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IntroductionCultured fish species are increasingly exposed to fungi and bacteria in the rearing environment, often causing disease and mortality causing aquaculture producers to rely on the use of chemical therapeutants which may have negative consequences for both human and environmental health. This study investigated the effect of humic acid as a treatment to reduce fungal and bacterial infections to increase survival during the incubation of rainbow trout (Oncorhynchus mykiss) eggs.MethodsHumic acid, an aqueous extract of oxidized lignite was added to water entering both stacked tray and jar type incubators at a dosage rate of 0.005 mg/L from fertilization until hatching.ResultsThe water treatment eliminated observable fungus and significantly improved survival until hatching in the treated incubator trays (77.2%) compared to the untreated controls (55.5%) (p ≤ 0.001). Egg survival was not improved in the incubator jars. The humic acid reduced bacterial diversity, but only in the stacked tray incubators and altered bacterial composition of the water after 20 days of treatment.DiscussionThe treatment increased the bacterial abundance of Burkholderiales, positively associated with healthy fish eggs and decreased the abundance of Flavobacterium and Aeromonas, known fish pathogens. These findings support the topical treatment of humic acid as a potential alternative treatment to prevent fungal infections and reduce mortality during incubation of rainbow trout eggs.

In a 10‐week study, we evaluated the effects of replacing 20%, 40% or 60% of fishmeal (present in control diet at 300 g/kg) on a digestible protein basis with yeast Saccharomyces cerevisiae or a yeast mixture of Wickerhamomyces anomalus and S. cerevisiae on growth performance, nutrient digestibility, nutrient retention and intestinal health of rainbow trout (Oncorhynchus mykiss). Triplicate tanks with 35 rainbow trout (144.7 ± 25.1 g mean ± SEM) were fed rations of 1.5% of total biomass per tank. Replacement of 60% of fishmeal with yeast mixture resulted in lower specific growth rate of 1.0% versus 1.2%/day for other diets. Apparent digestibility coefficients for crude protein and most amino acids were highest in fish fed fishmeal‐based diet, with similar values for fish fed the diet with 20% replacement with yeast mixture. Diet with 20% replacement with yeast mixture resulted in highest phosphorus digestibility. Replacement of 60% of fishmeal with S. cerevisiae resulted in oedematous mucosal fold tips in the proximal intestine. The results of this study suggest that these yeasts can replace up to 40% of fishmeal under current inclusion levels in diets for rainbow trout without compromising growth performance, nutrient digestibility or intestinal health.

Live yeast may be a sustainable protein source in salmonid diets while exhibiting a probiotic effect to counteract environmental stressors, such as increased water temperature that is being exacerbated by climate change. The objective of this study was to evaluate the effects of feeding a high dietary inclusion of live yeast and increased water temperature on growth, haematological and intestinal physiology of rainbow trout. For six weeks, 129 g fish in 16 tanks (n = 4) were fed either a diet based on fishmeal or based on live yeast (214 g kg−1 of diet or 7.6 log CFU g−1 of Saccharomyces cerevisiae) that replaced 40% of fishmeal protein while fish were reared in water temperatures of either 11 °C (cold) or 18 °C (warm). Fish weights, caudal blood and proximal and distal intestines were collected and analysed. Fish fed live yeast resulted in reduced growth (SGR and WG) and higher FCR, while growth in cold and warm water was similar despite differences in TGC. However, increased mortality, plasma cortisol, and intestinal oedema and villous damage indicated fish reared in warm water were subjected to chronic stress. Temperature had a significant effect on haematocrit and red blood cell counts that resulted in significantly higher haemoglobin levels in fish kept in warm water attributed to an elevated oxygen demand. In the proximal intestine, increased temperature resulted in reduced expression of pro-inflammatory cytokines, e.g. TNFα and IL8, that were further reduced in fish fed live yeast. In addition, feeding live yeast reduced gene expression of CLD6 involved in gut barrier function, which suggests that the level of yeast was too high and masked any beneficial effects on fish health. In conclusion, feeding a high inclusion of live yeast reduced fish growth and expression of intestinal genes, while increasing the temperature from 11 to 18 °C subjected fish to chronic stress that restricted growth, suppressed innate immunity and induced intestinal damage. Replacing 40% of fishmeal protein with live yeast did not counteract negative effects caused by increased temperature, thus alternative strategies need to be explored and implemented to protect the growth and health of rainbow trout from seasonal and long-term rises in water temperature.

