bility of an organism. Sublethal concentrations 

 of such poisons can also destroy one or several 

 links of the food chain in the sea, and so affect 

 the food supply for the population of animals or 

 plants important for human welfare. The normal 

 ecological environment may be so changed that 

 some planktonic organisms, most useful to shell- 

 fish as food, disappear and are replaced by a 

 luxurious growth of microorganisms not only 

 useless but even harmful to water-filtering mol- 

 lusks. Although great advances have been made 

 in the technique of bioassays, the results of 

 short-term tests lasting no longer than 72 hours 

 are of little use in determining the effects of 

 prolonged exposures of fish or shellfish to low 

 concentrations of poison. Furthermore, since the 

 criteria for the welfare of marine populations are 

 not known, it is impossible to set requirements for 

 purification of pollutants before they are permitted 

 to be discharged into the sea. The Federal 

 Water Pollution Control Act of 1961 authorizes 

 the Secretary of Health, Education, and Welfare 

 to organize comprehensive programs of investi- 

 gation which in the course of years will solve many 

 of the e.xisting pollution problems. 



Detailed descriptions of all types of pollution 

 that may affect the productivity of oyster bottoms 

 and methods of their detection and control are 

 beyond the scope of the present chapter, which is 

 limited to a discussion of the general principles 

 apphcable to the majority of situations and to a 

 description of the most important tj'pes of pollut- 

 ants encountered on oyster bottoms. Bibho- 

 graphical references listed at the end of the 

 chapter are limited to the more pertinent papers. 

 Discussions of more specialized pollution problems 

 are listed in a bibliograph}' prepared by Ingram 

 (1957) and also appear in papers published in 

 Tarzwell (1957, 1960). 



The production of oysters in the United States 

 is declining at a rapid rate (Galtsoff, 1956). As a 

 sedentary animal devoid of any means of loco- 

 motion after setting, the oyster is vulnerable to 

 environmental changes which weaken it and make 

 it less resistant to infection. Under natural 

 conditions unspoiled by human activities, the 

 oyster is in an equilibrium with its environment; 

 this adjustment, which may be called a steady 

 state, is the result of thousands of years of 

 adaptation and natural selection. It may be 

 upset by the sudden presence of materials not 



normally found in sea water or by excesses or 

 deficiencies of its normal components. 



Two types of pollution are commonly found on 

 03'ster grounds: domestic sewage and trade wastes. 

 In natural waters both types of pollutants undergo 

 gradual changes which lead to a degree of purifi- 

 cation, but at the same time deposit sediments 

 that cover oyster beds and change the character 

 of the bottom. Natural purification is not effec- 

 tive, however, in the case of detergents and radio- 

 active waste, which constitute a growing menace 

 to the safety and purity of our coastal waters. 



Domestic sewage 



Contamination of water by domestic sewage is 

 the oldest type of pollution; it probably began 

 during prehistoric times when man settled on the 

 shores of the rivers and bays and used natural 

 waters as the easiest and most convenient way of 

 disposing of the excrements and unwanted waste. 

 The problem has reached enormous proportions 

 with the population growth and the necessitj' of 

 disposing of quantities of domestic sewage in an 

 organized manner. 



The discharge of untreated domestic waste has 

 a threefold effect. It covers the bottom with a 

 sludge which smothers the oyster bed, affects the 

 normal functions of moUusks by reducing the 

 oxygen content of the water, and at the same time 

 greatly increases the bacterial content of the water. 

 Oysters, in common with other water-filtering 

 mollusks, retain and accumulate these bacteria in 

 their bodies. The degree of pollution is deter- 

 mined by the abundance of Escherichia coH found 

 in the water. The bacterium itself is not patho- 

 genic, but is used as an index of pollution. Pro- 

 cedures for determining the abundance of E. coli, 

 the so-called MPN (most probable number), are 

 described in great detail in Jensen (1959). They 

 are strictly followed by State and Federal Public 

 Health Officers and other officials responsible for 

 certifying grounds from which shellfish may be 

 liarvested for human consumption. Areas in 

 which the MPN of E. coli exceed the permissible 

 maximum of 70 per 100 ml. are condemned 

 and cannot be used for harvesting, but under 

 certain specified conditions the polluted oysters 

 and clams can be taken for planting to an unpol- 

 luted area. The presence of E. coli above the 

 prescribed MPN eliminates tlie utilization of 

 grounds for commercial fishery, but does not affect 

 the survival and growth of the oyster population. 



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FISH AND WILDLIFE SERVICE 



