was demonstrated by Teichmann (1957). The eel showed a reaction to this 

 substance at concentrations as low as 3 x 10-20 mg/1, i.e., when only 2-3 

 molecules of the substance could be present in the olfactory bulb. 

 The fish discern perfectly well the smells of many aquatic plants (Walker 

 and Hasler, 1949), as well as the smells of fish and other vertebrates 

 (Von Frisch, 1941; Sliultz, 1956; Walker and Hasler, 1949). In most cases 

 the sensitivity of fish to the smells of substances excreted by closely 

 related species is greater than to those of taxonomically remote species. 

 The repellent effect on fish may be produced by substances excreted from 

 the skin of other classes of vertebrates. Thus, buffotoxin extracted from 

 the skin of adult toads is preceived by fish, and produces a repellent 

 effect even at a dilution of 1:2.4x10^. These are but a few of the 

 reported studies noting the extremely high sensitivity of fish to changes 

 in smell and taste of water which they inhabit. 



Such a high sensitivity of fish to the organoleptic properties of 

 water can not but affect the distribution of fish in a water-body. It is 

 not difficult to imagine that tens and hundreds of substances, mostly of 

 an organic nature, entering natural waters with sewage may produce a re- 

 pellent or an attracting effect on fish changing feeding, wintering, and 

 spawning conditions; causing unnaturally high concentrations of fish in a 

 limited area, driving fish away from food, and thus making it difficult to 

 use the nutrient base, and reducing the bio-productivity of a whole water- 

 body. All of these complicated and intricate manifestations of the "ecolo- 

 gical ill-health" of a water body receiving organic substances, even in 

 strict conformity with the established standards, may be properly con- 

 sidered only on the basis of the knowledge of the reactions of avoidance of 

 the poisons, which change taste and smell of the water and food organisms. 

 Therefore, one of the main tasks in the field of experimental aquatic toxi- 

 cology is the thorough study of behavioral reactions. This study must in- 

 clude the reactions of detection and avoidance of chemical agents, the 

 study of mechanisms and the character of these reactions (repellent or 

 attracting), and the resolving power of the olfactory and gustatory organs 

 in fish. In summary, the idea is to establish physiological foundations 

 for a wide application in biological standards which have been well esta- 

 blished in the sanitary-hygienic standards related to chemical pollution 

 of water-bodies. At the same time, the study of avoidance reactions in 

 fish will enable an understanding of the peculiarities of distribution of 

 fish in water-bodies which are "loaded" with waste waters in accordance 

 with the biological standards. It should be noted that the distribution 

 of indicator organisms, fish included, may serve as one of the indices of 

 "ecological health" of a water-body. Thus, reference is made to the 

 "ecological establishment of the standards" or, to be more correct, to the 

 ecological principles of evaluation of the efficiency of biological 

 setting of standards. 



So, the ecological principle is very important for the evaluation of 

 the effectiveness of biological setting of standards for substances 

 entering waters. Even the most thorough observations and detailed descrip- 

 tions of the changes in aquatic ecosystems are not able to reveal harmless 

 concentrations of chemical substances discharged into a water-body, or to 

 establish what substance or a group of substances cause the noticed changes 



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