Lots of Carp, in the deeper areas you should get some during the colder months, yellowfish and mudfish, barbel are the main species with a few others in between...
If static for a while you can fish for carp and barbel in the dam during winter, but it is very important to stick to one spot, especially with the barbel in the dam as they do not move very much during winter so you need to allow them a long time to find the bait.
Carp not so much, a vaal dam or rietvlei trace can be used from the boat, an in 2 hours of fishing the same spot you should get a few fish, yellows and muddies in between that.
If spinning or fishing fly, one should use very small lures and flies to be successful in the dam during the colder months.