The use of terrestrial vegetables as a source of protein and lipid in aquaculture has led to a reduction of n-3 long-chain polyunsaturated fatty acids (LC-PUFA) in salmon fillets. Genetic modification of oilseed crops such as Camelina or rapeseed as well as production of heterotrophic microalgae has allowed the production of land-based sources of LC-PUFA with the potential to take the pressure off fish oil supplies, which mainly come from wild-caught oily fish – a finite resource. Although several studies have investigated the impacts that these new lipid sources have on fish performance and product quality, limited information exists regarding their impact on fish health or adaptation to stress (Napier and Betancor, 2023., Ruyter et al., 2022; Santigosa et al., 2020). The aim of the present study was to assess the impact of different novel lipid sources on Atlantic salmon ( Salmo salar ) parr on their stress response during smoltification by investigating the production of lipid inflammatory mediators (LIM) as well as the head kidney lipid and fatty acid composition and transcriptomic response.
Materials and Methods
A conventional nutritional trial was carried out both in freshwater Atlantic salmon parr from 37.6 ± 6.7 g up to smoltification ( 109.3 ± 30.3 g) . Fish were fed one of eight experimental feeds, in triplicate , containing different commercial and experimental lipid sources. Briefly, oils included two GM-camelina oils (high EPA and high EPA+DHA ; ECO and EDCO, respectively), a canola GM oil (high DHA; Aqua ), a microalgal oil (high EPA and DHA ; Ver ), a northern- and southern-hemisphere fish oil (reference feeds , NHFO and SHFO, respectively), krill oil (positive control; KO feed) and sunflower oil ( SFO; negative control) (Fig. 1) . Feeds were manufactured by BioMar (Brande, Denmark). All feeds used the same base pellet formulation with a fishmeal level reflecting current commercial practice for salmon and were formulated to satisfy all the known nutritional requirements of salmon, including a basal EPA+DHA content of >2.5% of total fatty acids.
Prior to sea water transfer, six fish per tank were humanely euthanized and samples from head kidney either quickly frozen for lipid extractions or stabilized into RNALater for RNAS eq analysis (Illumina Sequencing). Plasma was also collected from the six sampled fish, quickly frozen in liquid ni trogen until LIM by LC-MS-MS. Additionally, five fish per tank were subjected to a sea water challenge, by transferring the fish for 24h at a salinity of 35 ppt , after which the fish were euthanized and plasma and head kidney collected.
Results and Discussion
Despite experimental feeds being isolipidic, the kidney lipid content varied among the different dietary treatments. In this sense, fish fed NHFO displayed the largest fat content, whereas those fed EDCO displayed the lowest content, with the remaining treatments displaying intermediate values. These different lipid contents also had an impact o n the lipid class composition of head kidney, with fish fed NHFO showing the highest levels of neutral lipids, which are mainly used as energy source, specifically triacylglycerols. The opposite trend was observed in EDCO-fed fish , which in contrary displayed higher contents of structural or polar lipids than NHFO-fed fish. The head kidney fatty acid profile mainly reflected dietary input, not indicating any biosynthetic capacity, as expected. In this sense, the lowest n-3 LC-PUFA levels were detected in the group fed SFO and the highest in fish fed the microalgal oil.
The analysis of LIM demonstrated how the levels of those derived from shorter chain fatty acid (e.g. 18:3n-3 and 18:2n-6) were regulated by dietary input, rather than stress (salinity challenge). On the contrary, levels of those derived from n-6 (20:4n-6) and n-3 LC-PUFA (EPA and DHA) tended to increase/decrease mainly along with stress. Further RNASeq results will also be presented and related to LIM production in head kidney.
Napier JAN, Betancor MB (2023) Engineering plant-based feedstocks for sustainable aquaculture . Current Opinion in Plant Biology 71, 102323 .
Ruyter B. et al. (2022) A dose-response study with omega-3 rich canola oil as a novel source of docosahexaenoic acid (DHA) in feed for Atlantic salmon ( Salmo salar) in seawater; effects on performance, tissue fatty acid composition, and fillet quality . Aquaculture 561, 738733
Santigosa et al. (2021) A novel marine algal oil containing both EPA and DHA is an effective source of omega-3 fatty acids for rainbow trout (Oncorhynchus mykiss) . Journal of the World Aquaculture Society 51, 649-665 .
This work was completed as part of the biotechnology and biological sciences research council (BBSRC) funded project NOSIFISH “Novel Omega-3 Sources in Feeds and Impacts on Salmon Health” (BB/S005919/1 ).