Swordfish, Tuna etc are fish usually found in deep water. Those kind of fish are not affected by the mercury/PCB runoff usually affects fish in bays, deltas, rivers and lakes.
Swordfish and Tuna absorb mercury from the atmosphere that is released by industry pollution. All shellfish and regular fish have traces of mercury but larger amounts may target the human nervous system where usually nursing/pregnant women are mostly affected. Thats because young children and unborn babies have weaker immunity systems.
Here in the SF bay, there are fish that you should not eat period that are caught from certain parts. There are advisories all over the place where any adult should not eat more than 2 eight ounce meals of fish caught from the bay every month.