PETERSON, KLAWE, and SHARP: MERCURY IN TUNAS 



of absorption would be through the gills, but 

 it was also found that methylation of inorganic 

 mercury takes place in the slime of some 

 fishes (Jensen and Jernelov, 1969). The ultimate 

 fate of methylmercury formed this way has 

 not been investigated to the best of our knowl- 

 edge, but because the organic forms of mercury 

 have an affinity for lipids, perhaps the methyl- 

 ated form also enters the tissues of the fish. 

 It should be pointed out that research on 

 freshwater fishes demonstrates that methyla- 

 tion in fish slime is accomplished mainly, if 

 not exclusively, by certain microorganisms. 



Tunas and billfishes, which have high meta- 

 bolic rates and are extremely mobile species, 

 filter many thousands of liters of water over a 

 short period of time. Thus the possibility of 

 branchial extraction of mercury may be of 

 importance to the buildup of this substance 

 in tunas, especially in regions where the relative 

 concentrations of dissolved mercury are high. 

 The existence of specific oceanic areas with 

 high concentrations of mercury throughout the 

 water column has been demonstrated (Weiss 

 et al., 1972). As food organisms would also 

 tend to have greater mercury content in such 

 regions, it is readily conceivable that a dispro- 

 portionate increase in mercury levels could be 

 detected in tunas from such regions. Although 

 areas with high volcanic activity would be sus- 

 pected of having higher amounts of dissolved 

 mercury, there are no data to verify this. Indeed 

 very little is known about geographic variations 

 in concentration of mercury in the world oceans. 

 Furthermore, it is not yet known which of the 

 two pathways responsible for the presence of 

 mercury in tunas is more important. From ex- 

 periments with other fishes it has been demon- 

 strated that the mercury in some species origin- 

 ates principally from their food whereas in 

 other fishes the mercury originates mainly from 

 mercury dissolved in water (Jernelov, 1972b). 



Research dealing with types of mercury in 

 fishes indicates that in most instances practically 

 all of the mercury accumulated is in an 

 organic form, namely methylmercury (Westoo, 

 1966, 1967). The same seems to be true of 

 the tunas and swordfish, Xiphias gladius. A 

 recent analysis (Kamps, Carr, and Miller, 1972) 

 of the total mercury/methylmercury relation- 



ship, based on 11 samples of canned tuna and 

 20 samples of frozen swordfish steak from the 

 U.S. market, shows that mercury in the edible 

 portions of these fish is essentially all methyl- 

 mercury. However, it should be noted that 

 Pacific blue marlin, Makaira mazara, from 

 Hawaii were found to have total mercury levels 

 ranging from 0.35 to 14.0 ppm- whereas the 

 organic mercury for the same samples ranged 

 from 0.23 to 1.79 ppm (Rivers, Pearson, and 

 Shultz, 1972). Furthermore, studies on the 

 methylmercury and total mercury relationship 

 in lake trout, Salvelitius namaycush, from 

 Cayuga Lake, N.Y., indicate that 31-35% of 

 total mercury in 1-year fish is methylmercury 

 (Bache, Gutenmann, and Lisk, 1971). By the 

 fourth year, methylmercury in lake trout made 

 up 70% of the total. In 12-year olds, values 

 ranged from 67 to 88% . These findings, as well 

 as some other studies undertaken by a Japanese 

 researcher (Ui, 1971), indicate that the organic 

 to total mercury ratio may not be consistent 

 among or within species and that the relative 

 quantities of the two forms should be further 

 studied. 



Not all mercury which enters the body of a 

 fish is retained. For example, the uptake of 

 mercury from food ingested by northern pike, 

 Ehox lucius, a freshwater fish, does not exceed 

 20% (Jernelov, 1972b). Experiments carried out 

 recently by the National Marine Fisheries 

 Service indicate that an even lower figure is 

 applicable to skipjack tuna, Katsuwonus pelamis, 

 (Lasker and Leong, 1972).'^ It has been shown 

 that some fish have the ability to rid them- 

 selves of some of the mercury which enters 

 the tissues of their body. The mercury excre- 

 tion rate for tuna is unknown, but for some 

 other species of fish the biological half-life 

 for methylmercury is known to be on the order 

 of 2 yr (Jarvenpaa, Tillander, and Mietinen, 

 1970; Lockhart et al., 1972). 



- Parts per million by wet weight, i.e., 1 mg of 

 metallic mercury per kilogram of wet weight corresponds 

 to 1.0 ppm. 



•* Lasker, R., and R. L. Leong. 1972. Uptake and 

 excretion of mercury by skipjack tuna in aquaria. South- 

 west Fish. Cent., Natl. Mar. Fish. Serv., NOAA, La Jolla, 

 CA 92037. (Unpubl. manuscr.) 



605 



