FISHERY BULLETIN: VOL. 71, NO. 3 



A number of natural processes release a con- 

 tinuous but relatively minor amount of mer- 

 cury into the soil, groundwater, and air 

 around deposits of mercury compounds. This 

 effusion and volatilization of mercury is the 

 first of many steps in the generation of its 

 compounds and their subsequent entry into 

 bodies of water including the oceans. This 

 mercury is oxidized or recompounded in the 

 environment, deposited in the sediments if 

 insoluble or dispersed in the water if soluble. 

 Rainfall causes erosion of soils and facilitates 

 the mobilization of the mercury into the aquatic 

 environment. 



The second (and controllable) source of mer- 

 cury in rivers, lakes, and oceans stems from 

 the activities of man. Mercury compounds have 

 been used for millennia as a coloring agent, 

 as medicine, and for a multitude of industrial 

 uses. The most common industrial sources of 

 mercury contamination can be divided into 

 several classes. Fossil fuels, i.e., coal and oil, 

 have relatively low levels of mercury, but as 

 millions of tons of these fuels are used each 

 year, they release a substantial amount of 

 mercury into the atmosphere which eventually 

 enters the aquatic environment. Factories pro- 

 ducing plastics, chlorine, caustic soda, and/or 

 caustic potash have been charged with one of 

 the more obvious and largest discharges of 

 mercury and its compounds into rivers, lakes, 

 and oceans. The fungicidal properties of mer- 

 cury compounds have made it useful in pre- 

 servation of paint and as a coating for seeds 

 used in planting of crops. Mercury compounds 

 also have slimicide properties which have 

 brought about their extensive use in pulp mills 

 and other industries. Gold, silver, and rare 

 metals can be and are extracted by using 

 metallic mercury in an amalgamation process 

 in which the final step is driving off the 

 mercury by heat to leave the other metals 

 behind. The subsequent condensation of the 

 mercury vapors is not 100% effective, and 

 thus some of it enters the environment. Mer- 

 cury contamination also results from many 

 other industrial processes. However daily house- 

 hold, laboratory, and small business activities 

 contribute considerable quantities of mercury 

 and its compounds to sewage and sludge stores 



by the breakage of thermometers and electric 

 switches, discarding of or excretion of mer- 

 curials used for medication, and other seemingly 

 unimportant events. 



Studies show that the contaminant mercury 

 is usually concentrated near the sites of outfall 

 and that the levels decrease with distance 

 from the sources. 



MERCURY IN FISHES 



Methylmercury is one of the mercury com- 

 pounds most often found in the bodies of 

 fishes and other aquatic organisms (Westoo, 

 1966; Johnels et al., 1967). This compound is 

 known to be readily derived from inorganic 

 or metallic forms usually by biological inter- 

 mediaries (Jernelov, 1972a). Ionic mercury 

 binds readily to organic materials and can be 

 converted to methylmercury through bacterial 

 activity. Generally speaking, mercury in sedi- 

 ments of water bodies is converted faster under 

 aerobic conditions than in sediments with low 

 oxygen levels or where anaerobic conditions 

 prevail. Methylmercury can readily enter the 

 complex aquatic food chain which may include 

 many levels of concentration. 



Mercury compounds, regardless of whether 

 they originate from natural activities or are 

 introduced into the aquatic environment as 

 contaminants, can enter fish via two pathways 

 (Raeder and Snekvik, 1941; Jernelov, 1972b). 

 The most easily understood path is through the 

 food chain or the food web. All studies on 

 the food chain show a stepwise mercury con- 

 centration increase at each subsequent level, 

 culminating in the large predatory organisms, 

 i.e., tunas, billfishes, and sharks. The com- 

 plexity of the food webs, i.e., number of steps 

 and number of interrelations, and the metabolic 

 requirements of the various members, usually 

 limit the amount of mercury found at each 

 level. 



Another pathway, the relative importance 

 of which is poorly understood, is direct extrac- 

 tion of mercury from solution in the aqueous 

 media due to the affinity of mercury ions for 

 proteins. The possibility of the direct absorption 

 of mercury from water was postulated back 

 in 1941 by two Norwegian scientists, Raeder 

 and Snekvik (1941). The most obvious type 



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