With the aquaculture expansion, sustainability challenges have been emerging. Among them, there is great urgency in reducing the industry’s dependence on finite-marine ingredients. Plant lipid sources have been commonly used to replace fish oils in feed formulations. However, vegetable oils-based diets might markedly deviate from fish natural feeding habits and negatively impact their health. In the particular case of Atlantic salmon (Salmo salar), this issue acquires major relevance during the seawater transfer (SWT), a stressful event and challenging production moment often associated with great losses. To improve the fish overall health, supplementing diets with immuno-modulators, such as certain amino acids, is becoming a common approach (Andersen et al., 2015). Among the amino acids recognized to cover functional roles that may improve fish health, histidine (His) and its derivatives (Remø et al., 2014) were found to behave as buffer and antioxidant and to have anti-inflammatory properties in the tissues they are present (Andersen et al., 2015). In this study, we have investigated whether dietary lipid source and His supplementation could influence the non-specific immune response and antioxidant system in Atlantic salmon after SWT.
Materials and methods
The fish were given four experimental diets differing in lipid source (100% fish oil or blend of vegetable oils) and His content (10 or 14 mg His/g diet) in freshwater (FW), and a cross-over was done after SWT to investigate the effect of supplying His supplementation in FW vs SW (Figure 1). At the end of the FW phase and 2 and 4 weeks after SWT, fish were sampled for blood plasma and liver to assess immune parameters and oxidative stress biomarkers, respectively. Gut samples were also collected for gene expression measurements.
Results and discussion
Regardless of dietary treatment, post-smolts appeared to be under oxidative stress two weeks after SWT, as indicated by the lower antioxidant enzymes activity. Even so, they seemed to manage to protect themselves from oxidative damage, since no increase in hepatic lipid peroxidation (LPO) levels were observed between the two times. Additionally, the increased alternative complement pathway activity in plasma found at the end of the feeding trial further suggests that all groups recover well from the transfer.
Post-smolts fed fish oil-based diets displayed higher plasma bactericidal and alternative complement activities than those fed vegetable oil feeds. Increased LPO levels were also observed in fish fed fish oil diets, possibly due to the greater susceptibility of marine lipids to oxidation. Apart from a stimulating effect of His supplementation on plasma IgM levels four weeks after SWT, it did not seem to significantly influence the post-smolt Atlantic salmon immunity and oxidative status, independently of the dietary lipid source.
All dietary groups seemed to recover well from the stressful SWT moment. There were, however, some minor signs of improved immunity of fish fed fish oil-based diets compared to those fed plant lipids. The small differences in both plasma innate immune parameters and liver oxidative stress biomarkers between the dietary His groups suggest that it did not significantly influence the post-smolts health status. Further analyses on gut samples are currently underway to provide more insight into the fish health condition.
This work was partially supported by UIDB/04423/2020, UIDP/04423/2020, IF/00197/2015 and INFLAMMAA (reference PTDC/CVT-CVT/32349/2017), financed by Portugal and the European Union through FEDER and COMPETE 2020, and through the COMPETE and Operational Human Potential Programmes and national funds through Fundação para a Ciência e a Tecnologia (FCT, Portugal). The feeding experiment was funded by the Institute of Marine Research (Bergen, Norway) and the feeds were produced by BioMar AS.
Andersen, S., Waagbø, R., & Espe, M. (2015). Functional amino acids in fish nutrition, health and welfare. Frontiers in bioscience (Elite edition), 8, 143-169.
Remø, S. C., Hevrøy, E. M., Olsvik, P. A., Fontanillas, R., Breck, O., & Waagbø, R. (2014). Dietary histidine requirement to reduce the risk and severity of cataracts is higher than the requirement for growth in Atlantic salmon smolts, independently of the dietary lipid source. British Journal of Nutrition, 111(10), 1759-1772.