The plankton absorb it right out of the water. Then the krill eat the plankton and they absorb it into their bodies. The herring eat the krill and they absorb what the plankton and the krill ate. The large fish eat the herring and they absorb into their bodies what began with the plankton and then passed through the krill and the herring all along the chain. Finally the big fish and all their contaminants wind up in the bellies of the mammals - the orca, for example - who steadily build up dangerous levels of toxins such as mercury and PCBs.
It's a process called "bio-concentration" and it threatens the very survival of aquatic life, especially at the top of the food chain. Unfortunately many of these toxins concentrate in the flesh of the prey fish, instead of being eliminated, and give rise to a cumulative build-up as they move up the food chain.
Humans, at least those with a taste for marine mammals, can find themselves next in line. There's currently an uproar in Japan, a country that likes to dine on these creatures. School children in rural Japan get fed whale meat for lunch and it's now been discovered to have mercury contamination 10-12 times higher than recommended limits.
The meat is taken from short-fin pilot whales, a species of dolphin. With whaling season just around the corner, this meat was to be sent to schools throughout Japan. It's ironic but maybe the mercury might just be the salvation of Japan's pilot whales.
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