Another great bit of information Edvet. Looking at the three species of fish potentially adversely affected (Oncorhynchus mykiss, Oncorhynchus tshawytscha, Salmo clarki) it is interesting to note that the toxicology expressed in table 3 (J.A. Camargo et al. / Chemosphere 58 (2005) pp. 1261) is confined to eggs and fry. The Oncorhynchus tshawytscha fingerlings had a LC-50 96hr value of 1310 mg/l NO3-N. Expressed as an NO3 value - that would be a whopping 5799.1 ppm. For a salmonid species evolved in temperate regions that doesn't sound too bad. Considering tropical fish, we can assume evolution towards a more nitrate tolerate biome. Lastly, if you look at the results you can see that sodium nitrate was validated as a method of testing toxicity. I have not read the 1976 study by Colt and Tchobanoglous that was reviewed by these authors, which these results were based upon, but I suspect the osmotic impact of sodium on eggs and fry was a highly contributory influence. These are species that spawn upstream because they are not evolved to do so in saline environments. I have yet to find a valid example implying that nitrate dosing (at EI levels) could have toxicological effects.