COMPARATIVE ACUTE AMMONIA TOXICITY IN MARINE FISH AND PLASMA AMMONIA RESPONSE

Using a continuous-flow method a total of 14 acute toxicity bioassays were conducted using seabass (Dicentrarchus labrax), seabream (Sparus 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 env ironmental conditions (17-18 degrees C, 34 parts per thousand S, 8.15 pH and oxygen over 75% saturation), median 96-h LC50s averaged 1.7 mg l(-1) UIA-N (40 mg l(-1) TAN, total ammonia nitrogen) in seabass compa red with 2.5-2.6 mg l(-1) UIA-N (57-59 mg l(-1) TAN) in seabream and t urbot. Median LC50s did not change significantly from 24 to 96-h expos ure and were not related to fish size. Significant variations in fish sensitivity were observed from one group to another and seabass juveni les appeared to be more susceptible to ammonia than seabream and turbo t. In all species, mortality occurred over a relatively narrow range o f ammonia concentrations. Lethal threshold concentrations (LTC) were e stimated to be over 90% of 96-h LC50s. In starved fish, blood plasma l evels of ammonia, which were positively correlated with ambient ammoni a, can be used to estimate the extent of ammonia toxicity. The same in crease in plasma TAN vs. ambient ammonia level was observed in seabass , seabream and turbot. A mortality of 50% was observed after a 4-day e xposure when the increase in TAN was 4 times the initial level in seab ass and more than 10 times the normal level in seabream and turbot. Th ese results show, for the first time, that seabass unlike seabream and turbot have lower thresholds of physiological disturbances, which exp lains why they are more sensitive to ammonia.