We propose that in describing the reactions of fish to toxic irritants 

 in the embryonal and immediate postembryonal periods of development (prelar- 

 val and larval), the concept of resistance be universally used. Sensitivity 

 or susceptibility can be spoken of only if it is specially studied using 

 adequate methods of investigation. 



Returning to the primary point of the present report, I would like to 

 emphasize that over the past decade, new data have been obtained, indicating 

 the presence of clear age specifics in the sensitivity of fish to poisons. 

 However, the level of toxicoresistance is determined not only by the direc- 

 tion and intensity of metabolic processes of fish in various stages of onto- 

 genesis, but also by the nature of the toxic agent used. The resistance of 

 various species of fish to many organic poisons decreases with ontogenetic 

 development and reaches a minimum in sexually mature fish. However, this 

 process is not uniform and periods of high resistance (egg in stage of pul- 

 sating heart, larva in C2 stage and current year's brood) alternate with 

 periods of low resistance (egg in stage of gastrulation, larva at end of 

 larval period, immature individuals). Particular attention should be given 

 to the end of the larval and the beginning of the fry period of development, 

 when the resistance of fish to organic poisons drops sharply. As concerns 

 the resistance of fish to inorganic poisons and, in particular, to heavy 

 metal salts, it is minimal in the larval and fry period of individual devel- 

 opment. The resistance of the fry (embryonal period of development), both 

 to organic and to inorganic poisons, is significantly higher in comparison 

 to the larval and fry periods. The nature of the increased toxicoresistance 

 of the egg remains unclear. This factor makes the use of eggs as test ob- 

 jects (reference objects) undesirable in studies of the degree of toxicity 

 of various substances for various stages of the ontogenesis of fish and 

 biologic testing of natural and waste waters (larvae and fry are prefer- 

 able). 



REFERENCES 



Danilchenko, O.P. 1975. Effects of toxic substances on certain fresh-water 

 bony and cartilagenous fishes in the embryonal period of development. 

 Cand. Diss., Moscow State University, 150 pp. 



Grimm, O.A. 1896. Kaspiisko-volzhskoe rybolovstvo, St. Peterburg, 153 pp. 



Lukyanenko, V.I. and B.A. Flerov. 1963. Toxicoresistance of current year's 

 brood of carp. Materialy po biologii i gidrologii volzhskikh vodokhran- 

 ilishch. Izd. AN SSSR, Moscow-Leningrad. 



Lukyanenko, V.I. and B.A. Flerov. 1963. Materials on the age toxicology of 

 fish. Farmakol. i Toksikol., No. 5. 



Lukyanenko, V.I. and B.A. Flerov. 1965. Species peculiarities in the sen- 

 tivity and resistance of fish to phenol. Gidrobiologicheskii Zhurnal, 

 No. 2. 



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