Summing up our report on the age specifics of the sensitity and resist- 

 ance of fish to poisons, I would like to draw the attention of participants 

 in the symposium to still another "jery important, in my opinion, question. 

 I am speaking of the great need for a clear delineation between the concepts 

 of "sensitivity" and "resistance" of fish to poisons, which are quite dif- 

 ferent in their physiologic and toxicologic significance (Lukyanenko 1967). 

 Unfortunately, quite frequently in both domestic and foreign literature, the 

 concept of sensitivity and that of resistance of hydrobionts to various fac- 

 tors in the aquatic environment, as well as toxins, are either identified or 

 sensitivity is considered to be the reverse of resistance. The use of these 

 concepts as synonyms can lead and does lead to negative results, including 

 difficulty in understanding the degree of scientific foundation of the con- 

 clusion of various authors who have estimated the age differences of toxi- 

 coresistance of fish. 



There is a generally agreed idea, concerning the meaning of the concept 

 of resistance of an organism to abiotic factors in the environment, concern- 

 ing toxins of various natures. An estimate of the degree of resistance is 

 based either on the concentration of the substance causing death of a cer- 

 tain percentage of experimental animals (LC50 or LC]on) in a certain period 

 of time (24-48-96 hours or more), or the time of survival in a toxic solu- 

 tion of a predetermined concentration. Resistance is the capacity to sur- 

 vive low concentrations of a toxic substance for longer periods of time, or 

 to survive higher concentrations of the same substance for a fixed short 

 period of time by the operation of various regulatory mechanisms. Quite 

 understandably, the earlier these regulatory mechanisms are brought into 

 play (detoxication, excretion of the substance, etc.), supporting short-term 

 or long-term adaptation of the organism to the toxic agent, the longer will 

 be the time of survival of the organism and the more probable that, in the 

 case of interruption of the toxic effect on the organism, it will survive. 

 However, it is also obvious that regulatory mechanisms will be brought into 

 play earlier, the more sensitive the organism is to the toxin at the given 

 stage of individual development. 



In terms of their physiologic content, the concept of "sensitivity" is 

 close to or coincides with the concept of "excitability", the level of which 

 determines the threshold of excitability. In turn, a measure of excit- 

 ability is the minimum force of an irritant; in this case a chemical factor, 

 which exceeds the threshold of irritation. The greater the minimum force 

 of the chemical irritant necessary to call forth a reaction, the higher the 

 threshold of irritation, the lower the excitability, the lower the sensi- 

 tivity of the organism to the substance in question. Quite understandably, 

 the lower the threshold of irritation, the higher the excitablity, and the 

 higher the sensitivity. This is a generally known physiologic truth, in 

 light of which we must analyze the question of sensitivity of the organism 

 or cell to a toxic irritant. It follows from all of this that, in order to 

 estimate the level of sensitivity of the organism to a given toxin, the 

 question of the primary reaction of the organism to this irritant is of pri- 

 mary significance. I propose that there is no need to prove that neither 

 the concentration of the substance causing the death of a certain percentage 

 of experimental fish, nor the time of survival of fish at a fixed concentra- 

 tion, can be used in any way as an indication of the primary reaction to a 



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