Background In the last two decades, research has focused on testing cheaper and sustainable alternatives to fish oil (FO), such as vegetable oils (VO), in aquafeeds. However, FO cannot be entirely replaced by VOs due to their lack of omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA), particularly eicosapentaenoic (EPA; 20:5n-3) and docosahexaenoic (DHA; 22:6n-3) acids. The oilseed plant, Camelina sativa, may have a higher potential to replace FO since it can contains up to 40% of the omega-3 precursors α-linolenic acid (ALA; 18:3n-3) and linoleic acid (LA; 18:2n-6). Methods A 90-day feeding trial was conducted with 600 gilthead sea bream (Sparus aurata) of 32.92 ± 0.31 g mean initial weight fed three diets that replaced 20%, 40% and 60% of FO with CO and a control diet of FO. Fish were distributed into triplicate tanks per diet and with 50 fish each in a flow-through open marine system. Growth performance and fatty acid profiles of the fillet were analysed. The Illumina MiSeq platform for sequencing of 16S rRNA gene and Mothur pipeline were used to identify bacteria in the faeces, gut mucosa and diets in addition to metagenomic analysis by PICRUSt. Results and Conclusions The feed conversion rate and specific growth rate were not affected by diet, although final weight was significantly lower for fish fed the 60% CO diet. Reduced final weight was attributed to lower levels of EPA and DHA in the CO ingredient. The lipid profile of fillets were similar between the dietary groups in regards to total saturated, monounsaturated, PUFA (n-3 and n-6), and the ratio of n-3/n-6. Levels of EPA and DHA in the fillet reflected the progressive replacement of FO by CO in the diet and the EPA was significantly lower in fish fed the 60% CO diet, while ALA was increased. Alpha and beta-diversities of gut bacteria in both the faeces and mucosa were not affected by any dietary treatment, although a few indicator bacteria, such as Corynebacterium and Rhodospirillales, were associated with the 60% CO diet. However, lower abundance of lactic acid bacteria, specifically Lactobacillus, in the gut of fish fed the 60% CO diet may indicate a potential negative effect on gut microbiota. PICRUSt analysis revealed similar predictive functions of bacteria in the faeces and mucosa, although a higher abundance of Corynebacterium in the mucosa of fish fed 60% CO diet increased the KEGG pathway of fatty acid synthesis and may act to compensate for the lack of fatty acids in the diet. In summary, this study demonstrated that up to 40% of FO can be replaced with CO without negative effects on growth performance, fillet composition and gut microbiota of gilthead sea bream.

Researchers have adjusted dietary lipid:protein ratios and n-3 long-chain polyunsaturated fatty acids (LC-PUFA) to optimize the growth performance of Atlantic salmon. However, dietary impacts on the gut microbiome are lacking, especially under varying environmental conditions. To examine this response, post-smolt salmon (184 ± 5 g) were fed diets with lipid:protein ratios considered low (180, 570 g/kg) and high (230, 460 g/kg) along with low and high levels of n-3 LC-PUFA (7 or 14 g/kg) while fish were reared under low and high levels of dissolved oxygen (6.7 or 8.0 mg/L). At day 0, 35 and 116, digesta in the distal intestine were collected and analyzed for viable counts and 16S ribosomal RNA (rRNA) genes (V4 region) using Illumina MiSeq. The reduction in oxygen had negligible effects, except on viable plate counts of total bacteria and an initial effect on beta-diversity. In contrast, the high lipid (HL) diets had an increased alpha-diversity (e.g., Shannon and Chao-1) at day 0 and day 35 whereas high n-3 diets suppressed these indices at day 116. Generally, a reduction in alpha-diversity was observed over time and an interaction between lipid:protein ratio x n-3 was found. Between diets, beta-diversity and phyla abundance were similar as both Proteobacteria (44%) and Firmicutes (21%) dominated. However, at the genus level Aliivibrio, Streptococcus, Weissella, and Lactobacillus, were associated with low lipid (LL) diets while the high lipid diets were associated with less abundant bacteria, e.g., Chromohalobacter. At day 116, the relative abundance of the Tenericutes phylum increased 10-fold (36%). Fish fed the high lipid diet with high n-3 had reduced alpha-diversity, lowest abundance of lactic acid bacteria, and highest abundance of Mycoplasma, which may indicate a less healthy gut microbiome. Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analysis revealed that saturated and unsaturated fatty acid biosynthesis pathways were several folds higher in fish fed the high lipid diet, possibly to compensate for the lack of dietary n-3. In summary, our results show that the viable plate counts, alpha-diversity, beta-diversity, and predictive function of gut bacteria in Atlantic salmon post-smolts are influenced by dietary lipid:protein ratio and n-3 LC-PUFA over several time points with little effect by dissolved oxygen.

