The effect of different concentrations of external un-ionized ammonia (NH3) on the growth of juvenile turbot (Scophthalmus maximus L.) was investigated over a period of 20 days. When the concentration of un-ionized ammonia exceeded a threshold level of 0.108 mg NH3-N l-1, growth was found to be reduced, as indicated by a reduction in body and liver wet and dry weight gain. The observed reduction of growth could partly be explained by less food intake. Efficiency of food utilization was observed to be reduced as indicated by higher food coefficients and relative increases in the faecal amounts produced. No effect of un-ionized ammonia could be observed on the wet weight:dry weight ratio. In fish exposed to high levels of ammonia, the ratio of RNA to DNA in the liver was observed to be reduced. Darkening of skin, and sporadic and last swimming behaviour were observed among fish exposed to the highest concentrations of un-ionized ammonia.
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