Northwest salmon in hatcheries and estuaries are testing positive for PCBs and DDT, and researchers at the National Marine Fisheries Service aren’t sure of the source of the pollutants. The chemicals are at levels high enough to harm fish immune systems, but the scientists don’t yet know if they could hurt humans who eat the fish. U.S. Fish and Wildlife Service officials are concerned that the chemicals may be passed on to predators that feed on the salmon. Cormorants, bald eagles, and Caspian terns in the area have all tested positive for high levels of the toxic compounds.