Carbohydrates in dietary ingredients for European seabass : Impact on nutrient digestibility and waste production when reared in recirculating aquaculture systems

Summary

The growing demand for aquafeeds along with the limited availability of fish meal has necessitated the use of various alternative ingredients in feed formulations. Depending on the ingredient, such use may also entail higher and diverse carbohydrate inclusion in the diets with implications for fish digestion and environmental output in terms of waste production. The present study examined the impact of different dietary ingredients on nutrient digestibility and waste production focusing on faecal removal efficiency when used in recirculating aquaculture systems (RAS). For this purpose, a trial was performed with juvenile European seabass (Dicentrarchus labrax), testing seven ingredients with different carbohydrate levels and types (starch and non-starch polysaccharides, NSP): wheat dried distillers grain with solubles, DDGS; hydrolyzed feather meal I, HFM I; hydrolyzed feather meal II, HFM II; insect meal (black soldier fly larvae, Hermetia illucens), IM; single-cell protein, SCP; shrimp-shell meal, SSM; seaweed protein, SWP. The test ingredients were included in a basal (control, CTR) diet at a level of 15 %, and fish were fed restrictively for a period of 4 weeks. Apparent nutrient digestibility was measured, with a focus on carbohydrates, and faecal quantity was determined based on organic matter (OM) digestibility values. Waste production was evaluated based on the amount of removed faeces by settling, and faecal particle size distribution. The results showed that starch digestibility decreased with increasing dietary starch levels, whereas NSP digestibility varied depending on the ingredient source. Moreover, faecal waste production had a stronger correlation with dietary NSP compared to starch. High inclusion levels of both nutrients in the diet were correlated with reduced faecal removal efficiency by settling. Overall, the DDGS and SWP diets scored worse for all indicators used here to assess waste production, whilst SSM excelled at producing highly settleable faeces compared to the rest of the diets. Summarizing, the current findings suggest that the type of dietary ingredients, reflecting also the type and level of dietary carbohydrates, determines faecal quantity and quality, and therefore careful selection of ingredients for RAS aquafeeds should be considered in this context.