Dietary ingredient composition alters faecal characteristics and waste production in common carp reared in recirculation system

Feed is a common factor influencing both fish growth and waste production in aquaculture. We examined the effect of different feed ingredients on the quantitative and qualitative aspects of faecal waste produced by common carp. Ingredients rich in (i) starch (field peas, PEA); (ii) protein (feather meal, FeM); (iii) insoluble non-starch polysaccharides, NSP (sunflower cake meal, SFM); or (iv) soluble NSP (wheat dried distillers grain with soluble, WDG) were studied. Five experimental diets were produced, a control diet produced from a basal mixture (CON) and four other diets (PEA, FeM, SFM and WDG) were prepared by replacing 30% of the basal mixture in CON with the respective test ingredient (70:30). Common carp juveniles (95 g, 15 fish per tank) were fed (restrictively, 22 g/kg⁰.⁸/d) the experimental diets for 7 weeks, in triplicate groups. Growth, body composition, energy and nutrient balances were significantly different; with highest growth recorded in CON and WDG; lowest in the FeM and SFM groups. Apparent digestibility coefficient (ADC) of dry matter, nutrients (protein, fat, carbohydrate, ash and phosphorus) and energy in the diets and ingredients were significantly altered between the groups (p < .001). PEA had the best ADC for macro-nutrients and energy, while ash and phosphorus were highest with WDG; SFM had the lowest ADC values for DM, fat, carbohydrate, energy, and phosphorus. The quantity, proximate composition and recovery percentage of the feaces (collected by settling) was significantly different between treatments (p < .001). Faeces recovery and solid removal efficiency were the highest in WDG, comparable to CON; and lowest in SFM. Physical characteristics of the feaces (stripped) indicated by dry matter content and stability were significantly affected by the ingredients only at day 26 and 29, respectively. However, changes in dry matter and stability over time (days) were significant in SFM and WDG (for DM); and all groups except SFM (stability). Osmolality and viscosity of the stripped faeces was unaffected. In summary, SFM resulted in significantly lower performance, low DM and nutrient digestibility, increased faecal loss, lower stability and removal efficiency. FeM lowered growth, protein and fat digestibility; with altered faecal chemical composition. WDG and PEA performed on par with control, if not better in certain aspects with respect to mineral availability and faecal recovery. Overall, 30% inclusion of the test ingredients not only affected growth, but also the waste production and removal efficiency by altering the physical and chemical characteristics of the faeces in common carp.