Finding novel sustainable feed ingredients for the aquaculture industry is crucial to reduce the climate footprint and increase the production volumes in this industry. Marine by-products such as viscera, head, skin, backbone and cut-offs from fish are local and sustainable raw materials for feed production. Enzymatic hydrolysis of by-products facilitates use of hydrolyzed proteins as feed ingredients for the same species as it came from (Council Regulation (EC) No 142/2011, 2011). The aim of this study was to partially replace fish meal with salmon hydrolysate in feed for post smolt Atlantic salmon, and evaluate the effects on growth, digestibility and gut health. Salmon hydrolysate was also characterized and analyzed for prion content by proteomics.
Materials and methods
Salmon protein hydrolysate was produced by Nutrimar AS by enzymatical hydrolysis of salmon by-products followed by separation, concentration and spray drying of the liquid hydrolysate. The high molecular weight peptides in the hydrolysate was concentrated by dialysis (10 kDa cut-off) and analyzed for salmon prion protein content by targeted and shotgun proteomics analysis with LC-MS/MS. A feeding trial with Atlantic salmon (Salmo salar, Rauma strain, mean weight 141.5 g) post smolts was conducted to test isonitrogenous diets with different levels of salmon hydrolysate (FPH) replacing fish meal (FM) protein. The control diet with 30 % FM was compared with test diets containing 20 % FM with 9 % FPH (FPH-09), and 10 % FM with 18 % FPH (FPH-18). Atlantic salmon were fed until doubling of weight before collecting intestinal samples for histological evaluation of gut health as described in (Nordvi et al., 2023). Digestibility was examined by stripping feces and analyzing for protein, fat and ash content compared to inert yttrium oxide added to the diets.
Results and discussion
Proteomics analysis of the salmon hydrolysate used in this feeding trial did not detect any salmon prions. Salmon protein hydrolysate (FPH) increased the initial specific growth rate (SGR) significantly compared to the FM control diet (Figure 1). This is probably due to attractant effect of free amino acids and peptides that stimulate feed intake (Kousoulaki et al., 2018). The FM fed salmon showed compensatory growth by having the highest SGR in the second growth period (day 25-58). The weight difference was not significant after 58 feeding days but the stimulatory effect on feed intake is still industrially relevant for the sea-transfer phase and other changes in feed during fish farming.
The salmon hydrolysate amino acid profile is excellent for salmon in addition to being highly digestible as peptides. The apparent digestibility of protein was significantly higher in the FPH-18 diet compared to the other diets (Table 1), likely due to the free amino acids and peptides in the salmon hydrolysate (Hevrøy et al., 2005). Interestingly, the apparent ash digestibility increased linearly (p < 0.001) with increasing FPH content (Table 1). Histological analysis of midgut and hindgut from salmon fed the FM and FPH-18 diets did not reveal any significant differences between the diets, and the intestines looked normal and healthy.
Salmon hydrolysate is a promising novel feed ingredient for Atlantic salmon, due to its high protein digestibility and attractant effect that stimulates feed intake and growth. Atlantic salmon fed 9 % and 18 % salmon hydrolysate showed higher initial growth rate compared to a 30 % fish meal control diet in this feeding trial. Including 18 % hydrolysate in the diet significantly increased the protein digestibility, and the ash digestibility increased linearly with the hydrolysate inclusion level. Morphological analysis of gut samples revealed no significant differences among the diets. No salmon prions were found in the hydrolysate used in this feeding trial.
Council Regulation (EC) No 142/2011 introducing a system for the statistical monitoring of trade in bluefin tuna, swordfish and bigeye tuna within the Comunity  OJ L54/1. (2011).
Hevrøy, E. M., Espe, M., Waagbø, R., Sandnes, K., Ruud, M., & Hemre, G. (2005). Nutrient utilization in Atlantic salmon (Salmo salar L.) fed increased levels of fish protein hydrolysate during a period of fast growth. Aquaculture Nutrition, 11(4), 301–313.
Kousoulaki, K., Rønnestad, I., Rathore, R., Sixten, H. J., Campbell, P., Nordrum, S., Berge, R. K., & Albrektsen, S. (2018). Physiological responses of Atlantic salmon (Salmo salar L.) fed very low (3%) fishmeal diets supplemented with feeding-modulating crystalline amino acid mixes as identified in krill hydrolysate. Aquaculture, 486(December 2017), 184–196. https://doi.org/10.1016/j.aquaculture.2017.12.011
Nordvi, M. F., Løvmo, S. D., Whatmore, P., Sundh, H., Sigholt, T., & Olsen, R. E. (2023). Low intestinal inflammation model (HP48) in Atlantic salmon (Salmo salar) and inflammatory mitigation by Bactocell. Aquaculture, 563, 738920. https://doi.org/10.1016/J.AQUACULTURE.2022.738920