ation of its discharge into receiving waters. This way is not applicable 

 to the diffuse discharge from the agricultural lands. Here the method is 

 to determine the toxicity for aquatic organisms of each of the respective 

 toxicants, and prohibit or restrict the use of pesticides characterized by 

 a very high toxicity to fishes and aquatic invertebrates. The list of 

 pesticides used in modern agriculture is extensive, and the search for new 

 ones is so rapid that thousands of herbicides alone are patented every 

 year. It is not difficult to realize how labourous the task of preliminary 

 determination of toxicity of new pesticides is, especially if one accounts 

 for the present empirical approach of the water toxicologists to its solu- 

 tion. 



Currently, aquatic toxicology does not enjoy the general theory of the 

 action of pesticides on a living cell and the organism as a whole. In 

 both domestic and foreign literature, there are still very few studies de- 

 voted to the mechanisms of action of toxicants upon the cell, subcell, and 

 molecullar structures. Without an understanding of these mechanisms, it 

 is impossible to comprehend the development of toxicologic processes 

 brought about by different groups of toxicants. Nevertheless, it is this 

 understanding of toxicologic processes that must be the basis for choosing 

 the methods of evaluation of the toxicity of a substance, or a group of 

 structurally related substances. This understanding is also essential for 

 the determination of the maximum permissable concentrations (MPC) of these 

 substances in natural waters. The accepted practice in the UbSR 

 demonstrates that one of the most efficient means of providing protection 

 of waters from pollution is hygienic and fisheries standardization, i.e., 

 the establishment of maximum permissible concentrations (MPC) for toxic 

 substances entering water bodies. It is not merely coincidence that both 

 the medical profession and biologists have arrived at this solution. 

 Ichthyologists, hydrobiologists, sanitary and hygiene medical personnel 

 face the same problem, i.e., assuring clean natural waters, preventing 

 such a degree of pollution as to cause poisoning of animals and human 

 beings, and alteration of the normal course of biological processes 

 determining productivity of the waters, and their "self-purification" 

 which render water drinkable. It is quite understandable that the degree 

 of toxicity of a substance for animals, fish, and aquatic invertebrates 

 can be established only under experimental conditions. Harmless 

 concentration of a given substance for a given organism may be found in 

 the same way. The sensitivity and resistance of various land animals to 

 toxic substances is not the same as for aquatic organisms, thus, the 

 sanitary-hygienic and fisheries requirements for quality of water related 

 to toxic substances are different. 



Experimental data accumulated to date clearly show that the values of 

 the MPC of many substances for aquatic organisms, especially for fishes 

 (fisheries MPC), are lower, i.e., more "stringent" than for warmblooded 

 animals and humans (Sanitary-hygienic MPC). For example, the fisheries 

 MPC of copper (0.01 mg/1), nickel (0.01 mg/1) and zinc (0.01 mg/1) are 

 only one hundredth as high as the sanitary-hygienic MPC of the same metals 

 (1 mg/1). Similary, the toxicity of many organic substances, for fishes 

 and aquatic invertebrates (especially pesticides) is hundreds of times as 

 high as that of the warmblooded animals. The cause of these differences 



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