RE: "By using fish caught in the wild..." This is completely untrue. There is no way to tell what the contaminate levels are in an individual wild fish. It all depends on what the fish has been eating. On the other hand, companies farming fish have complete control over what their fish are eating and know exactly what is in the feed.
They can state with confidence that an entire pen of fish is contaminant-free, based on their diet. You cannot do this with wild fish unless you test every single fish. And fish feed companies have been working for years to source ingredients with low levels of PCBs and other contaminants, and recent tests show farmed fish has lower levels of contaminants than wild. And higher omega-3 levels, too. Wild is great. But it's not necessarily better.