Alternative feed sources: Effect on gut microbiota, immunity and health of rainbow trout

Shifting from conventional fishmeal- and soymeal-based aquafeed to low-cost, sustainable dietary alternatives is essential for expansion and resilience in aquaculture. Possible alternative feed resources include under-utilised organic wastes from agriculture and households and by-products from forestry. These resources usually have high nutrient content and may contain bioactive compounds (β-glucans, mannans, lignocellulose etc.) that elicit immunomodulatory responses, modulate gut microbiota and thus improve overall wellbeing in cultured fish. This thesis investigated the dietary potential of Neurospora intermedia, Yarrowia lipolytica and cello-oligosaccharides obtained from industrial by-products, municipal side-streams and forest by-products in diets for rainbow trout.  In a 30-day fish trial, N. intermedia was included in the diet by replacing 30% of the fishmeal-based control diet and diets containing fungi were processed with and without pre-conditioning (heat-treatment). The results showed high apparent digestibility coefficient of the N. intermedia diets and a gradual shift in overall gut microbiota, with increased abundance of Lactococcus from day 0 to 30. Preconditioning had no effect on digestibility or gut microbiota. Pre-treated Y. lipolytica yeast biomass in whole (WY) or autolysed (AY) form was incorporated in rainbow trout diets at 2% or 5% level in a 45-day trial. The 5% WY diet resulted in elevated expression of immune-related genes of the complement pathway, membrane receptor pathway, cytokines and adaptive immune pathway. There was a small impact of dietary Y. lipolytica on faecal microbiota in rainbow trout. The bioactivity of cello-oligosaccharides (COS) was examined by feeding rainbow trout diets containing 0-1.5% graded COS in a 45-day trial. Inclusion of 0.5-1.5% COS slightly increased lactic acid bacteria in faeces and marginally modulated gut immunity with respect to expression of complement and toll-like receptors. The COS diets also increased oxidative stress-reducing capacity in the gut and serum of the fish. These results indicate, N. intermedia, Y. lipolytica and COS can be used successfully as potential functional feed or additive for rainbow trout.