Impact of increasing mycoprotein Paecilomyces variotii (Pekilo) on growth performance, gut histomorphology, blood biochemistry and welfare indicators of post-smolt Atlantic salmon.

Abstract

In Atlantic salmon (Salmo salar) farming there is a need to increase the industrial sustainability and fish health status. Novel functional ingredients have potential to do so. For instance, microbial ingredients have cell wall components with immune-modulating properties that can improve fish health and welfare. Filamentous fungi Paecilomyces variotii, produced through the PEKILO® process (Enifer, Finland), is a protein source and potential functional feed ingredient to improve the health of Atlantic salmon due to its β-glucans, mannans and nucleic acids. To understand the effect of P. variotii on salmon growth and health, a control and three experimental diets with increasing inclusions of P. variotii were formulated and fed to vaccinated post-smolt Atlantic salmon for five weeks in saltwater. Diet 1 (D1) was the control diet, a commercial-like diet with soy protein concentrate (SPC), fish meal (FM) and wheat gluten meal (WGM) as main protein sources. On the contrary, in the experimental Diets (2, 3 and 4), the crude protein from SPC, FM and WGM was replaced at 5% (D2), 10% (D3) or 20% (D4) with P. variotii. Growth performance, gut histomorphology, biochemical blood parameters and welfare indicators were assessed to evaluate the effects of P. variotii on post- smolt Atlantic salmon. Results showed that P. variotii can replace up to 20% of the crude protein in diets without comprising fish growth performance. In addition, there were differences between D4 and D1 related to the length of simple folds and percentage area of goblet cells. Specifically, fish fed D4 had longer simple folds but with a lower percentage area of goblet cells (compared to D1). Interestingly, the size of the vacuoles in distal intestine also showed differences. Here, fish fed D2 had smaller vacuoles than fish fed D3. Regarding biochemical blood parameters (i.e., creatine kinase, chloride, lactate dehydrogenase, aspartate aminotransferase and total protein), all tested molecules were in normal ranges for post-smolt Atlantic salmon. Although, fish fed D4 had a lower level of creatine kinase (compared to D1), while fish fed D3 showed less chloride in their blood (compared to fish fed D2). By contrast, the evaluation of welfare indicators showed that fish fed D4 had a higher occurrence of skin disorders (compared to fish fed the other diets), while fish fed D1 and D2 exhibited more cases of cataract (compared to fish fed D3 and D4). Overall, the inclusion of crude protein (up to 20%) from P. variotii (PEKILOâ) can support fish growth performance and modulate both gut histomorphology (increasing villi length and reducing vacuoles) as well as biochemical blood parameters (by reducing creatine kinase levels). However, high inclusion of P. variotii (20%) could induce skin health challenges in fish.