4. Discussion
4.3 Effect of Microalgae diets to Fat content and Fatty Acid composition of liver and muscle . 44
4.3.2 Muscle total fat content and fatty acid composition
Results from our trial showed no significant difference on amount of muscle fat deposited in fish between the two dietary groups with fat percentage in fish from AA dietary group was 18.7%. Results agree with other studies for instance a study by Sprague et al (2015) who reported no significant difference on lipid percentage among four dietary treatments (northern fish oil, southern fish oil, 11% microalgae diet and 5.5% microalgae diet) that was fed to Atlantic salmon (post-smolts) for 19 weeks. Also Norambuena et al., (2015) found similar results despite using different microalgae species (Ulva ohnoi at 2.5% and Entomoneis spp at 5%) to feed Juvenile Atlantic Salmon for 84 days. Nevertheless a reason for no variation
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from our study between two dietary groups is still unclear although it might relate the observation that fish in this study had lower VSI% (leaner fish) and thus energy in diet it was mostly used for growth rather than fat deposition in the stomach area.
Deposition of total ω-3, from our study differed significantly in muscle tissues between the two dietary groups. Higher levels of total ω-3 in muscle tissues of fish in AA dietary group supports a theory that fillet composition adapts composition of the diet used. Results disagree with Kousoulaki et al (2015) study in which fillet total ω-3 was not affected by different inclusion levels of microalgae in diets. It should be noted that both studies agree with the fact that, fillet composition is influenced by the DHA content from the diet. However, variation observed between our study and Kousoulaki et al (2015) in deposition of total ω-3 can be related to differences in trial period length, microalgae inclusion levels and inclusion of plant oils such as rape seed oil, palm oil and linseed oil that also contain 18:3n-3 that is a DHA precursor. In our study trial period was 400 days at 6.25% inclusion level while Kousoulaki et al., (2015) study was conducted for 84 days at 0.1%, 0.6% and 1.5% inclusion levels, suggesting that length of the experiment and inclusion level influence deposited ω-3. In general, n-3 LC-PUFA are important for the somatic growth of marine fish with DHA the most highly retained PUFA in a variety of species (Sargent et al., 2002).
EPA and DHA from diets need to be deposited in fillet due to their significance to both fish and human health. Accumulation of two EFAs, EPA and DHA differed significantly with fish in FO dietary group having higher levels of EPA, while amount of DHA was noted to be in higher levels on fish in AA dietary group. Lower levels of EPA in muscle tissues in fish from AA dietary are not odd as same results have been obtained from earlier studies that fed microalgae diet in replacement of fish oil (Ganuza et al., 2008 ; Miller et al., 2007: Eryalçin et al., 2013). Also EPA requirements that is supposed to be half of DHA levels (Rodríguez et al., 1998) were not met (DHA: EPA 3:0.6) in this study. Lower levels of EPA in fish from AA group indicate that retro-conversion of DHA to EPA or synthesis through Δ6 and Δ5 desaturation of 18:3n-3 to produce EPA in AA fed fish was not sufficient to maintain body levels of EPA relative to a FO fed fish (Tocher and Ghioni, 1999).
Higher levels of DHA on fillets from fish in AA dietary group are related with higher levels of DHA in the diet. Results agree with previous studies, who observed higher flesh DHA levels in salmon parr-smolts when Schizochytrium sp. replaced fish oil as oil source at 100% inclusion
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level for over 9 weeks (Miller et al. 2007) and as dried product at 100% inclusion level for 51 days (Carter et al. 2003). However, caution should be applied when comparing results since the former authors utilised the richer oil extract as the sole oil source. In a more related study by Sprague et al., (2015) it was noted no significant difference on DHA levels between fish that were fed FO and the one were fed AA diet however DHA levels in diet were not balanced at the same level instead microalga diet had higher levels of DHA than in FO diet. Obvious difference from their study was between two algal treatments (11% and 5.5%) showing that higher levels of DHA in the diet led to higher deposition levels in fillets. In our study that was not the case because fish in the AA group were fed more DHA and less EPA than in fish from FO group with the sum of EPA+DHA balanced to the same level in both dietary group. Despite most studies focusing on DHA levels it should be noted that EPA is as important as DHA because its deficiency can also lead to slow growth, reduce survival rate and stress (Liu et al., 2002: Bou et al., 2017) and also because EPA plays a role as precursor of eicosanoids, its deficiency can be associated with cell immune and the stress responses (Ganga et al., 2005, 2006). Inclusion of algal to levels higher than 11% might be able to improve amount of DHA deposited in flesh to resemble or even be above to the amount that is deposited by fish oil diets (Sprague et al. 2015).
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Conclusion
Present study described effects of using microalgae Schizochytrium spp as a supplement to Atlantic salmon diets from 860 grams fish size to approximately 3 kg. Studied parameters included the ones assessed fish performance, fillet quality and fatty acid composition between fish in two dietary groups: AA and FO. In general, fish performance parameters showed fish from AA dietary group performed better than in FO group. Fish final weight varied significantly, with fish from AA dietary group having higher mean final weight of 3.3kg compared to mean final weight of 2.8kg of fish from FO dietary group. K value was 1.23 for fish from AA dietary and 1.19 for FO dietary group, SGR was also high in fish from AA dietary group (0.27). Other qualities like gaping score, fillet yield, HSI%, a*value and L*value showed no significant difference between the two dietary groups. Regarding fatty acid composition it was interesting to see higher levels of DHA in fillets from fish in AA dietary group and since this is the first study that involved feeding Schizochytrium spp to Atlantic salmon as n3-LC PUFA source for a long period of time (400 days) those were encouraging results. This study shows that it is possible for Schizochytrium spp to replace fish oil at a 6.25% as source of n-3 LC PUFA source in Atlantic salmon fish until they reach market size without causing negative effect to their performance, fillet quality and fatty acid composition. On the other hand main nutritional concern of using algal diet alone is low levels of EPA in fillets and therefore more studies are needed to counteract existing challenges.
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