Soybean meal as a protein source in gilthead sea bream (Sparus aurata L.) diets: effects on growth and nutrient utilization

The value of defatted soybean meal as a protein source for sea bream fingerlings (15.2±4.4 g on average) growing to market size (300-350 g) was evaluated by feeding extruded isonitrogenous and isoenergetic diets (46% protein and 22 MJ kg⁻¹) containing 20%, 30%, 40% and 50% soybean meal considering two phases. On day 87, the fish weight ranged between 66 and 81 g. The specific growth rate (SGR) of sea bream fed 50% soybean was lower (1.73% day⁻¹) than that of fish fed 20% (1.87% day⁻¹) and 30% (1.93% day⁻¹), but the food conversion ratio (FCR) was not significantly affected, and a quadratic significant trend was observed for the feed intake (FI) in relation to the dietary soybean level. At the end of the second phase on day 309, fish weight was between 303 and 349 g, but SGR and FCR were similar for all diets, and ranged between 0.64 and 0.69% day⁻¹, and 1.95 and 2.10% day⁻¹ respectively. The final biometric parameters were not affected by the diets, although the levels of some free amino acids in the muscle were affected. Sensory differences were detected by panellists in fish fed diet 20% as compared those fed diet 50%, which had a less marine flavour and was less juicy. The global growth results suggest the possibility of feeding sea bream weighing less than 80 g with 30% soybean meal, and for fish weighing more than 80 g, a 50% dietary soybean meal can be used until the fish reach commercial weight, with no negative effects on growth or feed efficiency. Nevertheless, when sensory analysis and economic aspects are considered, the maximum inclusion level of soybean was 20-22%.