In the first part of our report, we analyzed the age specifics of the 

 resistance of bony fish and cartilagenous fish in the larval and fry stage 

 of life, using the model of phenol intoxication of fish performed in our 

 laboratory. The fact of gradually decreasing resistance from larvae to fry 

 and from fry to immature individual, we found has been repeated by many re- 

 searchers in experiments with other organic poisons, including pesticides 

 and detergents. 



In contrast to organic poisons, toxic substances of inorganic nature 

 and, in particular, heavy metal salts, are most toxic for fish "in the lar- 

 val and fry stages" (Stroganov and Pazhitkov 1941). However, what are the 

 dynamics of toxicoresistance of fish in the larval and fry periods of life, 

 i.e., in the early stages of ontogenesis, we do not know due to the sparse 

 nature of studies of this problem. D. Blaxter (1975) considers, for 

 example, that the "sensitivity" of plaice larvae (meaning decreased resist- 

 ance) increases with age. If "young" larvae survive in 1000 pg Cu/liter, 

 32-42 day larvae died at a concentration as low as 300 pg Cu/liter. G. 

 Larson, et aV. (1977) studies the acute toxicity of inorganic chloramino 

 compounds for larvae with the yellow sac, fry and juvenile American brook 

 trout ( Salvelinus fontinalis ). The fry were less resistance than the larvae 

 and the lethal concentration (LC50) of inorganic chloramines at 96 hours ex- 

 posure for them was 82 yg/liter, for larvae with the yellow sac - 90-105 yg/ 

 liter. In the larvae, a decrease was noted in the resistance with increase 

 in body weight. 



In our laboratory in the last three years, we have performed a cycle of 

 studies involving students from the ARE - Abbas Said Abu El-Ess, and from 

 Iraq - Talyal Al Kubeysi and Adnan Musa Edzhad - on the age dynamics of 

 toxicoresistance of larvae and fry of sturgeons with respect to common 

 metals, cadmium and cobalt. 



The experiments were performed on 1, 5, 10, 20 and 30-day-old larvae, as 

 well as 40, 60, 90 and 120-day-old fry of the giant sturgeon, Russian stur- 

 geon and Caspian sturgeon. We used the following concentrations of salts: 

 cadmium chloride - 0.01, 0.1, 0.5, 1, 2, 4, 5, 8 and 10 mg/liter; cobalt 

 chloride - 0.1, 1, 4, 5, 8, 10, 16, 32 and 64 mg/liter. The indication of 

 resistance of the larvae and fry was the percentage of deaths and the time 

 of survival in a solution of a given concentration of toxic substance. The 

 duration of the experiments was 48 hours; observations were performed around 

 the clock. 



Summarizing the results of many series of experiments in this cycle, we 

 conclude that the level of toxicoresistance of larvae and fry of these stur- 

 geons differs significantly and that the larvae are significantly less re- 

 sistant in comparison to the fry. However, within each of these two age 

 groups of early ontogenesis, there is a significant change in toicoresist- 

 ance, as indicated by the percentage and time of death of fish at the same 

 concentration, as well as the threshold lethal concentration. For example, 

 the toxicoresistance of the Russian sturgeon gradually decreases from the 

 early stages of larval development to later stages, becoming minimal in the 

 transition period (from larval to fry), then increases once more from the 

 early age group to the later age groups, reaching a rather high level by the 



165 