Important operational changes that have gradually been assimilated and new approaches that are developing as part of the movement toward sustainable intensive aquaculture production systems are presented via historical, current, and future perspectives. Improved environmental and economic sustainability based on increased efficiency of production continues to be realized. As a result, aquaculture continues to reduce its carbon footprint through reduced greenhouse gas emissions. Reduced use of freshwater and land resources per unit of production, improved feed management practices as well as increased knowledge of nutrient requirements, effective feed ingredients and additives, domestication of species, and new farming practices are now being applied or evaluated. Successful expansion into culture of marine species, both off and on shore, offers the potential of substantial increases in sustainable intensive aquaculture production combined with integrative efforts to increase efficiency will principally contribute to satisfying the increasing global demand for protein and food security needs.

Welfare of farmed fish has become of increasing concern for consumers, producers, interest groups and authorities. To improve fish welfare, it is necessary to find indicators that can identify stressful situations early enough so that an intervention can take place before detrimental effects occur. By using heart rate bio-loggers in freely swimming rainbow trout (Oncorhynchus mykiss), combined with plasma cortisol levels and a range of haematological and blood chemistry parameters, we assessed the severity of stress responses induced by a range of common aquaculture practices before and during harvest. Following surgery, transportation and reintroduction with conspecifics in the sea cage, it took ~4 days for heart rate to stabilize and for a clear circadian rhythm in heart rate to emerge (i.e. average circadian fluctuation in heart rate of ~25 to 27 beats min−1). The presence or absence of this circadian rhythm in heart rate could inform researchers in the aquaculture industry whether or not specific farming routines induce unnecessary and prolonged stress. The elevations in heart rate caused by common farming practises such as crowding and transportation (e.g. heart rate increased by ~8 and 9 beats min−1 above what would normally be expected for that time of day, respectively) corresponded well with increases in plasma cortisol levels. Stressful farming practises or events (indicated by elevated plasma cortisol levels) such as air exposure during brailing and aquatic hypoxia triggered a hypoxic bradycardia until fish were released back into oxygenated water whereupon heart rate significantly increased to repay the accumulated oxygen debt. Repeated stress induced by multiple farming practises (i.e. combined stressors of crowding, brailing and transportation) clearly had a cumulative and long-lasting effect as heart rate peaked at ~25 beats min−1 above what would normally be expected for that time of day. Heart rate also remained significantly elevated by ~9 beats min−1 the following morning, suggesting that if rainbow trout need to be subjected to multiple stressors during consequtive days, it is recommended that sufficient time for recovery is provided between stressors. This study demonstrates that heart rate monitoring can be useful to assess stress levels of freely swimming fish in sea cages. Moreover, the use of implantable bio-loggers opens up a broad range of possible applications that will allow researchers to investigate the effects of environmental and/or anthropogenic stressors on the welfare of fish under conditions more realistic to the aquaculture industry.

Ultraviolet (UV) irradiation is commonly used to control pathogen loads in recirculation aquaculture systems (RAS), although these micro-organisms can be shielded by particles in the water, and some species tolerate very high UV doses. The objective of this study was to evaluate membrane filtration (MF) as an alternative, or complimentary, treatment to UV irradiation for pathogen control in RAS, as well as examine the operation and cost of each treatment. In a pilot-scale RAS, both MF and UV were used to treat wastewater for 30 days and water samples were collected biweekly and analysed for culturable bacteria, suspended solids, UV transmittance and other parameters. Bacterial control efficiencies were similar between both MF and UV treatments, which removed 99% of total bacteria and 98% of heterotrophic bacteria, respectively. Surface fouling was negligible for the UV while MF required biweekly cleaning to maintain operation. However, MF had the additional benefit of removing 96% of suspended solids, which resulted in increased UV transmittance. Capital and operating costs of MF were similar to UV, but only when MF treated a fraction of the wastewater compared with UV. We conclude that MF represents a potential complimentary technology to enhance UV irradiation, especially to minimise pathogens in RAS that are shielded by particles or tolerate UV.

