Effects of the dietary protein: lipid ratio on growth and nutrient utilization in gilthead seabream (Sparus aurata L.)

Gilthead seabream Sparus aurata L. (initial mean body weight: 42.5 g) were fed four experimental diets containing either 47 or 51% of dry matter (DM) as protein and either 15 or 21% as lipid for 12 weeks. Each diet was hand-d istributed to triplicate groups of 60 fish, three times a day until satiati on. The digestibility coefficients of the dietary components were determine d using chromic oxide as a marker. The levels of protein or lipid in the di ets did not affect the digestibility. Fish regulated their feed intake and attained the same weight at the end of the experiment. However, feed effici ency varied between diets, with best values obtained with both diets contai ning 21% lipid. When diets contained only 15% lipid, feed efficiency increa sed with dietary protein level. Nitrogen retention was significantly higher with high fat diets regardless of dietary protein level. Neutral lipid dep osition was significantly higher in liver for diets rich in lipids. It was elevated in muscle only in fish fed the diet containing 47% protein and 21% lipid and this deposition in muscle contributed to a significant increase in body fat content. Phosphorus load to the environment, measured as percen tage retention of ingested or digestible phosphorus, was significantly lowe r with both diets higher in lipids.