They typically screw into a plate on the grave marker and are secured with a small chain.

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The cycle goes on and the levels of toxicity increases until you get to the top of the food chain with pelagic fish such as swordfish that end up on our dinner plate.

CNN: STORY HIGHLIGHTS

Farmers have long called for the food supply chain, which can involve many traders and processors between farm gate and consumer plate, to be overhauled and simplified.

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"People are now asking about where the food on their plate is coming from and they're taking more of an interest in the supply chain, " he said.

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Mr Strong is close to John Mackey, the co-founder and chief executive of Whole Foods, a high-end supermarket chain though Mr Strong says that Mr Mackey already has too many other things on his plate.

ECONOMIST: Free cities