Ammonia, a potential toxic to fish: Applications to turbot.

Total ammonia nitrogen (TAN) is potentially toxic to fish, with moderate or severe effects on survival and growth depending on ambient levels and expo sure durations. In a first point, the reasons why NH3 is the most toxic for m of TAN (the sum of NH3 and NH4+) is developed. As soon as fish are expose d to abnormally high concentrations of exogenous TAN, NH3 immediately enter s the fish and passively diffuses in all body compartments (at steady state , NH4+ is the main form of TAN). Depending on time and ambient ammonia conc entrations, various steady states are observed in relation with fish excret ion capacity. Eventually, a threshold of blood plasma TAN and/or tissues co ntents is reached, above which all physiological functions are highly distu rbed and death occurs (chronic lethal concentration). In a second part, available data concerning acute and chronic toxicity of T AN to turbot (Psetta maxima) juveniles are summarized and compared with oth er fish species (marine or fresh water species). For short exposure, turbot tolerance to TAN is in the same range as in seabream and seabass, 96-h LC5 0s (lethal concentrations for 50% of the population) are 1-3 mg/l NH3. Such high concentrations are unusual in fish farming even at high stocking dens ities. In comparison, chronic lethal concentrations are significantly lower , averaging 0.8-1 mg/l NH3 for one-month exposure duration. In all marine f ish species, there is some risk of growth disturbances over 0.11 mg/l NH3, usual concentrations in intensive fish farming using semi-closed systems. T his safety level in terms of growth response is significantly lower in salm onids specially during fresh water rearing. To conclude, under optimal rearing conditions (environmental factors and fe eding conditions), turbot juveniles and other marine fish species tested ar e relatively tolerant to TAN. However, their sensitivity to TAN is highly d ependent on environmental factors such as ambient pH and oxygen availabilit y. Their tolerance may also be affected by exposure conditions : acclimatio n, fluctuating TAN levels and exposure duration.