the degree of toxicity of one chemical reagent or another, and in deter- 

 mining the resistance of organisms to toxicants. 



Reversibility of intoxication implies the recovery of organisms to their 

 normal physiological state after some pathological shifts brought about by a 

 toxic agent. The reversibility of pathological processes is possible only 

 at a definite concentration, and at a given duration of exposure to a toxic 

 substance. It may be said that pharmacological practice is based on this 

 phenomenon, since all pharmaceuticals employed are also toxins; but in a de- 

 finite combination they are of use for the organisms. Such combinations, at 

 which changes occurring under influence of poisons demonstrate reversi- 

 bility, should also be understood in the area of aquatic toxicology. 



Data from literature on this problem are fairly scanty and, in some in- 

 stances, contradictory. Evidence of these facts can be found in the works 

 by Jones (1947, 1951, and 1957), Schweiger (1957), Wuhrmann and Woker 

 (1950), and Stroganov and Pozhitkov (1941), in which reversibility of in- 

 toxication in fish as affected by cyanides, sulphides, chloromercury, ethyl 

 alcohol, salts of heavy metals, and phenols, has been investigated. 



The dynamics of phenol intoxication reversibility have been described 

 in a study by Lukyanenko and Fluorov (1963). Studies by Mann (1958), 

 Ludemann (1962), Chernysheva (1968) and others have been concerned with 

 reversibility of intoxication in fish as affected by insecticides. In 

 these reports, the possibility of restoring the vital activity of fish 

 which have been intoxicated with organophosphates is shown. Similarly, the 

 irreversible phenomena arising from contact with organochlorine compounds 

 is also demonstrated. A high degree of reversibility has been demonstrated 

 under the influence of detergents (Libmann 1960), but the resistance of 

 fish to various diseases decreases drastically. 



This study has employed unpurified multi-component wastes from sulphate 

 pulp production as toxicants in various modifications and dilutions. 

 Further, sewage from sewage treatment plants has also been used. Waste 

 waters utilized contained methyl mercaptans, sulphides, hydrosulphides, 

 sulphates, acids and alkalis, methyl alcohol, furfurol, acetone, ammonia 

 and other organic and mineral compounds. The water in the natural effluent 

 receiver is similar in chemical composition to the average composition of 

 wastes resulting directly from production. It is nearly oxygen-free and has 

 a high carbon dioxide content (25.1 mg/0- Different quantities of sulphur- 

 containing compounds have been found in wastes from boiling and evaporating 

 shops. They possess a strong hydrogen sulphide smell. These wastes contain 

 alkali and some fairly toxic organic substances, including terpentine, 

 methanol, acetic and other acids. Wastes from the heat-and-power stations 

 are distinguished by a considerable amount of mechanical suspensions, the 

 result of burning slurry lignin, bark, and fuel oil, and by their sulphur 

 trioxide and sulphur dioxide content. 



Atlantic salmon ( Salmo salar ), Cisco ( Coregonus albula ), roach ( Rutilus 

 ruti lus ), perch ( Perca f luviatilis ) and pike ( Esox~lucius ) were test 

 species. Fish of the first year of life (from the moment of hatching until 



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