain Bernadine Steyn said.
“The 16-year-old son was assaulted, during which he sustained a stab wound near his left eye.”
He was admitted to hospital, but had since been discharged. A cellphone, watch, keys, and two wallets were stolen.
The robbers slashed the tyres of their Ford Ranger before fleeing. No one had been arrested by 2pm on Thursday.