The predators can be other fish or birds.
When we arrived at our home, our lake already was stocked with fish, some of these were predators which kept a balance. These included peacock bass, or as they call it here tocanare and also triara. Before we decided to start tilapia farming we use to catch the triara by using a piece of styrofoam with a hook and line attached. To this we would put a small fish as bait. We tossed these into the lake in the evening and in the morning, we had caught a triara.
The birds can be problematic to control if you find they are taking your larger fish. Obviously those you don't want them to take and giant egrets will take amazingly large fish.
Some of the birds could be:
The cormorant is a diving bird which will be on the surface of the water and dive down for fish. These birds are notorious as they can decimate a lake of fish. We have only seen one here on our lake and he was encourage to leave.
The egret will wade into the water and stab at the fish. There are many different types and sizes of egrets. We have seen the smaller snowy egret work as a team herding small shoals of fish into the water's edge where they are easily plucked out. The giant egret prefers to work alone and in fact will often chase off others who want to encroach on its feeding ground.
The heron will wait at the shore for fish to come within striking distance. We have seen them using bait, namely fish pellets to lure a fish to the shore.
The last is the kingfisher. Here we have three different sized kingfishers and all hover above the water, spot their target, pull their wings back and with the accuracy of a missile plummet into the water, normally coming out with a fish.
Depending on your circumstances and the size of your fish, you may wish to encourage or discourage the birds. If you have an over population problem then these will be a help. If your fish have just arrived and are small, then they are easily preyed upon and should be discouraged with nets, noise or other birds such as geese. We found when we had geese they were aggressive against other waterfowl.