Replacement of fish oil with a DHA-rich algal meal derived from Schizochytrium sp. on the fatty acid and persistent organic pollutant levels in diets and flesh of Atlantic salmon (Salmo salar, L.) post-smolts
Introduction
It is widely accepted that fish consumption is an excellent source of the beneficial omega-3 long-chain polyunsaturated fatty acids (n−3 LC-PUFA), eicosapentaenoic acid (EPA; 20:5n−3) and docosahexaenoic acid (DHA; 22:6n−3), which have important roles in protecting against cardiovascular disease as well as neurological and inflammatory conditions among other health benefits (Calder & Yaqoob, 2009). Nevertheless, fish consumption, particularly oily fish, is also a major dietary exposure route for humans to persistent organic pollutants (POPs), including dioxins [polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs)], dioxin-like polychlorinated biphenyls (DL-PCBs) and polybrominated diphenyl ethers (PBDEs). These lipophilic compounds are easily absorbed and rapidly distributed to lipid-rich organs and tissues which can result in their bioaccumulation. Consequently, the beneficial effects may be offset by the negative risks associated with fish intake.
In farmed fish, such as Atlantic salmon (Salmo salar), the traditional marine-derived components of fish feed, fish oil and fish meal, are considered to be the major sources of POPs (Berntssen et al., 2010, Jacobs et al., 2002). Reducing the POP levels in fish oils, and ultimately the feeds and flesh of fish, without affecting the LC-PUFA content, has been achieved using decontamination techniques (Berntssen et al., 2010, Sprague et al., 2010) or by utilising less polluted fish oils from the southern hemisphere (Sprague et al., 2010). However, the increased competition from the pharmaceutical and nutraceutical industries for n−3 LC-PUFA coupled with the stagnated global supply of wild catch fisheries has led to increased prices and supply pressures resulting in the aquafeed sector investigating alternative lipid sources.
The partial and complete replacement of marine ingredients with agricultural plant products, mainly of oilseed origin, has been performed in salmon without any detrimental effects on growth performance or fish health (e.g. Bell et al., 2004, Bell et al., 2005). Such substitution reduces the levels of undesirable POPs (Bell et al., 2005, Bell et al., 2012; Berntssen, Julshamn et al., 2010). Conversely, since the fatty acid composition of fish tissue reflects that of the diet (Sargent, Tocher, & Bell, 2002), increasing the vegetable content in aquafeeds reduces the favourable LC-PUFA levels abundant in fish oils, thereby compromising the overall nutritional quality of the final product. Finishing diets can be employed to restore flesh n−3 LC-PUFA levels in vegetable fed fish, although this still relies upon the inclusion of fish oils (Bell et al., 2004, Bell et al., 2012), albeit at the risk of increasing POP levels in a previously low contaminated product (Bell et al., 2005, Bell et al., 2012). Since marine fish lack the conversion pathways to efficiently produce EPA and DHA at appreciable levels, these essential fatty acids must be obtained through the diet (Sargent et al., 2002).
Marine microalgae are primary producers of n−3 LC-PUFA, and are therefore a promising alternative to the traditional marine derived ingredients of fish feed. Several species have been identified as potential sources, among which the thraustochytrids have been preferred due to their ease for large-scale heterotrophic cultivation under controlled conditions to produce a high lipid product rich in n−3 LC-PUFA (Lewis et al., 1999, Ratledge, 2005). Furthermore, unlike crude oils and oilseeds, algal biomass produced by fermentation is generally free from environmental contaminants and heavy metals (Ratledge, 2005). Schizochytrium sp. is a fast growing thraustochytrid microalgae, rich in DHA, with a relatively simple culture process compared to other single-cell microalgae (Ganuza et al., 2008; Lewis et al. 1999). Subsequently, the aquaculture industry has investigated the potential of thraustochytrids, particularly Schizochytrium sp. in either dried biomass or oil extracted form, as an alternative lipid source in enriching zooplankton for feeding to finfish larvae (Barclay & Zeller, 1996), supplementing channel catfish diets, Ictalurus punctatus, (Li, Robinson, Tucker, Manning, & Khoo, 2009), or as replacement for fish oils in diets for sea bream, Sparus aurata (Ganuza et al., 2008), and Atlantic salmon (Carter et al., 2003, Miller et al., 2007). However, to date no such studies have been performed in post-smolt salmon to assess Schizochytrium sp. inclusion as a potential replacement for fish oil in grow-out feeds and its effects on the nutritional quality of the final product.
The present study therefore evaluated the replacement of fish oil with a DHA-rich algal meal, derived from Schizochytrium sp., at two different inclusion levels (11% and 5.5% of diet) on the fatty acid and POP compositions of Atlantic salmon diets and flesh compared to fish fed a fish oil diet of either northern or southern hemisphere origin.
Section snippets
Experimental set-up and diets
The trial was performed at Marine Harvest’s (Scotland) Feed Trial Unit (Ardnish, Inverness-shire, Scotland) using a commercial strain of 1700 1 + Atlantic salmon post-smolts. Fish (850 ± 100 g, mean ± SD) were initially stocked into four 125 m3 sea pens and acclimatised for 10 weeks during which they were fed a high rapeseed/fish oil (6:1, w/w) preconditioning diet in order to reduce flesh POPs and LC-PUFA levels. Fish (1534 ± 400 g) were then split between twelve pens (130 fish per pen) and acclimated for
Dietary fatty acid and POP compositions
The fatty acid compositions of the precondition and enrichment diets are presented in Table 1. Since the aim of the present study was to investigate the effects of the algal-feeds on LC-PUFA levels, fish were first fed a preconditioning feed consisting mainly of rapeseed oil to decrease n−3 LC-PUFA levels. Previous studies using vegetable-based diets have shown a decrease in the levels of flesh EPA and DHA due to the absence of these particular fatty acids in plant-based ingredients (Bell et
Conclusion
The replacement of fish oil with a DHA-rich Schizochytrium sp. microalgae significantly decreases both dietary and flesh fillet POP levels compared to fish oil based treatments. Moreover, flesh fillet DHA levels can be tailored to similar levels in fish oil fed fish when algal biomass is included at 11% of the diet. However, the absence of EPA in algal-based diets significantly impairs the overall nutritional value, in terms of g EPA + DHA per serving, to the final human consumer. Current and
Conflict of interest statement
None of the authors have a conflict of interest.
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