Chronic low-level ammonia exposure can inhibit growth and increase susceptibility to disease.

• A. True
• B. False
• C. It depends on temperature
• D. It depends on stocking density

Answer: (A)

Chronic low-level ammonia exposure is harmful because ammonia acts as a metabolic toxin for fish, and sustained exposure shifts the balance away from growth toward stress responses and repair. Ammonia primarily enters and affects the gills, where it disrupts ion transport and acid–base regulation. When the water contains ammonia, especially in the form NH3 that becomes more prevalent at higher pH, this toxic species diffuses into gill tissues, causing irritation and damage. That gill stress increases the energy the fish must devote to maintaining homeostasis—breathing harder, circulating ions, and repairing tissue—leaving less energy available for growth.

Over time, continuous gill and osmotic stress can slow feed intake and feed efficiency, so the fish grow more slowly even if food is available. Chronic ammonia exposure also activates stress pathways, elevating cortisol and other stress hormones; this prolonged state can dampen immune function, reduce mucus production on the skin and gills, and impair the activity of immune cells. As a result, fish become more susceptible to infectious diseases and parasites.

Temperature and water chemistry can influence the degree of toxicity by shifting the NH3/NH4+ balance and the fish’s metabolic rate, but the fundamental effects remain: sustained ammonia exposure undermines growth and weakens disease resistance.