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Abstract

Using a continuous-flow method a total of 14 acute toxicity bioassays were conducted using seabass (Dicentrarchus labrax), seabream (Spams aurata) and turbot (Scophthalmus maximus) juveniles weighing from 6 to 163 g (wet weight). Median LC50s of un-ionized ammonia-nitrogen (UIA-N) and median LT50s (plus their confidence intervals) were calculated for 6, 12, 24, 48 and 96-h exposures for each trial. Under optimal environmental conditions (17–18 °C, 34%. S, 8.15 pH and oxygen over 75% saturation), median 96-h LC50s averaged 1.7 mg 1−1 UIA-N (40 mg 1−1 TAN, total ammonia nitrogen) in seabass compared with 2.5–2.6 mg 1−1 UIA-N (57–59 mg 1−1 TAN) in seabream and turbot. Median LC50s did not change significantly from 24 to 96-h exposure and were not related to fish size. Significant variations in fish sensitivity were observed from one group to another and seabass juveniles appeared to be more susceptible to ammonia than seabream and turbot. In all species, mortality occurred over a relatively narrow range of ammonia concentrations. Lethal threshold concentrations (LTC) were estimated to be over 90% of 96-h LC50s.In starved fish, blood plasma levels of ammonia, which were positively correlated with ambient ammonia, can be used to estimate the extent of ammonia toxicity. The same increase in plasma TAN vs. ambient ammonia level was observed in seabass, seabream and turbot. A mortality of 50% was observed after a 4-day exposure when the increase in TAN was 4 times the initial level in seabass and more than 10 times the normal level in seabream and turbot. These results show, for the first time, that seabass unlike seabream and turbot have lower thresholds of physiological disturbances, which explains why they are more sensitive to ammonia.