SECTION 5 



CHRONIC EFFECTS OF LOW LEVELS OF HYDROGEN CYANIDE 

 ON FRESHWATER FISH 



Lloyd L. Smith, Jr. 



INTRODUCTION 



The toxicity of cyanides to fish has long been recognized. Compounds 

 containing the cyanide radical are frequently present in effluents of 

 many industries including electroplating plants, steel mills, petroleum 

 refineries and gas works. In aqueous solution the cyanide radical from 

 simple alkali cyanides such as NaCN hydrolyzes to form free cyanide (CN 

 ion and molecular HCN). The molecular (un-ionized) component predominates 

 at pH values found in most natural waters, with less than 4 percent of 

 free cyanide occurring in the ionic form below pH 8 at 25 °C. As the pH 

 of aqueous simple cyanide solutions is increased, the percentage of free 

 cyanide present as the CN~ ion is increased to satisfy the equilibrium 

 reaction of HCN $ H + + CN . 



Little information is available on the long-term effects of hydrogen 

 cyanide on fish. Neil (1957) and Broderius (1970) found that free cya- 

 nide concentrations of 10 ug/liter, expressed as CN, impaired the swim- 

 ming performance of salmonid fishes. Leduc (1966) measured the growth of 

 juvenile cichlids ( Cichtcaoma bimaculaturt) . 



The work reported here was designed to determine the effect of low 

 levels of HCN on the fathead minnow, Vimzphalm, piomelai t f rom egg 

 through the juvenile period of the second generation, and on brook trout, 

 SalveJiinLU) {^ont^naLU , adults through egg maturation to spawning and 

 development of the second generation to advanced juvenile stage. 



MATERIALS AND METHODS 



Experiments with fathead minnows were started with eggs from labora- 

 tory stock originating at the Duluth laboratory of the U.S. Environmental 

 Protection Agency. The brook trout adults utilized were from the State 

 of Wisconsin hatchery at Osceola, Wisconsin. Water for experiments was 

 taken from the laboratory well (Table 1). 



Eighty fathead minnow larvae were placed in each of 15 20-liter glass 

 tanks, and sodium cyanide solution was introduced to the chambers with a 



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