The process of biomagnification is what results in the high levels of mercury and PCBs in predator fish. The higher a fish is on the food chain, the higher levels of such chemicals you can expect to see. It is true that the cats and carp feed on the bottom and are most directly exposed to mercury and PCBs as these chemicals are bound to the sediment. IDNR and IDEM would tell you that the fish consumption advisory (FCA) isn't exact science but more a guide to healthy fish consumption. The moral to the advisory is "moderation".