Guar gum in rainbow trout (Oncorhynchus mykiss) feed:: The influence of quality and dose on stabilisation of faecal solids

In a dose-response study to identify optimal binder inclusion parameters, a control diet and five diets incorporating binder (0.2, 0.3 and 0.4% of a mid-viscosity guar gum (MV) and 0.2%, and 0.3% of a high-viscosity guar gum (HV)) were fed to triplicate groups of rainbow trout at a ration of 1.2% BW d(-1). Feed conversion, growth, and with one exception, apparent digestibility of macronutrients were not significantly effected by the addition of binder. All binder containing diets improved stability of fish faeces significantly with the most effective (HV 0.3%) leading to increases in viscosity and elastic modulus of up to 266 and 209%, respectively. The high-viscosity guar gum (HV) performed significantly better than the mid-viscosity (MV), and improvements were positively dose- and quality dependent, except in the case of MV 0.4%, which yielded a slightly less marked improvement than MV 0.3%. The possibility of inactivation by microbial fermentation is discussed. Furthermore, compared to the control, suspended particles from binder-stabilized facces remained significantly larger and therefore removable by mechanical means after exposure to defined hydrodynamic stress. According to the stability data recorded, the best result was achieved by supplementing feed with 0.3% high-viscosity guar gum. This resulted in a reduction in post-microscreening residual load of about 52% for a 100 mu m gauze. The improvement correlated strongly (r(2)=0.95) with measured faecal stability data, suggesting that final particle size in self-cleaning trout farms is in effect a measure of mechanical stability of facces in the face of given turbulences. It is suggested that the economic and ecological advances deemed essential in fish farm process management may be achieved through systematic manipulation of the physicochemical properties of facces by certain NSPs. (C) 2007 Elsevier B.V. All rights reserved.