The impact of feed processing on the digestibility of two major grain proteins fed to post-smolt Atlantic salmon (Salmo salar) was examined using a simple factorial experimental design of two protein types (wheat and corn gluten) and two feed processing methods (hot [cooking] extrusion and cold extrusion). A series of diets were each prepared from the same basal mash, including two basal diets (each diet was prepared using the same processing methods as the test diets) were also included in the experiment. Triplicate tanks of salmon (286 ± 20.9 g) were fed each diet for one week and then stripped of faeces, which was repeated two more times. Faeces, diets and ingredients were assessed for protein, amino acid, and energy digestibilities and by derivation the ingredient digestibilities for these parameters were also determined. Ingredient protein digestibility was affected by both ingredient type and feed processing method, with an interaction effect between parameters also present. Protein digestibilities were highest in diets that were cold extruded, with corn gluten protein digestibility being significantly affected by feed processing method, but wheat gluten comparatively less affected. Energy digestibilities typically reflected the protein digestibilities, but the interaction effect was not evident. The digestibility of the sum of amino acids in corn gluten was negatively affected by hot extrusion but this processing method had limited impact on wheat gluten digestibility. The digestibility of individual amino acids was highly variable between ingredients and processing methods, though was typically consistent with the digestibility observations of crude protein and sum of amino acids. Wheat gluten had superior digestibility characteristics than that of corn gluten across most parameters. This study demonstrates that the feed processing method has a clear influence on the nutritional value of corn gluten but in contrast has negligible effect on the digestibilities of wheat gluten.

A study was conducted on 500 juvenile rainbow trout (122 ± 4 g) fed either a control diet or a treatment diet containing 300 mg/kg of a microencapsulated blend of organic acids and essential oils to elucidate effects on intestinal morphology and microbiome. Proximal intestinal villi length was significantly increased in fish fed the treatment diet. Despite no differences in gut inflammation scores, edema, lamina propria inflammation and apoptosis were completely absent in the distal intestine of fish fed the treatment diet. Next-generation sequencing of the 16S rDNA showed no differences in alpha and beta diversity, and gut bacteria were mainly composed of Firmicutes, Bacteroidetes and Proteobacteria. On the genus level, LefSe analysis of indicator OTUs showed Bacteroides, Sporosarcina, Veillonella, Aeromonas and Acinetobacter were associated with the control diet whereas Streptococcus, Fusobacterium and Escherichia were associated with the treatment diet. Aeromonas hydrophila and Acinetobacter spp. are opportunistic pathogens and several strains have been found to be resistant to antibiotics. The increase in villi length and reduction of specific pathogens indicates that feeding a microencapsulated blend of organic acids and essential oils improves gut health and may serve as a part of an effective strategy to reduce antibiotic use in aquaculture.

In a 4-week experiment, 15 cannulated rainbow trout were fed three diets based on fish meal (FM), Saccharomyces cerevisiae yeast (SC) and Wickerhamomyces anomalus and S. cerevisiae yeast mix (WA). Fish were fed daily, and blood samples were collected on day 7 of each week at 0, 3, 6, 12 and 24 hr after feeding. In the final week, fish were exposed to a 1-min netting stressor. All essential and non-essential plasma amino acid levels except methionine were similar between fish fed diets FM, SC and WA. Plasma methionine and sarcosine were significantly higher in fish fed diets SC and WA, possibly due to the crystalline methionine level, form or feeding regime. Hydroxy-proline and 3-methyl-histidine were higher in fish fed diet FM, which can be explained by the higher levels present in fish meal compared with yeast. In stressed fish, there were no dietary effects on plasma amino acid levels, but significant increases in taurine and cystathionine were found in stressed compared with unstressed fish. These results demonstrate that yeast-based diets produce similar plasma amino acid profiles to fish meal and suggest that yeast may be a suitable fish meal replacement in diets for rainbow trout.

