Bioaccumulation at work brought on by the mechanics of the food chain. When one organism eats another, about 90% of the energy involved in the transaction is lost. That means that the higher up the food chain you are, the more you have to proportionally eat in order to supply your energy demands (this is why it's greener to be a vegetarian than a regular meat-eater, you're getting 10% of the energy from the veg directly instead of 10% of the energy from the cow that was already getting just 10% of the energy from its feed. Basically, 10% vs. 1% energy gained). Because you're consuming more, you're exposing yourself to more of these toxins that can accumulate in the body, so you wind up with an overall higher concentration than organisms below you in the chain. Same reason people worry about mercury in fish.
In August 2003, the Bush Administration relaxed emmissions controls. In 2004, the EPA had to change their tuna recommendations. It don't take long for apex predators to bioaccumulate.