chium rosenbergii , concluded that NH4''" is toxic. The work of these re- 

 searchers raises questions about an ammonia criterion based solely on NH3. 

 In addition, it is also known that prior acclimation, temperature, and dis- 

 solved oxygen may also affect the toxicity of ammonia to fishes. Consider- 

 ing the large number of industrial and agricultural discharges which contain 

 ammonia, and the tremendous expenditure of energy and resultant cost to 

 treat these discharges for ammonia reduction to meet statutory requirements, 

 it is reasonable to ask whether a single water quality standard for ammonia 

 can be justified. Certainly some of the factors that increase or decrease 

 the toxicity of ammonia should be considered further. 



EFFECT OF ACCLIMATION 



The question of whether fishes can acquire an increased tolerance to am- 

 monia by acclimation to low ammonia concentrations is an important one. In 

 certain real-world environmental situations, such as a stream receiving ef- 

 fluent from a sewage treatment plant, fishes may be subjected to high am- 

 monia concentrations for short and/or intermittent periods of time. If a 

 fish had an increased ammonia tolerance, developed due to acclimation or 

 conditioning to low ammonia levels, it would perhaps be able to survive what 

 might otherwise be acutely lethal ammonia concentrations. 



There is some information in the literature reporting that the effect of 

 previous exposure of fishes to low ammonia concentrations reduces or does 

 not affect their tolerance to lethal ammonia levels. Steinmann (1928) re- 

 ported that the minnow Alburnus bipunctatus was more susceptible to ammonium 

 hydroxide if previously exposed. Observations by McCay and Vars (1931) in- 

 dicated that bullheads ( Ameiurus nebulosus ) subjected to several successive 

 exposures to ammonia, alternated with recovery in fresh water, acquired no 

 immunity from the earlier exposures to the later ones. Fromm (1970) accli- 

 mated goldfish ( Carassius carassius ) to low (0.5 mg/liter) or high (5.0 or 

 25.0 mg/liter) ambient NH3 for periods of 20 to 56 days and found that urea 

 excretion rate in subsequent 24-hour exposures to concentrations ranging 

 from 0.08 to 2.37 mg/liter was independent of the previous acclimation con- 

 centration or duration. 



There is a larger body of information, however, which indicates that 

 prior exposure of fishes to low concentrations of ammonia increases their 

 resistance to lethal concentrations. Vamos (1963) conducted an experiment 

 in which carp (species not specified) were exposed to 0.67 and 0.52 mg/liter 

 NH3 for 75 minutes, revived in fresh water for 12 hours, and then subjected 

 to ammonia at a concentration of 0.7 mg/liter NH3. Control fish, exposed 

 only to the latter ammonia concentration, developed ammonia-poisoning symp- 

 toms within 20 minutes, but the previously exposed fish did not exhibit 

 these symptoms until 60-85 minutes. M'^lci'cea (1968) subjected carp ( Rhodeus 

 sericeus amarus Bloch) for 4 days, and minnows ( Phoxinus phoxinus L. ) for 3 

 days to "acclimation" solutions of ammonium sulfate (0.26 mg/liter NH3). 

 The "adapted" carp and "unadapted" control group were then exposed to lethal 

 concentrations of ammonium sulfate (5.1 mg/liter NH3). The mean survival 

 time of the adapted carp was 88 minutes and that of the unadapted carp was 

 78 minutes. The minnows were subjected to lethal toxic concentrations of 



120 



