Abstract

Simple Summary: The Chilean meagre (Cilus gilberti) has been domesticated to enhance aquaculture in South America. However, hypoxia, a known chronic stressor in fish that affects their immune response to pathogens, could pose a significant challenge. This study focused on evaluating the impact of acute and intermittent hypoxia on the antibacterial capacity of the epidermal mucus of C. gilberti. Overall, the mucus of fish under both conditions exhibited activity against two marine pathogens belonging to the genus Vibrio sp. During hypoxia events, this activity decreased, but upon restoration of normal oxygen concentration, mucus activity levels recovered to match those of the normoxia group. Notably, oxygen deficiency affected the innate immune response of fish to the lipopolysaccharide (LPS) of Vibrio anguillarum. While LPSs stimulated mucus activity under normoxic conditions, no stimulation and even downregulation were observed in fish exposed to hypoxic conditions. Lysozyme displayed a similar pattern of activity, while no modulation of peroxidase activity was detected after periods of hypoxia. This work constitutes the first study about the environmental impact on the defense mechanisms of C. gilberti. It aims to contribute to enhancing the welfare and health of cultured fish, thereby fostering greater productivity within the future Chilean meagre aquaculture industry. Comprehending the immune defense mechanisms of new aquaculture species, such as the Chilean meagre (Cilus gilberti), is essential for sustaining large-scale production. Two bioassays were conducted to assess the impact of acute and intermittent hypoxia on the antibacterial activity of juvenile Chilean meagre epidermal mucus against the potential pathogens Vibrio anguillarum and Vibrio ordalii. Lysozyme and peroxidase activities were also measured. In general, fish exposed to hypoxia showed a 9-30% reduction in mucus antibacterial activity at the end of hypoxic periods and after stimulation with lipopolysaccharide. However, following water reoxygenation, the activity of non-stimulated fish was comparable to that of fish in normoxic conditions, inhibiting bacterial growth by 35-52%. In the case of fish exposed to chronic hypoxia, the response against V. anguillarum increased by an additional 19.8% after 6 days of control inoculation. Lysozyme exhibited a similar pattern, while no modulation of peroxidase activity was detected post-hypoxia. These results highlight the resilience of C. gilberti to dissolved oxygen fluctuations and contribute to understanding the potential of mucus in maintaining the health of cultured fish and the development of future control strategies.