PHYSICAL CHARACTERISTICS OF SINKING AND FLOATING EXTRUDED AND EXPANSION-STEAM PELLETED FEEDS AND THEIR EFFECTS ON WATER QUALITY AND GROWTH OF RAINBOW TROUT IN A COMMERCIAL SETTING

To determine the effects of feed pellet processing (extrusion and expansion-steam pelleting) and on feed physico-chemical characteristics, fecal stability, water quality, and growth performance in rainbow trout, three types of trout feed pellets (compressed sinking, extruded sinking, and extruded floating) were prepared, analyzed, and fed to juvenile rainbow trout (initial weight = 285.8±15.2 g) at three feed rates for 124 days. Results showed that all feeds had similar chemical composition and water absorption curves with soaking time, but extruded feeds had a significantly higher degree of starch gelatinization than compressed feed, which led to extruded feeds having much higher water stability, fecal durability, and lower P discharge. Extruded floating feed produced better growth and feed conversion in rainbow trout than the two sinking feeds. Feed rate also contributed to better growth, but the improvements were not seen beyond feed rate 2. The present study is the first to show that the extrusion process not only produces pellets having better quality than the expansion-steam pelleting method but also improves fecal size and durability in water, and therefore, use of extruded feeds has the potential to improve waste collection and removal and reduce contribution to pollution in effluent.