In the river they tend to just swim around in the channels looking for food, and that big "black" looking fish is normally an nkupe cruising for food. The smaller ones do tend to run in small shoals and can be found around fallen trees and rocks, looking for passing titbits.
For the smaller fish (up to about 2kg) park above the tree, and cast into the current downstream with a good sinker, and small hook on a running sinker rig. The bites are very strong and they are good fighters for their size.
For the bigger fish (over 2.5 kg), drifting down the river with a small fillet is quite succesful, but they are far and few between unless you find an area that they seem to have invaded due to food being consistently available.

The fish in Kariba are harder to find (and catch), as they are often lone fish swimming around. The occassional fish has been caught on a spinner whilst spinning for bream or small tiger, and these will give you a serious fight, so I hope you have an electric trolling motor to follow it.

They are occasssionally caught on worm or fish fillet,  on a ledger rig whilst fishing for other species.
They are incredibly strong fighters, as can be imagined with that large roundish body, and so some skill is required to catch them.