Introduction
The functionally essential and human health promoting omega-3 fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are still primarily provided through finite marine raw materials (fish meal/oil) and supply is increasingly strained by a growing aquaculture industry. In the face of this EPA/DHA scarcity (Glencross et al., 2025; Tocher et al., 2019), the challenge is how to allocate the limited supply of these valuable fatty acids most effectively in order to a) minimize their use wherever possible while b) simultaneously ensuring nutraceutical EPA/DHA levels in the final product. In contrast to their marine counterparts, many freshwater fish possess the entire pathway for EPA/DHA biosynthesis from the C18 precursor ALA (α-linolenic acid), suggesting the potential for entirely marine-ingredient-free and low EPA/DHA grow-out diets combined with EPA/DHA-containing finishing diets to achieve economically and nutritionally optimal EPA/DHA resource allocation. In this sense, a 13-week finishing diet trial was performed with African catfish (Clarias gariepinus), a freshwater species capable of EPA/DHA biosynthesis (Oboh et al., 2016).
Materials & Methods
Two experimental diets were formulated in an isonitrogenous and isolipidic manner (43% crude protein, 13% crude fat) on the basis of non-marine animal protein sources. Diets only differed with regard to the added oil source with the high EPA/DHA diet (HI) being based on salmon oil (8% inclusion, EPA+DHA: 4.5 g kg-1 feed) and the low EPA/DHA diet (LO) based on rapeseed oil (8% inclusion, EPA+DHA: 0.5 g kg-1 feed). Fish (start weight: 153 g) were reared in 12 independent recirculating aquaculture systems (1120 L system volume) for 13 weeks (12 weeks feeding + 1 week simulated purging without feeding) at 27 °C in three experimental groups (n = 4): HI diet for 12 weeks (HI), LO diet for 12 weeks (LO) and switch from the LO to the HI diet after the first 6 weeks of feeding (LO-HI). Apart from tracking growth performance, fish were continuously sampled (week 0, 2, 4, 6, 7, 8, 10, 12, and 13) to investigate the development of filet fatty acid profiles, hepatic expression of genes relevant in the biosynthetic pathway from ALA to EPA/DHA (elovl2, elovl5, fads2) and in amino acid metabolism (glud1a and gpt2), filet proximate composition and lipid classes, as well as dissolved total inorganic nitrogen excretion.
Results
Growth and feed conversion performance did not differ significantly between the HI and the LO-HI treatment (body weight gain - BWG: 645%/641%, FCR: 0.83/0.82), whereas the fish in the LO treatment showed significantly reduced performance (BWG: 555%, FCR: 0.94) as well as a significantly higher body weight loss after the purging period (4.7% vs. HI – 4.0% and LO-HI - 3.9%). The HI-fed fish recorded the consistently highest filet EPA and DHA content, reaching 3.3 mg EPA g-1 dry matter (DM) and 11 mg DHA g-1 DM at the end of the trial, whereas the filet EPA and DHA content of the LO-fed fish declined until week 8 before stabilizing to finally achieve 39% (1.3 mg EPA g-1 DM) and 48% (5.3 mg DHA g-1 DM) of the filet EPA and DHA content of the HI-fed fish. Filet EPA and DHA content in the fish from the finishing diet group (LO-HI) increased steadily from week 8 onward to achieve 75% (2.5 mg EPA g-1 DM) and 77% (8.4 mg DHA g-1 DM) of the respective levels found in the HI-fed fish, which were nevertheless still significantly higher. Expression of hepatic elovl2 and elovl5 was significantly elevated in the LO-fed fish compared to the HI-fed fish at the end of the feeding period, with fads2 also showing a trend towards higher expression. Dissolved total inorganic nitrogen release was significantly higher in the LO group (40.4 g kg-1 feed) than in both of the other groups (35.9 g kg-1 feed), coinciding with significantly higher hepatic glud1a expression.
Discussion
Despite the capacity of African catfish to biosynthesize EPA and DHA from ALA as previously suggested (Oboh et al., 2016) and again implied by the gene expression and fatty acid results of the present study, fish in this trial showed significantly reduced growth and feed conversion performance when fed exclusively with the low EPA/DHA diet (LO). Combining this insight with the significantly higher dissolved nitrogen release of these fish points towards the hypothesis that exceedingly low dietary EPA/DHA concentrations stimulate active EPA/DHA biosynthesis to cover physiological requirements which in turn costs additional energy, potentially supplied at least partly through amino acid catabolism, causing a reduction in overall growth performance. However, fish in the finishing diet treatment (LO-HI) performed equally well to the fish exclusively fed the high EPA/DHA diet. While not quite achieving the filet EPA/DHA levels of the latter, filet EPA/DHA levels in the finishing diet treatment still recovered significantly compared to the LO treatment. Considering the lower costs of rapeseed oil compared to salmon oil or other fish oils, this result suggests that a finishing diet strategy for African catfish could be beneficial from a feed cost perspective while at the same time enabling high product quality regarding filet EPA/DHA content.
Acknowledgement
This research was funded by the Federal Ministry of Research, Technology and Space as part of Agricultural Systems of the Future (CUBES Circle), grant number 031B1523B, as well as the support of VVI CENAKVA Research Infrastructure (ID 90238, MEYS CR, 2023–2026).
References
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Oboh, A., Betancor, M.B., Tocher, D.R., Monroig, O., 2016. Biosynthesis of long-chain polyunsaturated fatty acids in the African catfish Clarias gariepinus: Molecular cloning and functional characterisation of fatty acyl desaturase (fads2) and elongase (elovl2) cDNAs. Aquaculture 462, 70–79. https://doi.org/10.1016/j.aquaculture.2016.05.018
Tocher, D.R., Betancor, M.B., Sprague, M., Olsen, R.E., Napier, J.A., 2019. Omega-3 long-chain polyunsaturated fatty acids, EPA and DHA: Bridging the gap between supply and demand. Nutrients. https://doi.org/10.3390/nu11010089