With their immune systems compromised by toxins such as PCBs, the Gulf of Maine marine mammals are susceptible to disease.

As the number of dead and stranded seals washing up on southern Maine beaches rises by the day, researchers are linking the sudden die-off to decades of chemical pollution that made the seal population vulnerable to toxins and disease.

“We don’t think there is any possibility that these animals are biologically healthy,” said Susan Shaw, a marine biologist based in Blue Hill.

Shaw has studied the effect of man-made toxins such as polychlorinated biphenyl – PCB – on the long-term health of harbor seals for almost two decades. Her findings, based on flesh samples, show that the population of harbor seals in the Gulf of Maine is loaded down with toxic, immune-suppressing chemicals, conclusions that are in line with a thick body of scientific evidence from studies of whales, dolphins, porpoises and other marine animals in the U.S., Canada and Europe.