Catfish farming is the largest and most successful segment of the U.S. aquaculture industry, contributing significantly to the country’s food supply and rural economies. However, this industry is confronted with major challenges, particularly due to toxic ammonia buildup in aquaculture systems, which reduces fish growth and physiological performance and even causes mortality. Therefore, to maintain and improve the productivity of catfish farming, it is crucial to implement effective strategies to alleviate ammonia-induced toxicity.
This research was, therefore, undertaken to investigate the potential mitigation of ammonia toxicity in channel catfish (Ictalurus punctatus) by altering the pH of culture water. First, to provide primary information on the sensitivity of this species to ammonia toxicity under realistic fish culture operation, a 10 day-LC50 test was conducted, which was found to be 21.69 mg/L (total) ammonia. Thereafter, to get insight into the protective effect of water pH against chronic ammonia exposure, catfish were cultured under three pH levels (7.1, 7.8, and 8.5) and simultaneously challenged high environmental ammonia (HEA, 5.42 mg/L representing 25% of 10-day- LC50). As such, there were six experimental groups (with three replicated tanks) viz. pH7.1(control), pH7.8, pH8.5, pH7.1(control)+HEA, pH7.8+HEA, and pH8.5+HEA. Following two months, weight gain (%), specific growth rate, and feed conversion were significantly reduced in pH7.1(control)+HEA compared to pH7.1(control) (Fig. 1). Interestingly, the growth parameters in pH8.5+HEA groups were significantly higher than pH7.1(control)+HEA, signifying that HEA inhibited growth performance at normal rearing pH (7.1), but the toxic effect of HEA was alleviated by rearing the fish at a high pH of 8.5. Ammonia excretion rate (Jamm) was strongly inhibited in pH7.1(control)+HEA.
In contrast, HEA-exposed fish reared at a high pH level (8.5) were able to increase Jamm efficiently, which was associated with upregulated branchial expression of ammonia transporters, Rhesus glycoproteins ‘Rhcg’. These responses prevented a build-up of excess ammonia in plasma. A series of histopathological alterations were observed in the gills, with the most severe lesions in pH8.5 and pH7.1(control)+HEA. Ions (Na+, K+) levels were disrupted in pH7.1(control)+HEA but remained stable in pH7.8+HEA and pH8.5+HEA. Overall, our findings suggest that raising catfish at a higher pH level (typically at 8.5) can ameliorate the inhibitory effects of HEA by protecting the gill morphology as well as mitigating HEA-induced ammonia excretory and ion-regulatory disruption. In conclusion, we recommend fish farmers consider raising catfish in higher pH environments when facing elevated ammonia levels in the culture units.