Toxins that are present in fish and seals that swim in polluted water are consumed by the Orca and concentrate in its blubber. Eventually the toxic burden caused by the concentration of contaminants suppresses the Orca's immune and reproductive systems, and it becomes susceptible to a number of diseases or is killed by the chemical itself. It is worth noting that humans eating the flesh of whales, as they still do in Japan, will further concentrate these toxins in their bodies.
Orca calves are especially vulnerable to poisoning as the toxins are passed through to the fetus during pregnancy. Calves also nurse on milk that is loaded with toxins released by the mother as she breaks down her fat stores to produce milk. Studies have shown that calf mortality is higher than normal in the Northern and Southern communities, where PCB levels in blubber have been shown to be dangerously high. Three out of four calves will die before their first year. In fact, mortality rates for all whales in these pods are higher than normal and for the last three years the pods have declined by 15%. Scientists expect population declines to continue as fewer reproducing females remain alive to produce calves. Eventually, half of the maternal lineages in the pods will die out.
PCBs are not the only toxin that concentrates in orca bodies. Three dead orcas washed ashore near Vancouver Island were also found to have dangerously high levels of organochlorine in their flesh, a chemical that is deadly to humans. The unnatural deaths of pod members can result in further casualties and the eventual death of more pod members. The Exxon Valdez oil spill resulted in the immediate death of seven orcas from ten resident pods in Prince William Sound, Alaska. Researchers are confident that the deaths of these important individuals were responsible for the break up of certain subpods and the eventual death of former pod members. (It is widely accepted that pod membership enhances whale survival through the location of scattered food sources and group support.) Disruption, dislocation and death of remaining pod members is especially likely when females die, since maternal groups form the basis of all orca society.