Mercury Higher in Ocean Fish Then in Freshwater Fish

Mercury Higher in Ocean Fish Than in Freshwater Fish           

While it is true that freshwater concentrations of mercury are far greater than in ocean water, eating salt water / ocean fish poses a greater health threat to humans, according to a new Duke University study*.  

 It turns out that in freshwater, sunlight can more easily disintegrate the methylmercury which latches onto decayed plants and animal matter.   But in saltwater the methylmercury remains tightly bonded to the chloride.   In this form it is ingested and accumulates in the tissues of marine life, making sea water and ocean fish more toxic than fresh water fish.     

 So what’s the big deal? Fish and shellfish have a natural tendency to store methylmercury in their organs, which makes them the leading source of mercury ingestion for humans. Some say contaminants found in fish can last years in the human body – hence a warning to women of child bearing age.  A potent neurotoxin, methylmercury can cause kidney problems, neurological disorders, and even death, says Heileen Hsu-Kim, PhD, an assistant professor of civil and environmental engineering at Duke’s Pratt School of Engineering. 

 Methylmercury is particularly harmful to children because their brains and nervous systems are still growing.  Cognitive thinking, memory, attention, language and fine-motor and visual spatial skills can be affected.  In light of this fact, the U.S. Environmental Protection Agency identified four fish (shark, swordfish, king mackerel or tilefish – the list should be longer) which are not safe for children or women who may become pregnant, are pregnant or are nursing. 

 Nine fish the Environmental Defense Fund finds unsafe for anyone (as of 4/13/09):

            Alewife

           Bass, striped (wild)

            Bluefish, Croaker (white)

            eel (American)

            Eel (European)

            Shad

            Sturgeon, wild (imported)

            Weakfish

Close behind on the list are:

            Mackerel, king

            Tuna, bluefin

            Marlin

            Shark

            Swordfish

 Dr. Hsu-Kim believes scientists and policymakers should focus on the effects of mercury in oceans instead of freshwater.  (Currently, the Environmental Protection Agency and Food and Drug Administration make no distinction.)   The Global Biogeochem Cycles journal published a study which would also support this direction, having found increases of methylmercury in the Pacific Ocean.

 Are there any mercury-free fish left?   While eating seafood is our main source of exposure to methylmercury, it is also our best source of omega 3’s.  Even though most contaminant levels are on the rise, we can still make informed decisions.  Generally speaking, the smaller the fish, the less mercury it has. There are several guides to fish consumption choices to which the Advanced Wellness Center would add its recommendation to stay away from the bottom feeders regardless of mercury levels.    Shellfish tend to harbor parasitic organisms which also negatively affect the human body.  Until the Hsu- Kim study, wild atlantic salmon and sardines were thought to be amongst the best fish to eat.    You decide.  

 Guides to Fish Consumption:

Green Guide Fish Picks

Environmental Defense Fund Seafood Selector Health Alerts (www.edf.org/page.cfm?tagID=17694)

EPA Fish Advisories (has links to seafood advisories specific to your area).

Sources:

Duke University News & Communications, June 22, 2010

Organic Authority June 30th, 2010 – Barbara Feiner

EPA & FDA Fish Advisories

Mother Nature Network, Is There Such a Thing as Mercury-Free Fish? 12/2008

¹ Sunderland E. M., Krabbenhoft, D. P., Moreau, J. W., Strode, S. A. & Landing, W. M., Global Biogeochem Cycles (a journal of the American Geophysical Union) doi:10.1029/2008GB003425 (2009).