| Water pH constitutes a fundamental ecological parameter that regulates physiological homeostasis and determines the productivity of fish within aquaculture systems. In intensive, high-density recirculating aquaculture operations, abrupt declines in water pH—often stemming from excessive stocking densities, unbalanced feeding protocols, and uncontrolled application of chemical additives—pose critical risks to fish health and compromise system sustainability. Acute acidification episodes destabilize the aquatic milieu, disrupt endocrine regulation, and precipitate extensive metabolic perturbations. In China, landlocked rainbow trout (Oncorhynchus mykiss) and anadromous steelhead trout (Oncorhynchus mykiss), representing two distinct life-history strategies within the same species, have become the principal salmonid species cultured in offshore marine aquaculture. These ecotypes exhibit marked physiological plasticity differences, including thermal tolerance, osmoregulatory capacity, and hypoxia resilience, which likely underpin their distinct responses to acidification stress. Teleost fishes have evolved intricate physiological adaptations to counteract environmental acidification, with the antioxidant defense system playing a central role. Acidic stress conditions induce accumulation of reactive oxygen species (ROS), eliciting the upregulation of key antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT). These enzymes act synergistically to neutralize ROS and mitigate oxidative damage, as evidenced by alterations in lipid peroxidation biomarkers such as malondialdehyde (MDA). Concurrently, the innate immune system is mobilized to attenuate cellular damage and combat opportunistic infections. Acidification further disrupts ionic homeostasis by inhibiting branchial ATPase activities, thereby compromising osmoregulatory function. Additionally, gastrointestinal pH alterations may impair digestive enzyme activities, resulting in reduced nutrient assimilation efficiency. This study systematically compared the physiological and biochemical responses of rainbow trout and steelhead trout to acute acidification stress. Three experimental pH gradients (6.5, 6.0, 5.5) were established using a controlled chemical acidification protocol (37% hydrochloric acid: sodium bicarbonate = 1 mL: 0.45 g), with pH 8.0 serving as the control. The acidification rate was maintained at 0.25 pH units per hour to simulate realistic stress scenarios. Physiological parameters—including hepatic antioxidant enzymes (superoxide dismutase [SOD], catalase [CAT], malondialdehyde [MDA]), serum non-specific immune markers (alkaline phosphatase [AKP], acid phosphatase [ACP]), branchial osmoregulatory ATPases (Na+/K+-ATPase, Ca2?-ATPase, Mg2?-ATPase), and intestinal digestive enzymes (amylase [AMS], lipase [LPS], trypsin [TPS])—were quantified at three critical intervals: pre-stress (0 h), peak stress (12 h), and post-recovery 24 h. Results showed that acidification significantly influenced hepatic superoxide dismutase (SOD) and malondialdehyde (MDA) levels, serum alkaline phosphatase (AKP), branchial Na+/K+-ATPase and Mg2?-ATPase, as well as intestinal amylase (AMS) and trypsin (TPS) activities in both ecotypes. At pH 6.0, rainbow trout exhibited significantly higher hepatic SOD, MDA, serum AKP, branchial Na+/K+-ATPase, Ca2?-ATPase, and Mg2?-ATPase activities compared to other treatment groups (P < 0.05), whereas their intestinal AMS and TPS activities were significantly lower at pH 5.5 (P < 0.05). At pH 5.5, steelhead trout exhibited significantly higher hepatic SOD and MDA levels, serum AKP, branchial Na+/K+-ATPase and Mg2?-ATPase activities compared to other treatment groups (P < 0.05). Conversely, their intestinal AMS and TPS activities were significantly elevated at pH 8.0 and 6.5 relative to other treatments (P < 0.05). At pH 6.0, significant differences (P < 0.05) were observed in rainbow trout between 12 h and recovery 24 h for hepatic SOD, MDA, serum acid phosphatase (ACP), AKP, branchial Na+/K+-ATPase, and intestinal lipase (LPS) and TPS activities. At pH 5.5, significant differences (P < 0.05) were detected between 12 h and recovery 24 h for rainbow trout serum AKP activity and steelhead trout hepatic SOD, serum ACP, AKP, branchial Na+/K+-ATPase, and intestinal LPS activities. Collectively, rainbow trout exhibited the most pronounced responses at pH 6.0, with most parameters returning to baseline after recovery, whereas steelhead trout demonstrated stronger stress responses at pH 5.5. Significant interaction effects between species and acidification levels were observed for hepatic SOD, MDA, catalase (CAT), branchial Na+/K+-ATPase, Ca2?-ATPase, Mg2?-ATPase, and intestinal AMS, LPS, and TPS activities (P < 0.05). Principal component analysis revealed that different pH treatments significantly altered the overall physiological and biochemical states of both ecotypes, with distinct clustering patterns between the two species. Collectively, these findings indicate that landlocked rainbow trout are more vulnerable to acute acidification stress, whereas anadromous steelhead trout possess greater physiological resilience in acidic environments. These findings provide theoretical insights into the environmental adaptation mechanisms of salmonids with divergent life-history strategies and inform aquaculture management practices in acidified water systems. Implementing pH buffering strategies at ≥6.5 could mitigate stress in rainbow trout systems, whereas steelhead trout cultures may tolerate brief exposures to pH 5.5 with appropriate recovery intervals. |
