Percent of methemoglobin (MHb) in the blood of fingerling chinook salmon (Oncorhynchus tshawytscha) acclimated to fresh water and sea water was observed after 48 hours of exposure to nitrite in a standing bioassay. Percent mortality was also recorded. MHb in fresh water (32 mg/1 ca++) with 30 mg/1 nitrite increased from 16.2% to 44.6% with a corresponding mortality of 80%. MHb in sea water (400 mg/1 ca++) remained near 19% with 100 mg/1 nitrite and no mortalities were observed. Increasing the calcium concentration from 32 to 200 mg/1 in fresh water with 30 mg/1 nitrite decreased the mortality from 80% to 0% while the MHb level remained near 40%. Decreasing the calcium concentration from 400 to 0 mg/1 in artificial sea water with 100 mg/1 nitrite increased mortalities from 0% to 90% while the level of MHb remained at 20%. The data presented here suggest that calcium acts antagonistically toward the toxicity of nitrite in chinook salmon fingerlings.
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