During subsequent ontogenetic development, resistance of fish to phenol 

 continues to drop. The survival time of zope larvae in the stage of mixed 

 feeding in phenol solutions of 100 and 150 mg/liter was found to be 48 and 

 30 hours, respectively. This is 1/5 the time of survival of the embryos in 

 the stage of beginning of pulsation of the heart (240 hours) and 1/2 the 

 time of survival of hatched embryos. Whereas, during the embryonal period 

 of development, the toxicoresistance of the zope undergoes significant 

 changes throughout the entire larval period of development; i.e., at the 

 beginning, middle and end, it remains more or less at the same level. Then, 

 in the early fry period of development, the stability of the zope to phenol 

 drops greatly (by a factor of more than 10) and the mean survival time in 

 phenol solutions of 150 and 100 mg/liter becomes 2-3 hours. However, the 

 least resistance was noted for mature zope, which survived only 6-8 hours in 

 a phenol solution of 25 mg/liter, i.e., 1/4-1/6 the concentration used in 

 the experiments with the fry. Let us recall that the eggs, embryos and lar- 

 vae survive and develop without any significant deviations from the norm in 

 a solution of this concentration. In order to cause death of eggs in this 

 same time interval, the concentration of phenol must be increased to 1000 

 mg/liter, i.e., by a factor of 40. 



Thus, the resistance of the zope in the early stages of ontogenesis to 

 one of the most widespread organic poisons, phenol, undergoes significant 

 changes. The least resistance is that of the eggs in the stage of gastrula- 

 tion; the greatest, that of the eggs in the stage of pulsation of the heart. 

 Subsequently, the level of toxicoresistance decreases continually from 

 hatching embryo to larvae, from larvae to fry and fry to adults. An analo- 

 gous variation was observed in experiments with eggs, hatched embryos, lar- 

 vae, fry and mature individuals of another species of the genus Abramus , the 

 common bream. 



In experiments with still another species of carp ( Carassius carassius ), 

 we succeeded in comparing the toxicoresistance of four age groups: current 

 year's brood, 1-, 2- and 3-year fish (Lukyanenko and Flerov 1963). The 

 criterion of resistance was the time of survival of experimental fish in 

 toxic solutions of phenol (17-800 mg/liter). As was to be expected, the 

 most resistant carp was the current year's brood, which survived many times 

 longer than older fish. For example, in a phenol solution of 50 mg/liter, 

 the mean survival time of the current year's brood was 137.4 hours, of fish 

 1-2 years old - 34.9 hours, of fish which had completed 2 years of life - 

 12.4 hours, of fish over 3 years old - 5.7 hours. Analysis of these mate- 

 rials indicates that the survival time of the current year's brood in com- 

 parison to carp 1+ years old is 3.9 times greater, than that of carp 1+ 

 years old in comparison to carp 2+ years old 2.8 times greater. The dif- 

 ference between the next two age groups (2+ and 3+ years) is still less, a 

 factor of 2. The impression is gained that, as age increases, the resist- 

 ance of the fish, after reaching a certain level, undergoes only moderate 

 changes. However, there is no doubt that fish in the younger age groups are 

 more resistant to phenol than fish in the older age groups. 



This is also indicated by the results of a comparative study of the 

 level of toxicoresistance of the current year's brood and two-year-old rain- 

 bow trout ( Salmo irideus Gibb) which we performed (Lukyanenko and Flerov 



159 



