Ammonia results from decomposition of effluents organic matter, e.g. feed wastes and fish faeces. Its un-ionized form can be toxic because diffuses easily through fish respiratory membranes, damaging gill epithelium and impairing gas exchanges. The objective of this work was determining the 96-hour CL50 of un-ionized ammonia for newly hatched pejerrey Odontesthes bonariensis larvae. Trials were set up completely randomized design, with three different concentration of un-ionized ammonia (0.57, 0.94, and 1.45 mg L-1 NH3-N) and a control treatment (n = 3). Experimental units were 20-L, aerated aquaria stocked with 20 larvae (average weight 3.9 mg). Pejerrey larvae exposed to un-ionized ammonia during 96 hours present 50% mortality at 0.71 mg L-1 NH3-N.
pejerrey; Odontesthes bonariensis; toxicity; nitrogen; larvae