Fish meal is mainly derived from wild caught fish where landings have reached a maximum capacity and cannot expand further without detrimental effects on fish stocks. In contrast, the aquaculture industry is the fastest-growing animal food-producing sector worldwide [1,2]. Therefore, great efforts have been made to find sustainable, healthy and low-cost solutions for replacing fishmeal for carnivorous fish like Atlantic salmon . Salmonids that are fed diets in which fish meal is completely replaced with plant-based proteins show reduced growth, lower FCR, altered gut microbiota, metabolic and intestinal dysfunctions [3,4]. Biodegradation processes such as fermentation have proven efficient in improving the nutritional quality of several plant-protein based diets . Nevertheless, knowledge on this topic is still limited and the inclusion of fermented products is less than 0,5% in common Atlantic salmon feeds . The aim of this study was to assess the efficacy of substituting fish meal for fermented products (rape seed, soy and seaweed) and map how these diets affect fish performance, welfare and gut- microbiota in Atlantic salmon post-smolts.
Materials and methods
This 12-week diet-trial was performed at the NIVA research station, Solbergstrand. PIT tagged Atlantic salmon (Salmo salar) were stocked in 9 tanks with full strength sea water in flow-through. Fish were allowed >14 days for acclimation before the trial started. At the start of the experiment, the mean fish weight was 250 ± 48 g. Two diets, EP199 (fermented rapeseed and soy, 15% inclusion), EP299 (fermented seaweed and soy 30% inclusion), were evaluated against a control diet (CTRL) (Table 1). Diets were produced by SPAROS R&D and all diets were tested in triplicate tanks. Fish were fed ad libitum with automatic feeders according to the daily feeding regime: 10:00, 12:00, 14:00, 00:00, 02:00 and 04:00. Water quality was monitored daily. Feed waste was continually collected in order to calculate feed conversion ratio (FCR) and feed intake. All fish were weighed at the start, middle and end of the experiment to assess condition factor and specific growth rate (SGR). 9 fish from each diet were sampled at the start, after 6 weeks and at the end of the trial (12 weeks). From these samples microbial composition and diversity in the gastro-intestinal tract was mapped using 16S rRNA amplicon sequencing of the V4-V5 region. Sequence reads were analyzed using DADA2  and community composition and ordination analysis identified the most important drivers of observed diversity Blood plasma samples were analyzed for the immune health biomarkers (Lysozyme, protease, superoxide dismutase and peroxidase activity) using commercial kits. To assess if diet affected the fish’s ability to respond and recover after a stress event a standardized stress test was performed at the end of the trial according to .
Results and discussion
Preliminary results from this 12-week diet trial revealed that post-smolts fed with the EP199 diet had a higher condition factor compared to fish fed with CTRL and EP299 diets after both 6 and 12 weeks, and a better SGR and FCR compared EP299 fed fish at the end of the trial (Fig 1.). In contrast, fish fed EP299 besides having a lower condition factor also had a lower FCR and SGR compared to fish fed CTRL and EP199 diets after 12 weeks. Preliminary results from sequencing of the microbial community in the gut show no difference in microbial diversity (Richness and Shannon-diversity) between fish fed different diets, more data on the effects of diet on microbial composition will be presented. Post-smolts fed with EP199 also had increased levels of lysosome, peroxidase and protease activity in the plasma compared to fish fed a CTRL diet. Furthermore, EP199 fed fish had a higher stress responsiveness, measured as a larger increase in plasma levels of cortisol after a stress test. Overall, these results suggest that fish fed a diet with reduced fishmeal and a 15% inclusion of fermented rapeseed and soy perform as good or better than fish fed a CTRL diet. Furthermore, results suggest that the EP199 fed fish may have a better primed immune system and responsiveness to stress. Future studies are needed to assess and optimize inclusion of fermented seaweed products for Atlantic salmon feed.
We would like to thank the staff and students at Solbergstrand for help carrying out this experiment. This study was funded by the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 818431 (SIMBA).