Field validation of innovative feeds targeting gut integrity and feces quality in rainbow trout Oncorhynchus mykiss

Solid waste production from rainbow trout (Oncorhynchus mykiss) farms is known to increase in wintertime, possibly as a result of reduced dry matter digestibility at low temperatures. At the same time the quality of the feces changes, in that their stability in water seems to decrease and they fall apart in water as whitish flakes. Gut histology performed on trout sampled in wintertime highlighted some pathohistological changes, which may be related to the above mentioned effects. A specifically formulated diet targeting enhanced digestibility and gut protection has been proved in lab scale to significantly reduce volume of feces produced under simulated winter conditions. Following further refinement of the formulation, a field trial was set up with 57000 rainbow trout (average initial weight 150-200 g) fed in duplicate either a control (CD) or an experimental (Defender-GE) diet. The trial was carried out under natural winter conditions in Northern Italy (average temperature 6and lasted 80 days. The fish fed Defender-GE showed higher growth rate (+14% when expressed as Thermal Growth Coefficient) and lower Feed Conversion Ratio (-24%) compared to the control ones. As a result, the feeding cost per kg biomass gain decreased by 12% for the units fed Defender-GE. Hystological analyses confirmed that this latter diet exerted a protective effect towards the intestinal epithelium, possibly enhancing lipid and therefore energy absorption. Feces quality dramatically improved in fish fed Defender-GE, with higher stability of fecal pellets,as such in turns reducing suspended solids in water. A specifically formulated diet may therefore be a tool to enhance gut integrity and improve feces quality in rainbow trout under winter conditions, while supporting growth rates and reducing feeding costs.