Some work has been done on treatment of methemoglobinemia. Ascorbic 

 acid administered intravenously reduced methemoglobin in rainbow trout blood 

 (Cameron 1971). Methylene blue administered either by injections (Bortz 

 1977) or by addition to test water (Wedemeyer and Yasutake 1978) also re- 

 duced methemoglobin levels. Removal of fish to nitrite-free water results 

 in a reduction of methemoglobin levels, although to a smaller extent than 

 found for methylene blue treatment (Wedemeyer and Yasutake 1978). Methylene 

 blue reduces methemoglobin levels rapidly, within a few hours. The treat- 

 ment appears to be temporary, in that methemoglobin levels gradually rise 

 again (Bortz 1977). 



Methemoglobinemia, then, is one mechanism by which nitrite is toxic to 

 fishes. It is probably not the only mode of toxic action. Observations by 

 Smith and Williams (1974) that mortality occurred for some rainbow trout 

 with blood methemoglobin levels lower than other rainbow trout which sur- 

 vived led them to suggest that those fish died from a toxic reaction to ni- 

 trite itself rather than from methemoglobinemia. Crawford and Allen (1977) 

 observed that in seawater with added nitrite, chinook salmon had high (74%) 

 methemoglobin levels but very low (10%) mortality; in freshwater with added 

 nitrite, lower (44%) methemoglobin levels were found in the salmon, but 70% 

 mortality occurred. They further observed that fish dying in freshwater 

 often had red gill lamellae, not the brown color typically caused by methe- 

 moglobinemia. This indicates that the toxicity of nitrite in freshwater may 

 be attributable to something else besides or in addition to methemoglo- 

 binemia. More research is needed to determine what this mechanism is. 



The effect of chloride and calcium also needs more study to elucidate 

 the mechanism by which these ions reduce nitrite toxicity. It has been sug- 

 gested (Perrone and Meade 1977) that chloride may compete with nitrite for 

 uptake through gills, or for entry into the red blood cell, thus suppressing 

 methemoglobin formation. Calcium does not appear to be affecting methemo- 

 globin formation, because raising the calcium level of freshwater did not 

 reduce methemoglobin levels in chinook salmon (Crawford and Allen 1977). 

 These are important areas for further research. 



In conclusion, it is apparent that the toxicity of nitrite to fishes is 

 highly dependent on the chemical composition of the test water, and that 

 more research is needed to define the mechanism(s) of nitrite toxicity and 

 to learn more about ways to protect fish from nitrite poisoning. 



REFERENCES 



Anthonisen, A.C., R.C. Loehr, T.B.S. Prakasam, and E.G. Srinath. 1976. In- 

 hibition of nitrification by ammonia and nitrous acid. J. Water Pollut. 

 Control Fed. 48(5): 835-852. 



Archer, M.C., S.D. Clark, J.E. Thilly, and S.R. Tannenbaum. 1971. Environ- 

 mental nitroso compounds: Reaction of nitrite with creatine and creati- 

 nine. Science 174: 1341-1343. 



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