Mercury in Sharks

All over the country at waterfronts and museums, there is a poster detailing which fish and what amount of them are safe to eat due to high levels of mercury. With the revolution of the way people think about heavy metals in our environment, a large portion of the conversation has turned to the levels of contamination in fish and how to best protect humans from any adverse effects. However, while conversation about mercury levels in fish has increased over the past years, one aspect is being overlooked. Sharks are affected the most by toxic methyl-mercury however the conversation rarely focuses on this aspect of the chain.

            The summer before my senior year in high school, I spent one month in Fiji conducting research in Pacific Harbour at the Shark Reef Marine Reserve. Every day I would slide on my wet suit and scuba gear, jumping into the clear waters to observe the behaviors of bull sharks, grey reef sharks, black tip sharks, and white tip sharks.
Being a visitor in these amazing creatures’ habitat was eye opening, and since my trip, I have been dedicated to spreading awareness about the dangers sharks face and have been working to debunk the myths that sharks are ruthless man-eaters.

            In our seminar, we have been discussing the effects of mercury in our environment, and having known that large fish can be affected the most by the toxin, I was curious to find out if sharks were also part of the equation. As it turns out, they are, and mercury actually takes a toll on some of the shark’s most important body systems.

            Mercury finds its way to the sharks just like every other fish; through the food chain. It enters the water from factory runoff and toxic rain, and then accumulates inside the smallest members of the ocean food chain, phyto and zooplankton. Small ocean invertebrates such as krill and shrimp consume the plankton and then accumulate a small amount of mercury themselves. Small fish such as salmon or reef fish like clownfish eat the krill and shrimp, accumulating the mercury from them as well as the plankton. On the next level, small mammals such as seals or larger fish such as tuna will eat the smaller fish, ingesting the mercury of the three levels below them. In the last level which is often overlooked, sharks take the largest hit of the cycle, accumulating the mercury from their diet of seals, then seals diet of smaller fish, then the smaller fish’s diet of krill and then finally the krill’s diet of plankton.

            Because in the United States shark finning and consumption is illegal, the issue of mercury poisoning in sharks doesn’t make headlines. Just like consuming tuna, consuming shark meat, which is even more toxic, is dangerous. Because the mercury levels in tuna affect humans, it is something that has been brought to attention. However, sharks have been left out of the discussion even though they suffer even more than tuna and swordfish.
            High levels of mercury in sharks inhibit secretion in their rectal glands[1]. These glands are important because they control osmosis and regulate fluid volume outside cells. These glands are also important because they regulate the salt in the shark’s bodies by excreting extra fluid out of the body. With an imbalance in salt, the sharks are unable to ingest seawater, which is important to keep them alive. By running salt water through their gills, sharks are able to bring oxygen into their bloodstream. If they have a salt imbalance and cannot ingest more salt water, they will die.

           

Mercury also affects the sodium, potassium, and chloride channels in the shark’s cells. These are all important aspects of cell regulation and management necessary to keep the shark healthy. Most importantly, the chloride channels regulate homeostasis in the sharks, keeping their bodies at equilibrium. Just like in humans, sharks need to regulate their body temperature and pH levels. Damaged chloride channels also affect organic solute transport within cells. The chloride channels provide the proper gradient that allows larger organic molecules to pass through the cell membrane. Without these necessary molecules, the cells are unable to make vital proteins to keep the sharks alive and their genomes intact.

            The most important question we can ask is what is being done? And the answer is almost nothing. Because this has been something that has accumulated over many years of factory production and mercury use, time needs to pass so that the bioaccumulation will decrease. Unless the smallest members of the food chain are mercury free, the sharks won’t be mercury free. The only thing that will keep this an issue is if we continue to put mercury into our environment. Backtracking would make this already large issue even bigger, and would start causing more harm to sharks and all other aspects of our oceans.

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[1] http://ajpcell.physiology.org/content/290/3/C793.long

http://www.scielo.br/scielo.php?pid=S0034-71082000000400005&script=sci_arttext

http://www.psychologynoteshq.com/wp-content/uploads/2011/11/sodium_channel.png

http://upload.wikimedia.org/wikipedia/commons/c/c5/MercuryFoodChain-01.png