Dietary starch, non-starch polysaccharides and their interactions affect nutrient digestibility, faecal waste production and characteristics differentially in three salmonids: Rainbow trout, Atlantic salmon and Arctic charr

Waste management has emerged as a critical issue in aquaculture. In this study, we examined the impact of dietary starch and non-starch polysaccharides (NSP) content on nutrient digestibility, faecal waste production, faecal removal efficiency and the faecal characteristics in three salmonid species, namely rainbow trout ( Oncorhynchus mykiss ), Atlantic salmon ( Salmo salar ) and Arctic charr ( Salvelinus alpinus ). Four diets were formulated according to a 2 × 2 factorial design. The first factor, starch, was tested by including 0% gelatinised wheat flour (low starch) or 20% gelatinised wheat flour (high starch) in a plant-based basal diet. The second factor, NSP, was tested by adding 0% NSP source (low NSP) or 10% NSP source (high NSP). High NSP level was achieved by adding an equal mixture of soya hull (5%) and wheat bran (5%). Diets were tested in triplicates for each species and feeding was done restrictively. Experimental duration was 42 days for rainbow trout and Atlantic salmon and 49 days for Arctic charr. Among the three species investigated, Arctic charr had the lowest digestibility values for most nutrients, whereas rainbow trout and Atlantic salmon values were comparable. High starch level in the diet reduced the macronutrient (protein, fat and ash) digestibility in all three species. High starch and high NSP levels in the diet increased faecal waste production, with the effect being more pronounced for the NSP content of the diet. High dietary starch levels increased the proportion of smaller-sized particles, while high NSP content increased the ability of faecal particles to withstand mechanical stress. The high starch level in the diet lowered faecal removal efficiency but increased by high NSP content. The highest and lowest faecal removal efficiency was recorded for Atlantic salmon and Arctic charr, respectively. The amount of non-removed faeces accumulating in the system was increased by the high starch levels in rainbow trout and Atlantic salmon but remained unaffected by the NSP content of the diet across the species. No interaction effect between starch and NSP content of diets was observed for faecal removal efficiency and non-removed faeces. Among the three species investigated, Arctic charr had the maximum amount of non-removed faeces per kilogram of dry matter feed. • Considerable differences exist between salmonids (rainbow trout, Atlantic salmon and Arctic charr) regarding nutrient digestibility and faecal characteristics. • Arctic charr faeces is dominated by particles of smaller size and lower stability. • High dietary starch reduces the size and stability/cohesiveness of faecal pellets. • High dietary NSP increases the size of faecal pellets, thereby improving faecal removal efficiency by sedimentation. • The amount of non-removed faeces is unaffected by NSP inclusion in the diet.