The difference in resistance of aquatic invertebrates is determined by 

 many factors. The most important of these are morpho-physiological (size 

 of external coverings, their permeability, peculiarities of their respira- 

 tory system, their surface) and behavioral (ability to avoid toxicant, 

 general activity in a toxic environment) characteristics. 



When there are sublethal concentrations (less 5 mg/ liter) on Daphnia 

 longispina , behavior and llt C trace food consumption is initially disrupted, 

 and later on embryogenesis and fertility are disrupted. Thus, physiologi- 

 cal indices in intertebrates are good specific tests for water toxicity. 



FISH 



Three stages of phenol intoxication in fish have been observed: (1) 

 disorderly general motor activity, (2) loss of equilibrium and, (3) cessa- 

 tion of motor activity and respiration. The main stages of poisoning are 

 the same for freshwater fish, but the degree of manifestation and duration 

 of intoxication varied in different species of fish. 



The species characteristics of susceptibility (initial reaction to the 

 action of the toxicant) and resistance of freshwater fish to phenol have 

 been found. 



Regarding susceptibility, the fish species may be arranged in the 

 following order of increasing sensiti vity--crucian carp, blue bream, bur- 

 bot, bream, perch, pike, ruffe, roach, trout. Regarding resistance, the 

 following descending order is observed--crucian carp, roach, bream, blue 

 bream, pike, ruffe, perch, burbot, trout. It has been observed that high 

 susceptibility did not always correlate with low resistance and vice versa. 

 For example, roach ( Rutilis rutilis L. ) is highly susceptible and highly 

 resistant; burbot ( Lota lota L. ), is on the contrary, has low suscepti- 

 bility and low resistance. 



Fry are the most resistant, mature fishes are the least. Differences 

 in fish age are leveled with an increase in concentrations. Resistance to 

 phenol decreases with significant increase in body weight. Resistance of 

 fish to a toxicant is considerably less in summer than in winter. 



The role of basic environmental factors in fish resistance to phenol 

 has been demonstrated. Resistance of fish falls with a decrease in the 

 dissolved 2 content and with an increase in temperature. Water hardness 

 and pH influence the resistance of fish to phenol practically not at all. 



The adaptation of fish to the toxicant has also been investigated. 

 Fish preliminarily exposed to sublethal concentrations of phenol for 

 different time periods (from a week to more than a year) when put into a 

 solution with extremely toxic concentrations showed a lesser resistance to 

 lethal concentrations than those not exposed. This proves that a fish can 

 not individually adapt to phenol. The adaptation is realized by selecting 

 the most resistant individuals. The first generation of fish (guppies) was 



186 



