The economy of commercial-based fish farming is predominantly based on stocking the fish at a higher density and rearing them with a high-protein diet. This scenario, along with limited renewal of clean water (e.g., in recirculatory and pond-farming), eventually results in a build-up of toxic ammonia in the water, which can reduce growth, increase vulnerability to diseases, and even cause mortality. Within the aquaculture system, there is no fast remedy to alleviate high ammonia-induced toxicity, so prevention rather than correction should be the focus.
Therefore, we tested the hypothesis of whether pre-acclimation to a low concentration of ammonia could enable the fish to develop an ’ammonia stress-avoidance’ memory, enhancing their tolerance to subsequent lethal and sub-lethal ammonia threats. To test this, we selected channel catfish (Ictalurus punctatus), a key species in the U.S. aquaculture industry, as the model organism. Catfish were pre-exposed to 2.17 mg/L (total) ammonia (10% of determined 10-day LC50’ value, 21.7 mg/L) for 21 days. Thereafter, each of these pre-exposed and parallel control (without pre-exposure ‘naïve’) groups were recovered for 7 days in clean water. Following this recovery phase, each group was subsequently exposed to a lethal (100% 10-day LC50) and sub-lethal (25% 10-day LC50 for 21 days) ammonia concentration. Results show that during the lethal ammonia challenge, the pre-exposed group had a significantly longer survival time than the naïve group (Fig. 1). This indicates that catfish can develop an ‘ammonia stress-avoidance memory’ that enables the fish to resist a subsequent ammonia threat. In addition, following 21 days of sub-lethal ammonia exposure, the ammonia excretion from fish was significantly inhibited in the
naïve group, which resulted in a significant accumulation of toxic ammonia in the plasma. Interestingly, pre-exposed fish were able to excrete ammonia efficiently and retained ammonia load in the plasma within the basal level. Overall, these data suggest that ‘ammonia stress-memory’ was evident for both lethal and sub-lethal endpoints. Conclusively, based on our findings, we can state that stocking and rearing the ammonia pre-exposed catfish in aquaculture facilities can be used as an imperative strategy to enhance the fish’s ability to alleviate the toxicity induced by the water-borne ammonia.