Figure 1 shows the toxicity curves for these tests (LC50 in mg/liter NH3 

 vs. time). There was a statistically significant correlation between the 

 NH3 concentration at which the fish were acclimated and their subsequent re- 

 sistance to acutely toxic NH3 concentrations. The higher the NH3 concentra- 

 tion at which the fish were acclimated, the more tolerant the fish were to 

 acutely lethal levels during the 96-hour test period. The shapes of the 

 curves also show that there is a general trend for fish acclimated at higher 

 ammonia concentrations to take longer to arrive at an eventual asymptotic 

 LC50 value. 



We also performed some experiments to determine whether the length of 

 time of acclimation to low ammonia concentrations affected the fish's re- 

 sponse in subsequent exposure to lethal NH3 levels. Duration of acclimation 

 to ammonia in these experiments ranged from 29 to 154 days; the subsequent 

 lethal tests were all 96-hour bioassays as described above. Results showed 

 that there was a significant relationship between 96-hour LC50 and length of 

 time of prior acclimation; the longer the acclimation period, the more tol- 

 erant the fish were to high ammonia levels. Our calculations took into con- 

 sideration the fact that fish weight also increased as acclimation duration 

 increased. We also investigated whether there was an effect on fish's tol- 

 erance to ammonia if they were placed in fresh (ammonia-free) water for 

 periods of 2, 14, and 28 days after acclimation and before exposure to 

 lethal concentrations. From limited data, our experiments indicated that 

 fish rapidly (less than 2 days) started to lose the tolerance to ammonia 

 built up by acclimation once they were placed in ammonia-free water. 



In summary, there is reasonable evidence that fishes with a history of 

 prior acclimation to some sublethal concentration of ammonia are better able 

 to withstand an acutely lethal concentration, at least for some period of 

 hours and possibly days. The concentration limits for both acclimation and 

 subsequent acute response need definition and explanation. 



EFFECT OF TEMPERATURE 



There is limited information in the literature on the effects of temper- 

 ature on ammonia toxicity to fishes. Generally, the toxicity of total am- 

 monia decreases with lower temperatures, attributable mainly to a decrease 

 in the concentration of NH3. Woker (1949), testing chub ( Squalius cephalus ) 

 within the range of 10-25 C, concluded that water temperature had practi- 

 cally no effect on the manifestation time of toxic symptoms resulting from 

 ammonia. On the other hand, Colt and Tchobanoglous (1976) observed that the 

 tolerance of channel catfish ( Ictalurus punctatus ) to ammonia increased as 

 the experimental temperatures were increased up to the fish's reported opti- 

 mum temperature for growth (29-30 C). It is reasonable to expect that at 

 temperature conditions which are marginal for any given fish species, the 

 species will not be able to function optimally to resist toxic effects of 

 ammon i a . 



We have conducted eight 96-hr ammonia bioassays on 2- to 12-g rainbow 

 trout at elevated temperatures within the range 12-19 C. Test conditions 

 were similar to those employed in the acclimation experiments reported 



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