Black soldier fly (Hermetia illucens) meal is a potential alternative to fishmeal and plant proteins in diets for farmed fish since it can be produced on organic waste substrates, requires little energy and water inputs and contains high levels of essential amino acids. Recent studies have partially replaced fishmeal with black soldier fly meal, however, research on their impact on gut microbiota of fish is limited. In a five week experiment, juvenile rainbow trout (Oncorhynchus mykiss) were fed either a reference diet based on fishmeal or three diets with 30% inclusion of black soldier fly meals in the form of pre-pupae, larvae or defatted-larvae. The combined luminal content and mucosa were collected from the distal intestine of three fish per tank with four tanks per diet (n = 12) and 16S rRNA gene amplicons were sequenced using the Illumina MiSeq platform. Feeding the insect-based diets increased the alpha-diversity of bacteria and abundance of lactic acid bacteria, which may be due to the addition of dietary chitin. Compared with fishmeal, feeding insects resulted in higher abundance of phyla Firmicutes and Actinobacteria with lower abundance of Proteobacteria. Fish fed the full-fat meals had higher abundance of Corynebacterium that was attributed to its ability to produce lipase and the high content of dietary lipids as a substrate. Bacillaceae was increased in fish fed both larvae diets and unchanged in the pre-pupae diet, which indicated that life-cycle stage of the insect influenced the gut microbiota. Based on these results, we found that feeding black soldier flies increased diversity and altered the composition of gut bacteria of rainbow trout, which were further influenced by life-cycle stage and lipid content of the insect meal.

Requirements for omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), for Atlantic salmon are typically represented as an absolute level in the diet (e.g. g/kg or % of diet). Data for other species suggests that requirements for n-3 LC-PUFA are actually relative to dietary lipid (e.g. % of total fatty acids). A 2 × 2 factorial design of dietary lipid level × n-3 LC-PUFA level was designed to examine this question. Atlantic salmon post-smolts of 187 ± 4 g were fed one of four diets for 116 days that either had a low or high lipid level (180 or 230 g/kg) and a low or high n-3 LC-PUFA level (7 or 14 g/kg). Fish fed the diet with high-lipid + high n-3 had greater final weight and weight gain than the high-lipid + low n-3 diet, but no differences were noted between the two low-lipid diets. Significant effects of n-3 and a lipid*n-3 interaction were observed. However, no effects on feed intake, FCR and survival were found. Feeding high n-3 diets generally increased n-3 levels and retention in the whole body, especially EPA and DHA. Relative expression of lipid metabolism genes in the liver showed that fish fed high lipid + high n-3 had lower levels of expression of fatty acid synthesis genes (fads2d5, fads2d6 and elovl2). Upregulation of lipid transcription factor (srebp2 and lxr) and fatty acid beta-oxidation (hoad and aco) genes in fish fed low lipid + high n-3 further suggest that the proportion of dietary n-3 and energy level in those diets were lower than the high-lipid + high n-3 treatment. In conclusion, the significant interaction between lipid and n-3 levels on growth clearly shows that n-3 LC-PUFA requirements are relative to the lipid level in diets for Atlantic salmon. These results support the notion that requirements for this species should be defined based on a percent of total fatty acid content, implying that the absolute amount of n-3 LC-PUFA needs to increase as lipid content of the diet increases.

Yeast is a potential alternative to fish meal in diets for farmed fish, yet replacing more than 50 % of fish meal results in reduced fish growth. In a 4-week experiment, 15 rainbow trout (Oncorhynchus mykiss) were cannulated and fed three diets each week: 30 % fish meal as a control (FM); 60 % replacement of fish meal protein, on a digestible basis, with Saccharomyces cerevisiae (SC); and 60 % replacement with Wickerhamomyces anomalus and S. cerevisiae mix (WA). Blood was collected at 0, 3, 6, 12 and 24 h after feeding. In the final week, fish were exposed to a 1-min netting stressor to evaluate possible diet-stress interactions. Significant increases in pH, TCO2, HCO3 and base excess were found after fish were fed the SC and WA diets compared with FM, which elevated blood alkaline tides. Yeast ingredients had lower buffering capacity and ash content than fish meal, which explained the increase in alkaline tides. In addition, fish fed the WA diet had significantly reduced erythrocyte area and fish fed SC and WA diets had increased mean corpuscular haemoglobin levels, indicating haemolytic anaemia. Higher levels of nucleic acid in yeast-based diets and potentially higher production of reactive oxygen species were suspected of damaging haemoglobin, which require replacement by smaller immature erythrocytes. Acute stress caused the expected rise in cortisol and glucose levels, but no interaction with diet was found. These results show that replacing 60 % of fish meal protein with yeasts can induce haemolytic anaemia in rainbow trout, which may limit yeast inclusion in diets for farmed fish.

