The median lethal concentration (LC50) of aqueous ammonia at reduced dissolved oxygen (D.O.) concentrations was tested in acute toxicity tests with rainbow trout (Salmo gairdneri) fingerlings. Fifteen 96-h flow-through tests were conducted over the D.O. range 2.6–8.6 mg/L, the former concentration being the lowest at which control fish survived. There was a positive linear correlation between LC50 (milligrams per litre un-ionized ammonia) and D.O. over the entire D.O. range tested; ammonia toxicity increased as D.O. decreased. Ammonia LC50 values were also computed for 12, 24, 48, and 72 h; the correlation with D.O. was greater the shorter the time period.Key words: ammonia toxicity, dissolved oxygen, rainbow trout, Salmo gairdneri

Rainbow trout accumulated methylmercury linearly during 24 days when continually exposed to methylmercury. Exposure was by means of water solutions (0.07‐1.33 μg Hg/liter), food consumption (8.0‐380.5 ng Hg/g fish per day) or both. Methylmercury accumulated from one source had no influence on the rate of uptake from the second source. Methylmercury accumulated from both sources was quantitatively additive, which validates a frequently used assumption. Food consumption rate and therefore growth rate had no influence on the rate of mercury accumulation from water. Nearly 70% of the methylmercury ingested and 10% of the methylmercury passed over the gills was assimilated.

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The acute toxicity of ammonia to fathead minnows Pimephales promelas was measured in 35, 96‐hour, flow‐through tests. The fish were from both wild and hatchery‐reared stocks, and ranged in size from 0.1 to 2.3 g. The 96‐hour median lethal concentrations (LC50) ranged from 0.75 to 3.4 mg/liter un‐ionized ammonia (34 to 108 mg/liter total ammonia‐nitrogen). The toxicity of ammonia decreased as temperature increased over the range 12 to 22 C. There was no significant relationship between ammonia toxicity and dissolved oxygen concentration, over the range of the latter from 3 to 9 mg/liter. Toxicity was not related to the size or the source of test fish. Received July 12, 1982 Accepted June 20, 1983

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