sures to NO2-N concentrations of 67 to 100 mg/liter. Manifestations of the 

 acutely toxic effects of nitrite can thus vary widely, depending on fish 

 species. 



Little information has been reported on the effects of nitrite exposure 

 for periods of time longer than 1-4 days. We have conducted 36-day expo- 

 sures on cutthroat trout (S. darki ) fry (Thurston e^ aj_. 1978) and found 

 LC50 values at 36 days to be only slightly lower than 96-hour values. 

 Wedemeyer and Yasutake (1978) exposed steelhead trout (S. gairdneri ) to low 

 NO2-N concentrations (0.015-0.060 mg/liter) over a 6-month period and found 

 no serious deleterious effects. Growth and ability of the fish to adapt to 

 seawater were not impaired. Varying degrees of gill hyperplasia and lamel- 

 lar separation were observed early in the test but the fish seemed to re- 

 cover and after 28 weeks these abnormalities were no longer observed. 



Fish size has also been thought to be a factor influencing fishes' sus- 

 ceptibility to nitrite. Rainbow trout sac fry, and 2-g fry, were found to 

 be less susceptible than were larger (12-, 14-, and 235-g) rainbow trout 

 (Russo et ^. 1974); 4.5-g fingerling rainbow trout were reported to be more 

 tolerant than were 100-g yearlings (Smith and Williams 1974). Coho salmon 

 ( Oncorhynchus kisutch ) fry (0.65 g) were less susceptible than were 

 yearlings (22 g) (Perrone and Meade 1977). We have now conducted 20 96-hour 

 nitrite bioassays on rainbow trout over the size range 2 to 387 g. These 

 experiments were all conducted under similar water chemistry conditions 

 (Table 1). The results are given in Table 2; over this larger range of fish 

 size than that reported previously, there does not appear to be any rela- 

 tionship between fish size and susceptibility to nitrite. This is illu- 

 strated in the graphs of LC50 vs. fish weight and length, shown in Figures 

 1 and 2. 



We have also studied the effect of chloride ion (CI") on nitrite toxi- 

 city to rainbow trout (Russo and Thurston 1977). We conducted a series of 

 nitrite toxicity tests in which we added CI" (as NaCl) in concentrations 

 ranging from 1 to 41 mg/liter. A significant reduction in nitrite toxicity 

 resulted from increased levels of CI" (Figure 3), and this effect was 

 linearly correlated (Figure 4). The 96-hour LC50 was raised from 0.46 

 mg/liter NO2-N in the presence of 1 mg/liter CI" to 12.4 mg/liter NO2-N at 

 41 mg/liter CI". Similar conclusions have been reported for coho salmon 

 (Perrone and Meade 1977) and for steelhead trout (Wedemeyer and Yasutake 

 1978). We have conducted some nitrite bioassays with addition of bromide 

 (Br"), sulfate (SO42-), phosphate (PO43-), and nitrate (NO3"); the results 

 of these tests indicate that these other anions also exhibit, in different 

 degrees, an inhibitory effect on nitrite toxicity. It is apparent that the 

 toxicity of nitrite is highly dependent on the chemical composition of the 

 water. 



Crawford and Allen (1977) studied the effect of calcium (Ca^"^) and of 

 seawater on nitrite toxicity to chinook salmon (0. tshawytscha ) . The acute 

 toxicity of nitrite in seawater was markedly less than that in freshwater, 

 logically so because of the chloride effect discussed above. Crawford and 

 Allen also found that increasing the calcium concentration both in fresh- 

 water and in seawater decreased the toxicity of nitrite. 



228 



