Dietary incorporation of soybean meal and exogenous enzyme cocktail can affect physical characteristics of faecal material egested by rainbow trout (Oncorhynchus mykiss)

Soybean meal (SBM) has a relatively low digestible nutrient/energy density compared to other common fish feed ingredients. Feeding high levels of SBM has been shown to affect growth and has also been predicted to result in increased solid waste outputs. The effect of SBM and an enzyme cocktail (Superzyme CS) on growth and waste outputs of rainbow trout was, therefore, examined in a 16-week trial. Three extruded feeds, containing 0%, 10% or 20% SBM, were top-coated with the enzyme cocktail at 0, 1 and 2.5 g/ kg to produce eight diets. The diets were fed to near-satiety to fish reared at 15 °C. Growth and feed utilization by the fish and apparent digestibility of the diets were examined. Waste outputs were estimated using a nutrient mass-balance approach and the physical characteristics of faeces freshly egested by fish examined.SBM and enzyme supplementation had no effect on growth and very small effects on digestibility. Mass-balance estimates of waste outputs were only slightly affected by SBM and enzyme incorporation. Total solid, solid P, and solid N wastes averaged 109, 3.7 and 3.9 kg per t of biomass gain (for fish growing from 5 to 110 g), respectively. Dissolved P waste and dissolved N waste averaged 1.1 and 26.4 kg per t of biomass gain, respectively. Cohesiveness of faeces was significantly reduced (P < 0.001) by the enzyme, and SBM had significant a negative effect on both sinking speed and cohesiveness (P < 0.01) of faeces.The reduction in faecal material cohesiveness and sinking speed as a result of dietary SBM and enzyme inclusion can potentially reduce solid waste recovery on land-based fish culture operations, but could potentially minimize localized impacts of some cage culture operations.