Biomagnification: Why a dead shark is more dangerous than a live one |
By Andrea Goth Contrary to what most people seem to believe, sharks are more dangerous dead than alive if they find their way onto your plate or soup bowl. This is the result of a process called biomagnification. Biomagnification is a process by which toxic substances become increasingly more concentrated or plentiful as they move up the food chain from one organism into the body of another. The story of biomagnifications begins with pollution. Industry produces harmful man-made toxins, like PCBs, dioxins, and furans, as well as heavy metals, like mercury, arsenic, and cadmium. These highly poisonous substances enter the ocean environment as pollutants.These toxins are particularly nasty because they are ‘persistent toxins’: they do not degrade in the environment or through the biological processes of living organisms. These toxins are also lipophilic, which means that they are ‘fat-loving’, lodging themselves in the fatty tissues of the body, and are not water-soluble. Therefore, they are not excreted from the body through the urine. Imagine mixing oil and water. It may appear to mix for a moment, but the oil and water will quickly separate into two layers. If you were to add a second oil, it would naturally mix with the first oil and avoid mixing with the water, even if you were to shake the mixture. Lipophilic pollutants, like the second oil, would rather mix with the fat in an organism than return to the water. Every organism in an environment contaminated with these persistent toxins will acquire and maintain an increasing concentration of the toxins in their bodies. This process is called bioconcentration. Unfortunately, the story doesn’t end there. Animals don’t just acquire and keep these persistent toxins through bioconcentration. They also consume more of these toxins when they eat other organisms that are similarly storing the toxins in their bodies. The higher on the food chain an animal is the more toxins it will consume and accumulate. This is the double-whammy where bioconcentration meets biomagnification. Let’s look at the example of a simplified food chain. On the bottom of the food chain are the photosynthetic algae that have accumulated a certain amount of persistent toxins. A little angelfish comes along and eats some algae, like it has every day of its life. With every bite of algae, it also consumes all the toxins the algae has bioconcentrated. A hungry tuna comes along and snaps up the angelfish. The tuna has also eaten all the persistent toxins that the angelfish had bioconcentrated, along with all the bioconcentrated toxins from all the algae the angelfish ever ate. Moreover, this tuna has eaten hundreds of innocent little angelfish throughout its life, each of which has eaten pounds and pounds of algae. This is one pretty toxic tuna. Then, a large shark comes along and has itself a tuna sushi lunch. In the process, it has ingested all the toxins that the tuna obtained through bioconcentration PLUS all the toxins in all the angelfish it has ever eaten PLUS all the toxins in all the algae those angelfish ate. All of the various fish and sea life that the shark eats in its life similarly comes with its considerable storage of poisonous persistent toxins. These toxins are stored and built up in the shark’s body until they reach a concentration hundreds of times higher than the concentration of toxins in the water. Imagine the level of pollutants a shark can accumulate over the course of its lifetime! Sharks typically live between 20 and 70 years, depending upon the species. As apex predators, sharks have the highest levels of toxins because they ingest all the toxins present in the organism levels below them on the food chain over a very long life. This is biomagnification at its worst. Now, imagine this shark is caught by a fisherman and ends up on your plate in a seafood restaurant or in your bowl of shark fin soup. Basically, you’ve just made yourself the top predator on the food chain – and are happily consuming more toxins than any other animal in the ocean! And those persistent toxins will now be with your for life. Blech. Sharks tend to be quite toxic and as such are generally unfit for human consumption. Shark fins consistently rank at the top of the list of all fish for mercury contamination. For this reason, the US Food and Drug Administration, and similar agencies of other governments, warn people against eating shark. In particular, the USDA says that pregnant women, children, and women of child-bearing age should not eat shark. Comments (2)
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