From the surface the only visible thing of the fish farms are the floating rings. These are made out of plastic tubes that is bent and assembled into rings. And there are nets on top of the rings to prevent birds from eating the precious fish.
Below the surface is an net witch keeps the farmed fish in, and the wild fish out. And since the farmed fish is predators and there are no wild prey for them to eat inside the cages, they are given pellets to eat. And these pellets are mostly made out of wild caught fish.
There are of course both advantages and disadvantages with farming fish in the open ocean or in fjords. But that is nothing that I will get into in this cache description.
If you want to know more of the pros and cons of fish farming you can use any search engine and learn more by yourself.