Many poisonous chemicals are dumped into the sea including some being pesticides such as DDT . These are then passed up through the food chain and become concentrated in whales and seals. Female whales then pass these pollutants directly on to their young through their milk which can result in death or disability. Other pollutants such as PCBs are fat soluble and are stored in the blubber and other fat reserves of dolphins. When the dolphin uses these fat reserves as food such as, in times of illness, large amounts of poisonous chemicals are released into it's blood.
Pollutants cause the lowering of an animals resistance to disease.
These are a few examples of how long some litter takes to break down.