MERCURY IN TUNAS: A REVIEW 



C. L. Peterson, W. L. Klawe, and G. D. Sharp' 



ABSTRACT 



Mercury in the aquatic environment comes from both natural processes and industrial 

 activities. The latter probably have not significantly altered the mercury content of 

 the high seas where most tunas are captured. 



Mercury compounds enter aquatic organisms through the food web and/or by direct 

 extraction from solution. The relative importance of these pathways in tunas is unknown. 

 Mercury occurs in tuna principally in the form of melhylmercury. 



Generally, tunas appear to have higher mercury levels than those fish species which 

 occupy a lower level in the food chain. 



Mercury content of tunas varies according to fish size. However, other factors such 

 as area of capture, differential growth rates, varying analytical techniques, and different 

 sampling methods may account for some of the observed variation. 



The U.S. Food and Drug Administration has established an "in-house" standard of 

 0.5 ppm of mercury for fishery products sold in the United States. Other countries 

 have established limits as high as 1.0 ppm. 



Seawater contains a wide array of dissolved 

 elements and salts including mercury. Some 

 of these occur in quantities that can be re- 

 covered on a commercial scale whereas the 

 minute concentrations of others can be de- 

 tected only by highly sophisticated techniques. 

 Mercury does not play a prominent role among 

 the metals dissolved in seawater because it 

 occurs there only in very small quantities. 

 Mercury in its various forms is also present 

 in the earth's crust from which it can be 

 dissolved by water. In the absence of water, 

 especially under conditions of increased heat, 

 mercury will enter the atmosphere in the 

 form of vapor, from which it can dissolve 

 in water at the water-air interfaces, i.e., rain 

 or the sea surface. It is difficult to conceive 

 that life could have evolved on this planet 

 in an environment devoid of this ubiquitous 

 element so it is of no great surprise that 

 mercury is present in the bodies of living 

 organisms. Furthermore, as is the case with 

 many elements and their compounds, living 

 organisms are even capable of concentrating 

 mercury within or outside of their bodies. 



' Inter-American Tropical Tuna Commission, Scripps 

 Institution of Oceanography, La Jolla, CA 92037. 



Tunas are no exception — mercury occurs in 

 various parts of these pelagic species in con- 

 centrations substantially exceeding those dis- 

 solved in the waters of the world oceans. 



An epidemic of deaths and serious physical 

 impairment among Japanese during the 1950's 

 and 1960's (which was later related to the 

 ingestion of seafood contaminated by mercury 

 from local industries), the discovery of high 

 levels of mercury in freshwater fishes and 

 wildlife in Sweden in the 1960's, and the 

 discovery of unusual mercury concentrations 

 in fishes from parts of the Great Lakes system 

 in 1970 led investigators to examine mercury 

 in tunas and billfishes. The findings resulted 

 in the seizure of some tunas and swordfish 

 in the United States whose mercury content 

 exceeded government standards, and signifi- 

 cantly affected fishing strategy and marketing 

 of these species. 



SOURCES OF MERCURY IN 

 THE MARINE ENVIRONMENT 



Mercury is most often encountered in nature 

 as mercury sulfide or cinnabar. Other com- 

 pounds and metallic mercury contribute a 

 small percentage of the total available mercury. 



Manuscript accepted February 1973. 

 FISHERY BULLETIN: VOL. 71, NO. 3, 1973. 



603 



