DISCUSSION 



Most mortality among fathead minnows occurred in the first 28-day ex- 

 posure to HCN in both parent and F-j generations (Table 4). The number of 

 eggs produced and fry which survived were reduced at 196 yg/liter and at 

 higher concentrations. It is estimated that the highest "no-etfect" 

 level of HCN is between 12.9 and 19.6 ug/liter based on egg production. 

 The lethal threshold for juvenile fathead minnows (defined as the HCN 

 concentration at which no fish die for 48 hours after continuous exposure 

 for 96 hours or longer) as determined by unpublished data from our labora- 

 tory is 119 yg/liter HCN at 25 °C, pH 8.0 and 6.0 mg/liter DO (Table 8). 

 Comparison of this acute toxic level to the "no-effect" level indicates 

 that the safe level for fish is between 11 and 16 percent of the acute 

 toxicity concentration of HCN. 



When adult brook trout, prior to spawning, were exposed to HCN, at 

 all concentrations greater than 5.7 ug/liter, a reduction in the produc- 

 tion of fertilized eggs occurred. When spawning was successful, egg 

 viability was not affected adversely at 43.6 ug/liter and lower. Growth 

 rate of juvenile brook trout during the first 90 days after hatching was 

 reduced at concentrations of 33.3 yg/liter HCN and higher (Figure 3). At 

 77.2 yg/liter it was 15.7 percent of controls. On the basis of acute 

 threshold toxic levels of 88 yg/liter at 10 °C (unpublished laboratory 

 data) for juvenile fish and 5./ yg/liter HCN as a safe concentration for 

 successful spawning, the "no-effect" level is approximately I percent of 

 the acute toxic level . 



From these chronic exposure tests of two fish species, it is evident 

 that safe levels of HCN in the environment are much lower than the concen- 

 trations which will kill fish on short exposure. Where there is con- 

 tinuous exposure to low levels of cyanide from steel mills or other 

 sources of cyanide, some fish populations can be adversely affected by 

 concentrations higher than 7-12 yg/liter. 



ACKNOWLEDGEMENTS 



The author wishes to acknowledge the contributions of David Lind to 

 the experiment of fathead minnows, and to Walter Koenst for the experiment 

 on the brook trout. 



REFERENCES 



American Public Health Association, American Water Works Association and 

 Water Pollution Control Federation. 19/1. Standard methods for the 

 examination of water and wastewater. 13th ed. 



Broderius, S.J. 1970. Determination of molecular hydrocyanic acid in 

 water and studies of the chemistry and toxicity to fish of the 

 nickelocyanide complex. M.S. thesis, Oregon State University, 

 Cor vail is. 



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