Yeasts can be used to convert organic food wastes to protein-rich animal feed in order to recapture nutrients. However, the reuse of animal-derived waste poses a risk for the transmission of infectious prions that can cause neurodegeneration and fatality in humans and animals. The aim of this study was to investigate the ability of yeasts to reduce prion activity during the biotransformation of waste substrates—thereby becoming a biosafety hurdle in such a circular food system. During pre-screening, 30 yeast isolates were spiked with Classical Scrapie prions and incubated for 72 h in casein substrate, as a waste substitute. Based on reduced Scrapie seeding activity, waste biotransformation and protease activities, intact cells and cell extracts of 10 yeasts were further tested. Prion analysis showed that five yeast species reduced Scrapie seeding activity by approximately 1 log10 or 90%. Cryptococcus laurentii showed the most potential to reduce prion activity since both intact and extracted cells reduced Scrapie by 1 log10 and achieved the highest protease activity. These results show that select forms of yeast can act as a prion hurdle during the biotransformation of waste. However, the limited ability of yeasts to reduce prion activity warrants caution as a sole barrier to transmission as higher log reductions are needed before using waste-cultured yeast in circular food systems.

BackgroundIn the last two decades, research has focused on testing cheaper and sustainable alternatives to fish oil (FO), such as vegetable oils (VO), in aquafeeds. However, FO cannot be entirely replaced by VOs due to their lack of omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA), particularly eicosapentaenoic (EPA; 20:5n-3) and docosahexaenoic (DHA; 22:6n-3) acids. The oilseed plant,Camelina sativa, may have a higher potential to replace FO since it can contains up to 40% of the omega-3 precursorsα-linolenic acid (ALA; 18:3n-3) and linoleic acid (LA; 18:2n-6).MethodsA 90-day feeding trial was conducted with 600 gilthead sea bream (Sparus aurata) of 32.92 ± 0.31 g mean initial weight fed three diets that replaced 20%, 40% and 60% of FO with CO and a control diet of FO. Fish were distributed into triplicate tanks per diet and with 50 fish each in a flow-through open marine system. Growth performance and fatty acid profiles of the fillet were analysed. The Illumina MiSeq platform for sequencing of 16S rRNA gene and Mothur pipeline were used to identify bacteria in the faeces, gut mucosa and diets in addition to metagenomic analysis by PICRUSt.Results and ConclusionsThe feed conversion rate and specific growth rate were not affected by diet, although final weight was significantly lower for fish fed the 60% CO diet. Reduced final weight was attributed to lower levels of EPA and DHA in the CO ingredient. The lipid profile of fillets were similar between the dietary groups in regards to total saturated, monounsaturated, PUFA (n-3 and n-6), and the ratio of n-3/n-6. Levels of EPA and DHA in the fillet reflected the progressive replacement of FO by CO in the diet and the EPA was significantly lower in fish fed the 60% CO diet, while ALA was increased. Alpha and beta-diversities of gut bacteria in both the faeces and mucosa were not affected by any dietary treatment, although a few indicator bacteria, such asCorynebacteriumandRhodospirillales, were associated with the 60% CO diet. However, lower abundance of lactic acid bacteria, specificallyLactobacillus, in the gut of fish fed the 60% CO diet may indicate a potential negative effect on gut microbiota. PICRUSt analysis revealed similar predictive functions of bacteria in the faeces and mucosa, although a higher abundance ofCorynebacteriumin the mucosa of fish fed 60% CO diet increased the KEGG pathway of fatty acid synthesis and may act to compensate for the lack of fatty acids in the diet. In summary, this study demonstrated that up to 40% of FO can be replaced with CO without negative effects on growth performance, fillet composition and gut microbiota of gilthead sea bream.