Thus, to be reliable, the index applicable to the rapid determination of 

 biological effects of pollutants must take into account the peculiarities of 

 both compounds and organisms. An example of one such approach to the prob- 

 lem can be found in the relationship of toxicity of organic in compounds in 

 fish to values of their concentration gradients on the blood-brain barrier 

 (Filenko and Parina, In press). It may be assumed that compounds of a homo- 

 logous series have equally effective toxic potentials, but varying tissue 

 accumulation capabilities, and that this is the principal reason for differ- 

 ent resulting toxicity. 



However, such general biological indices as survival and fecundity are 

 still the most reliable. To decrease the time required for assessment of 

 toxicity of a compound, instead of using the more reliable chronic experi- 

 ments, acute toxicity tests of the compounds over a period of 24-96 hours 

 usually are used. Application of such data for other conditions, concentra- 

 tions, and species specific coefficients and factors can be used (Steinberg 

 1974). This approach is primarily useful as a quick screening methodology. 

 When experiments are shortened, a portion of the reliability of response can 

 be retained by increasing the number of experimental tests. Therefore, it 

 becomes a question of the acceptability of the degree of simplification of 

 conditions, and the reduction of the length of the experiment to that which 

 is essential, and which involves a sufficient number of tests to make a 

 reasonably reliable estimation of the probable effect of the material on 

 the specific index in question for a period which exceeds the length of the 

 time of observation. 



An attempt to investigate aspects of this problem and some associated 



difficulties, are described in this paper. It should be noted, however, 



even the most carefully made predictions cannot equal the reliability of re- 

 sults from experimental verification. 



METHODS 



The experimental design utilized the water flea, Daphnia magna (Straus) 



in densities of 10 animals per 500 ml. The toxicity of individual compounds 



that are potential industrial and agricultural pollutants of water was 



assessed. The calculation of regression equations was made by the least 

 squares method. 



RESULTS AND DISCUSSION 



The toxic effect of compounds on Daphnia was assessed by organism sur- 

 vival. The typical mortality curve for varying concentrations of compounds 

 are shown in Figure 1. To demonstrate the regularity of this phenomenon, 

 the coefficients for different equations that could describe the mortality 

 of Daphnia in time were calculated. The results of such calculations for 

 trimethyl tin chloride (TMTCh) are given in Table 1. The exponential, 

 power, logarithmic, and parabolic functions were calculated. Tne fit of 

 theoretical and experimental points was examined using correlation coeffi- 

 cients. The larger the coefficients, the greater the correspondence to a 



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