1956) using the phenol intoxication model. The elevated resistance of the 

 current year's trout brood in comparison to 2+ year old individuals was re- 

 flected both in the absolute values of CLM (minimal lethal concentration), 

 CMT (maximum tolerant concentration) and LC50 (concentration causing death 

 of 50% of experimental fish), as well as the mean time of survival at all 

 concentrations of phenol tested (5, 7,5, 12.5, 15, 20, and 25 mg/liter). In 

 experiments with the current year's brood, the CLM was 15 mg/liter, LC50 - 

 11 mg/liter, CMT - 7.5 mg/liter, while in experiments with 2+ year old fish 

 the figures were 10 mg/liter, 7.5 mg/liter and 5 mg/liter, respectively. 

 The differences between two age groups of trout in terms of time of survival 

 at a given concentration of phenol were still more sharply expressed. The 

 mean time of survival of two-year-old trout in a phenol solution at 12.5 

 mg/liter was only 95 minutes, i.e., less than 1/6 the survival time of the 

 current year's brood - 601 minutes. No less demonstrative were the differ- 

 ences found in comparison of times of survival of the current year's brood 

 (272 minutes) and two-year-old fish (40 minutes) in a solution of 15 

 mg/liter phenol, survival being almost 7 times longer for the current year's 

 brood. 



The increased resistance of younger age groups, which we found in our 

 experiments with phenol in highly resistant carp and more susceptible trout, 

 indicates that what we have here is a general regularity of reactions of 

 fish of different levels of organization to organic poisons. In order to 

 test this assumption, we performed experiments (Kokoza 1970) on fry, 35-70 

 days of age, of three species of sturgeon: the Russian sturgeon, Caspian 

 sturgeon and sterlet, representing the evolutionarily more ancient group of 

 cartilagenous fish. The experiments involved phenol at 50 mg/liter. We 

 will not take the time to present the results of this series of experiments 

 in detail, but rather shall note only the clearly expressed specific differ- 

 ences in the level of toxicoresistance, manifested in the fry period of 

 development. The mean survival time of 40-45 day old fry of Russian stur- 

 geon (12 hours 24 minutes) was 4 times greater than that of sterlet fry of 

 the same age (3 hours 05 minutes), and 2.6 times greater than that of 

 Caspian sturgeon of the same age (4 hours 40 minutes). Sexually mature 

 Russian sturgeon, which survived in a phenol solution of 40 mg/liter for 5 

 hours 30 minutes, were also characterized by higher toxicoresistance in com- 

 parison to the Caspian sturgeon (1 hour 20 minutes) and sterlet (1 hour 35 

 minutes) (Lukyanenko 1967). 



However, in this case, we would like to concentrate our primary atten- 

 tion, not on the specific differences of toxicoresistance of the sturgeons 

 during their fry period of life, but rather on age differences, i.e., to 

 compare the time of survival of mature individuals of each of the three 

 species and 1-2 month fry of the same species. This comparison showed 

 clearly that the resistance of mature fish, as indicated by survival time in 

 phenol solutions of similar concentrations (40 and 50 mg/liter), is only 1/2 

 to 1/3 the resistance of fry. In other words, the conclusion which we have 

 reached, that of decreasing level of resistance of fish with increasing age 

 in terms of organic poisons, is true not only for the evolutionarily young 

 and highly organized bony fish, but also for the cartilagenous fish, lower 

 on the evolutionary scale. 



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