The diversification of future aquaculture feed formulas is expected to rise due to the increasing fish feed production. Changes in formulas and the corresponding adjustments to processing parameters can create variations in the pellet physical quality (PPQ ). Despite these potential changes in PPQ , the impact of varying PPQ on the physiological response of fish is often undervalued and overlooked.
In this study, we investigated the effect of protein sources and extrusion processing conditions through PPQ on the feed intake , gastrointestinal emptying and digestibility of spotted seabass. Four diets were formulated following a 2×2 factorial design, with two protein sources (fishmeal [FM] or cottonseed protein concentrate [CPC]) and two extrusion processing conditions ( PC-M, preconditioning water content 30 % and die temperature 120 °C vs. PC-H, preconditioning water content 22 % and die temperature 140 °C) . Both experimental diets were similar in macronutrient composition. Fish (46 g) were fed twice daily to apparent satiation for 20 days. Feed intake, nutrient digestibility and fish performance were evaluated. At the end of the experiment, chyme was collected after fish were fed restrictively one meal (0.95 % BW/d) to determine gastrointestinal emptying and water fluxes. Chyme was quantitatively collected from stomach and intestine at 2, 4, 6, 8, 12 and 24 h postprandial.
Altering the protein source affected PPQ as well as nutrient digestibility. CPC-pellets exhibited a longer hydration time, a higher hardness, pellet durability index (PDI ) and expansion rate than FM-pellets. CPC-pellets lowered nutrient and energy digestibility compared to FM-pellets. In addition, CPC-pellets led to a decrease in postprandial plasma glucose level and hepatosomatic index than FM-pellets. Processing conditions steered PPQ and influenced the physiological response of fish. PC-M led to a shorter hydration time, a reduced hardness and PDI , but a higher expansion rate than PC-H . PC-M resulted in a 23 % increase in stomach water flux at 4 h postprandial and a 2 % rise in stomach chyme water content than PC-H. Moreover, PC-H led to a respective 0.6% and 2.1% increase in protein and fat digestibility compared to PC-M. Feed intake and gastric emptying of fish remained unaffected by variations in PPQ in this study.