Catfish farming is the largest and most successful sector of U.S. aquaculture, supporting food security and rural economies. However, this industry is confronted with major challenges, particularly due to toxic ammonia buildup in aquaculture systems, which reduces overall performance and even causes mortality. Therefore, to maintain and improve catfish productivity, it is crucial to implement effective strategies to alleviate ammonia-induced toxicity.
This study examined whether altering water pH mitigates ammonia toxicity in channel catfish (Ictalurus punctatus). Fish were cultured at pH 7.1, 7.8, or 8.5, with and without high environmental ammonia (HEA, 5.42 mg/L, 25% of the 10-day LC50). As such, there were six experimental groups (with three replicated tanks each): pH7.1 (control), pH7.8, pH8.5, pH7.1+HEA, pH7.8+HEA, and pH8.5+HEA. Following two months of exposure, comprehensive physiological, biochemical, and molecular responses were evaluated. Ammonia exposure at pH 7.1 and 7.8 caused pronounced plasma ion imbalance, with reduced Na⁺, K⁺, Ca²⁺, and Mg²⁺ levels, indicating disrupted ion regulation. In contrast, fish at pH 8.5 under HEA showed near-normal ionic composition, supported by elevated gill ion transporter activities. Na⁺/K⁺-ATPase was elevated, enhancing sodium and potassium regulation, while increased H⁺-ATPase activity improved proton extrusion and acid–base balance. In addition, ammonia at pH 7.1 caused severe depletion of glycogen, lipids, and proteins, reflecting high metabolic costs, whereas fish at pH 8.5 maintained higher energy reserves, indicating reduced burden and improved energy allocation under chronic exposure. Gene expression analysis of
ammonia transport-related genes revealed
strong modulation by both HEA and pH. Rhbg and Rhcg transcripts remained unchanged under HEA at pH7.1 and pH7.8, consistent with impaired ammonia excretion capacity. However, fish at pH8.5+HEA displayed significant upregulation of both Rhbg and Rhcg (Fig. 1), suggesting that alkaline conditions facilitate branchial ammonia excretion. Taken together, these findings demonstrate that maintaining culture water at moderate alkaline pH (8.5) mitigates the detrimental effects of chronic ammonia exposure by stabilizing plasma ion composition, enhancing gill ion transport capacity, preserving energy reserves, and upregulating Rh glycoprotein expression. The results provide compelling evidence that water pH adjustment represents a practical, low-cost management strategy to improve ammonia tolerance, growth, and resilience in catfish aquaculture systems.