Modulating effect of elevated water hardness on growth performance, ammonia dynamics and ion-regulatory capacity in channel catfish (Ictalurus punctatus) following chronic challenge with high environmental ammonia and salinity stress

The aim of this study was to assess whether rearing fish at increased water hardness could assist channel catfish (Ictalurus punctatus) juveniles to efficiently cope with later (sub)lethal ammonia exposure or salinity stress. Fish were subjected to 100, 494 (10% of 96 h-LC₅₀) or 1250 mg/L (25% of 96 h-LC₅₀) CaCO₃ water hardness levels for two months. Afterwards, fish were challenged with high environmental ammonia (HEA; 12.2 mg/L total ammonia ~25% of 96 h-LC₅₀ value) or salinity stress (SST;10 ppt) separately for 21 days at their respective water hardness levels. Results show that after two months, weight gain (%), specific growth rate and feed conversion in 1250 mg/L CaCO₃ group were markedly worse than 100 and 494 mg/L CaCO₃ groups. This finding was accompanied with a down-regulation of hepatic IGF-I and growth hormone receptors (GHR) mRNA levels. During SST, growth was still significantly worse for those at 1250 mg/L CaCO₃ (1250SST). However, exposure to HEA inhibited growth for 100HEA and 494HEA relative to 1250HEA. Ammonia excretion rate (Jₐₘₘ) remained similar at different hardness levels; however, this was strongly inhibited at 100HEA, 494HEA and 100SST. This appeared to align to some degree with high water hardness having a protective effect on the gill histomorphology. In fact, fish at 1250HEA were able to increase Jₐₘₘ efficiently, which was associated with upregulated branchial expression of ammonia transporters (Rhesus glycoproteins ‘Rhcg’) and Na⁺/H⁺ exchanger, as well as augmented gill H⁺⁻ATPase activity. These responses prevented a build-up of excess ammonia in plasma. In contrast, both 1250 mg/L CaCO₃ and 1250HEA groups displayed inhibited branchial Na⁺/K⁺-ATPase activity indicating this unlikely has a major role in ammonia excretion. Gill Ca²⁺-ATPase activities reduced at the highest hardness (1250 mg/L CaCO₃) and 1250HEA groups, likely to prevent hypercalcemia in plasma. No differences among hardness levels were noted for ion-regulation or Ca²⁺- homeostasis under SST. Overall, findings suggest that although elevated water hardness up to 1250 mg/L CaCO₃ adversely affects the growth performance of channel catfish, it can ameliorate the inhibitory growth effects of HEA. This was partly attributed to 1250 mg/L CaCO₃ protecting the gills as well as mitigating HEA-induced ammonia excretory and ion-regulatory disruption. However, elevated hardness may not be as effective in alleviating salinity stress in catfish.