Feed and treat: What to expect from commercial diets

Abstract

Basic data describing the physical characteristics of fish fecal waste are important in the design of effective solid waste management in aquaculture, especially in land-based facilities such as recirculating aquacultural systems (RAS).This study describes the physical properties of feces from rainbow trout fed eight different commercially available and widely used diets in Germany. Additional data from an earlier but unpublished study pertaining to feces derived from two rather extreme all-vegetarian diets are also presented for consideration of the settling properties. The diets were tested on duplicate groups of 50 rainbow trout in a flow-through aquaculture system. The effects of the diets on the physical properties of fecal particles such as particle size distribution (PSD), modeled settling velocity and rheological character were examined and the effects of each diet on fish health, growth and feed utilization were determined. Specific growth rate (SGR) and feed conversion ratio (FCR) for the different diets ranged from 0.98%d−1±0.012%d−1 to 1.39%d−1±0.012%d−1 and 0.97±0.017 to 1.61±0.017 (mean±S.E.), respectively. The density of presoaked feces was significantly lower than that of intestinal feces and ranged from 1.01013±0.00692gcm−3 to 1.04547±0.00692gcm−3 (mean±S.E.). Stability data were in the range from 390.12±29.4Pa to 1214.79±29.0Pa for elastic modulus and from 62.12±6.1Pas to 232.68±6.0Pas for dynamic viscosity. Based on the stability and PSD data theoretical efficiencies for removal of fecal waste using a drum filter showed remarkable variation, ranging from 82.5 to 95.9% (60μm gauze). Based on the same data, theoretical removal by a sedimentation basin with routinely using overflow rates of 0.057cms−1 to 0.394cms−1 ranged from 62.8 to 93.8%. Both fecal density and PSD have an exponential impact on settling performance. Increasing fecal density improves the removal efficiency of a sedimentation basin by about 20%, however sedimentation was seen to be a less robust and efficient removal technique than drum filtration. Sedimentation systems also experience additional problems with respect to leaching. Turbulence that was mimicked in this study reflects to an optimal fish farm, which means disintegrating effects are mainly caused by fish motion. If disintegrating units e.g. pumps are used, which are known to promote further particle breakdown the effects would be amplified.The results demonstrate the central importance of density of suspended solids in defining removal efficiencies and suggest that manipulation of fecal density might offer a new and effective means of managing and optimizing waste output from aquaculture operations. This study describes the basic properties of fecal wastes generated by commercial diets and can be used as a basis for further research.