Effect of dietary starch, amylase and ash on nutrient digestibility, faecal waste production and faecal characteristics of rainbow trout, (Oncorhynchus mykiss).

Summary

Waste management is a critical issue in aquaculture. In this study, we examined nutrient digestibility, faecal waste production, faecal removal efficiency and the faecal characteristics of rainbow trout (Oncorhynchus mykiss) in response to the dietary starch level, amylase and ash supplementation and the interaction of dietary starch with amylase and ash. A basal mixture predominantly consisting of plant-based ingredients was formulated. The basal mixture was diluted with 100, 200, 300 and 400 g/kg gelatinised wheat flour, resulting in four diets of varying starch levels, referred to as 10-WF, 20-WF, 30-WF and 40-WF, respectively. Amylase was supplemented at 125 k-novo-α-amylase units (KNU)/kg to the 20-WF and 40-WF diets and referred to them as 20-WFα and 40-WFα. Two high ash diets were prepared by supplementing 20 g diamol/kg to the 10-WF and 30-WF diets and termed as 10-WFA and 30-WFA. Triplicate group of 25 fish (mean weight, 210 g) were restrictively fed one of the eight diets for five weeks. Higher dietary starch level led to increased faecal waste production, lowered faecal removal efficiency and resulted in greater accumulation of non-removed faeces in the system. Moreover, higher dietary starch level also diminished faecal stability, as evident by an increased share of particles <40 μm size in 40-WF upon exposure to mechanical stress. Amylase supplementation in the diet enhanced starch digestibility without influencing faecal waste production, removal efficiency, or the particle size distribution (PSD) of non-stressed faeces. Conversely, dietary ash supplementation increased faecal waste production and altered faecal characteristics by enhancing density and sinking velocity, although without improving its removal efficiency. In summary, this study underscores the potential of dietary manipulations to affect faecal waste production, removal efficiency, and various faecal characteristics, including viscosity, density, sinking velocity, particle size distribution, and stability.