holding the opposite point of view, that of reduced resistance of fish to 

 poisons in the early stages of ontogenesis, had performed experiments with 

 inorganic poisons, primarily heavy metal salts. This indicated to us that 

 the seeming disagreement, concerning the level of toxicoresistance of 

 various stages of ontogenesis of fish, resulted in fact from the different 

 nature of the toxic substances studied and, consequently, the differences in 

 mechanism of action of the poisons, organic and inorganic in nature, on the 

 developing eggs, larvae and fry. 



Considering the importance of this problem, both in the theoretical and 

 in the practical aspects, we undertook an experimental test of this assump- 

 tion, concentrating our emphasis on organic poisons. Since in most works on 

 the age variation of ichthyotoxicology, authors have used some single 

 "point" of embryonal, larval or fry development, we decided to study the 

 dynamics of toxicoresistance in each of the three periods of early ontogene- 

 sis. In our report, we summarize the results of many years of studies per- 

 formed on bony fishes (rainbow trout, bream, zope, carp) and cartilagenous 

 ganoids (Russian sturgeon, Caspian sturgeon, sterlet and giant sturgeon). 

 The toxins used represented a broad range of concentrations of phenol, cer- 

 tain pesticides (metaphos, yalan and propanid), as well as chlorides of 

 cadmium and cobalt, in order to determine the age specifics of toxicoresis- 

 tance of the fish to inorganic poisons. 



In our initial experiments, performed jointly with V.M. Volodin and B.A. 

 Flerov on the eggs, larvae, fry and mature individuals of two systematically 

 similar species of the genus Abramis ; the bream (A. brama) and zope (A, 

 ballerus ), exposed to the toxic effects of 12 different concentrations of 

 phenol (from 1 to 5000 mg/liter), we found that the toxicoresistance of 

 mature fish was significantly lower than that of the eggs, embryos and lar- 

 vae (Volodin, et^ aj_. 1965, 1966). This was reflected both in the lethal 

 concentrations for fish of the various age groups, and in the time of sur- 

 vival of each of the age groups studied with identical or similar concentra- 

 tions of toxic substance. 



The decrease in resistance of fish to phenol from young age groups to 

 older age groups agrees with the available data from the literature; how- 

 ever, in these same experiments we found that, within each of the three main 

 stages of early ontogenesis; embryonal, larval and fry, toxicoresistance 

 undergoes significant changes. For example, the least stable period of em- 

 bryongenesis was found to be the earliest - from the beginning of division 

 to the formation of the embryo, particularly the stage of gastrulation. Be- 

 ginning with the early formation of the embryo, resistance to phenol greatly 

 increases. Suffice it to say that, with a phenol concentration of 100 

 mg/liter, zope eggs in the early stages of development die 8 times more 

 rapidly than in the stage of formation of the embryo. After emergence from 

 the shell, resistance of the embryos decreases greatly and embryos without 

 shells die in half the time as those still in the shells. The significant 

 decrease in the resistance of embryos after hatching from the shell indi- 

 cates the great significance of the shell, preventing penetration of the 

 poison and its accumulation in the organisms during the embryonal period of 

 development. 



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