The incidence of different pellet size on growth, gut evacuation, feed digestibility and feed waste in gilthead sea bream (Sparus aurata)

The feeding behaviour of gilthead sea bream (Sparus aurata) consists in cracking and chewing feed. In farming condition, this results in crushing feed pellets with an occasional loss of some fragments which can vary in response to pellet dimension, thus affecting feed waste at the on-growing stage. However, few studies have addressed this issue and even less information on the further effect of different pellet size on growth, gut evacuation and feed efficiency are available on this species. Thus, a 122-day study was undertaken to assess the effects of three pellet size (2 mm, S; 4 mm, M and 6 mm, L) on growth, gut evacuation, feed waste and feed digestibility during the on-growing of gilthead sea bream (initial weight: 215.9 +/- 1.8 g). No significant effects of pellet size on growth (final body weight and SGR) were observed. Pellets diameters had no effects on feed digestibility (protein and dry matter) and feed efficiency parameters (FCR, PER, GPE, GLE) even if differences in the gastric evacuation rate were detected at different pellet size. At this regard, the shape-rate model developed to estimate the gastrointestinal evacuation pattern, evidenced a slower gastric evacuation rate in the 6 mm diet, while no differences in foregut evacuation rate were observed. Data on feed waste, highlighted how feed losses by chewing was practically absent in the S (2 mm) diet while in the M (4 mm) and L (6 mm) diets 24.3 and 17.3% of the entire meal was losses by chewing activity, respectively. The study reinforces previous observation that feeding pellets size of 4 and 6 mm in gilthead sea bream within 200-450 g could induced an excess of feed waste by chewing activity with economic and environmental implication. Despite the reduced feed intake observed, pellets size of 2 mm did not lead to any feed losses by chewing and was able to guarantee similar growth compared to the other diets. Further studies considering intermediate pellets size (3 mm) may be useful in order to further optimize the pellet size choice during the on-growing phase of this species.