The effect of dissolved oxygen on feed intake, growth and nutrient utilization in Atlantic salmon (Salmo salar)

Abstract

Atlantic salmon (Salmo salar) is a species of significant economic importance in aquaculture. Optimal growth and nutrient utilization are critical for the efficient production of Atlantic salmon. However, especially during late summer and early autumn farmed salmon can experience prolonged periods with low levels of dissolved oxygen (DO). Oxygen is considered to be one of the main limiting factors influencing fish metabolism and sufficient amount of oxygen is important for all energy-demanding processes. Previous studies have shown that hypoxia, or reduced levels of DO, can affect both the feed intake and growth rates in fish, potentially impacting the aquaculture productivity. However, the specific physiological and molecular mechanisms behind how Atlantic salmon adapt to such conditions are not fully understood. The aim of this study was to investigate if different oxygen levels would affect feed intake, growth and nutrient utilization in Atlantic salmon post-smolts. This study includes results from two trials with groups of different fish sizes: approximately 300g (OxySalar 1) and 700g (OxySalar 2). Triplicate tanks were kept at constant DO levels at 50%, 65% and 100% reared at an optimal growth temperature at 12 ℃ for 27 days (OxySalar 1) and 22 days (OxySalar 2). The fish were fed with a commercial diet to apparent satiety twice daily for two hours. Samples of the fish were taken at the start and at the end of the trials to document growth performance traits and the proximate body compositions of farmed Atlantic salmon exposed to constant prolonged low DO levels. Our results showed that the feed intake was reduced in mild hypoxia (65% DO) and moderate hypoxia (50% DO) throughout both trials. The reduction in feed intake was more pronounced towards the end of the trials, with the moderate hypoxia group reducing their feed intake by 18% in OxySalar 1 and by 12% in OxySalar 2 during the last five days. However, this reduction in feed intake did not correlate with a decrease in growth rates, as sub-optimal growth data were observed. The limitations in growth rated resulted further to minimal variations in body composition and nutrient utilization between DO levels. However, differences were found in the body composition between oxygen groups, particularly in fat content in the viscera, where DO100 had a greater capacity to store more energy in the form of fats. This may be attributed to the higher feed intake observed towards the end in the DO100 group. Although variations were found in the nutrient productive values, they were not significantly different and suggest that oxygen is not a limiting factor on nutrient retention. Our result suggests that a reduction in oxygen levels decreases the feed intake without affecting the feed efficiency.