How is mercury in fish related to the depth they live at? This is the question a few marine scientists at the University of Hawaii started asking back in 2007.
Mercury comes from coal; it’s an impurity in coal that goes into the air when coal is burned, and eventually falls to earth and enters waterways. Bacteria in the water convert mercury to a toxic form “methylmercury” that is easily absorbed by animals. The mercury that is absorbed by animals then gets concentrated up the food web. Large predatory fish often have high levels of mercury because they eat lots of small mercury-containing fish. Scientists, health officials, and the public have become increasingly concerned about mercury accumulation in fish because the toxic mercury gets transferred to people when we eat seafood. Scientists have shown that too much mercury in people can pose health risks, particularly for developing fetuses and young, growing children.
However, what has puzzled scientists is that mercury levels in large predatory fish can vary greatly, with some fish having much higher levels than others.
The scientists at the University of Hawaii wondered if this variation could be related to the depth at which the fish live and feed. They knew that mercury concentrations in the ocean increased with depth. So maybe mercury concentrations in fish also increase with depth.
To test their theory, the scientists collected several common predatory fish (e.g. tuna, swordfish, mahi-mahi) and their prey from Hawaii waters in 2007 and 2008. They analyzed the stomach contents of the predator fish they collected to verify what they eat and the depth they feed at. Then they analyzed tissue samples of both the predator and prey fish to determine their mercury concentrations, and compared this to the common depth they live at.
The scientists found that mercury concentrations were indeed greater in deeper water fish compared to shallow water fish. Prey fish that lived at deeper depths had more mercury than prey fish that lived at shallow depths. And likewise, deeper feeding predator fish like swordfish, bigeye tuna, and opah (or moonfish) had higher mercury levels than predator fish that tend to feed only in shallow waters, like yellowfin tuna and mahi-mahi1.
The scientists’ hunch was right and now they had an explanation for why some predatory fish have higher mercury levels than others!
But the scientists study didn’t stop there. Now they wanted an explanation for why the deeper feeding fish had higher mercury concentrations. So next they teamed up with chemists from the University of Michigan to try to find an answer.
They further examined the levels and forms of mercury in the tissues of Hawaii predator and prey fish, using a complex analysis called “stable isotope analysis”. Their analysis would help identify where the mercury found in the fish was coming from and how mercury in the ocean is cycled.
What they discovered was that chemical reactions driven by sunlight can destroy up to 80% of the toxic methylmercury form – the form commonly absorbed by animals – in shallow waters of the Pacific Ocean. They also found that it is in the deep oxygen-poor water where bacteria are converting significant amounts of mercury to the problematic methylmercury form2. So it is in the deeper waters where most methylmercury enters the ocean food web. And this is why animals that live and feed in deeper waters have more mercury.
This new scientific research will help scientists continue to understand which fish have high mercury levels that could pose a health threat and which ones have lower levels.
As well, this new study has important implications because mercury concentrations in the ocean may change over the next few decades depending on future industrial mercury emissions, as well as climate change. Scientists predict that mercury levels will rise in intermediate depths of the Pacific Ocean. Higher mercury levels in deep waters could lead to increased mercury concentrations in fish.
If we want to keep our fish safe, scientists say we need to reduce global mercury emissions. We have made some progress on this. The European Union and recently the U.S. have made regulations that limit mercury emissions by coal-burning power plants and limit mercury use in products. There are also ongoing international discussions about regulating industrial mercury emissions around the globe. But currently mercury emissions remain high in places like China and India, which continue to burn large amounts of coal to support their growing economies and lack mercury pollution regulations.
To learn more, please visit our mercury in seafood page.
Also check out our recent blog “Eating Seafood: Health Boon or Health Threat”
1: C. Anela Choy, Brian N. Popp, J. John Kaneko, and Jeffrey C. Drazen. (2009). The influence of depth on mercury levels in pelagic fishes and their prey. PNAS 106:13865-13869. http://www.pnas.org/content/106/33/13865.full
2: Blum J. D., Popp B. N., Drazen J. C., Choy A. C. and Johnson M. W. (2013) Methylmercury production below the mixed layer in the North Pacific Ocean. Nature Geoscience. http://www.nature.com/ngeo/journal/v6/n10/full/ngeo1918.html?WT.ec_id=NGEO-201310
Elizabeth Brown is a research scientist at Blue Ocean Institute.