Oxygen is one of the most important factors to consider in fish farming. Maintaining proper oxygen levels is of crucial importance for an improved performance as it impacts fish growth, development and welfare. Adequate oxygen supports disease prevention, efficient nutrient metabolism, and good water quality by preventing the accumulation of harmful substances. The aim of this work was to investigate the impact of three oxygen levels on meagre rearing through the assessment of differentially expressed genes in liver, gill and heart.
A 31 day long trial, performed at EPPO/IPMA (Olhão, Portugal), started with fish with 252.3 ± 29.5g, distributed along 9 tanks (1.5 m3) at a density of 10.9 ± 0.1 kg.m-3 per tank, salinity and temperature kept at 37 and 20.7 ± 1.0°C, respectively. Three oxygen levels were tested: normoxia (DO-1: 6-7 mg L-1); moderate hypoxia (DO-2: 4-5 mg L-1) and severe hypoxia (DO-3: 2.5-3 mg L-1). Oxygen was added automatically, when needed, to maintained the pre-defined levels. Fish were fed with a commercial diet four times a day, ad libitum. At the end of the trial, samples from liver, gills and heart were collected from 2 fish per tank for gene expression (6 fish per condition). Total RNA was extracted, quantified and quality verified before cDNA synthesis for RT-qPCR. Liver samples were also collected to evaluate the activity of oxidative stress-related enzymes.
Fish final weight was 391.2±62.0a, 404.0±60.9b and 411.5±56.3b g on DO-1, DO-2 and DO-2, respectively. Liver superoxide dismutase (SOD) presented an increased activity in DO-3 when compared to the Control, however SOD activity in DO-2 and DO-3 were similar, suggesting the activation of this response on moderate hypoxia conditions.
The hypoxia related gene HIF-3α decreased in severe hypoxia in heart when compared with the moderate hypoxia treatment, and in gill the expression decreased in normoxia and severe hypoxia when compared to moderate hypoxia. HIF-3α expression was higher in moderate hypoxia in both tissues and no other genes differed significantly. This suggests a possible adaptation of meagre to cope with low oxygen levels.
These results suggest a modulation on metabolic activity and provide insights into the mechanisms underlying the adaptive response of meagre to hypoxic conditions. However, further research is ongoing to investigate the expression of other stress and hypoxia-related genes and the functional consequences of their regulation in meagre.
Acknowledgments: This study was funded by project NOVIFEED - Novel tools for intelligent feeding management in Atlantic salmon and meagre farming (PT-INNOVATION-0099).