beluga whales are cetaceans - porpoises, whales, orcas, and company.
The point is that PCB's interfere with reproduction in a lot of these mammals. These mammals concentrate PCB's because of two reasons:
1. they eat near or at the top of the food chain
2. they have a thick layer of subcutaneous fat for insulation.
The problem for these mammals does not appear to be cancer, although PCB's are carcinogenic in mice, for example. Dead orcas in areas around Puget Sound are treated as a hazmat item because PCB's concentrations are high, and those with highest PCB levels do not reproduce well and tend to die younger.
Since you don't seem to be a scientist here is a newspaper version of what I'm talking about:
http://seattlepi.nwsource.com/local/69418_whale07.shtml