Growth performance and adiposity in gilthead sea bream (Sparus aurata): risks and benefits of high energy diets

A factorial experiment was conducted for 7 weeks over the summer period (23 -26 degrees C) with gilthead sea bream fingerlings fed under restricted (1- 1.75%) and unrestricted (2.5-3%) conditions with two practical diets (55% p rotein-9% lipid; 46% protein-17% lipid). Specific growth rates ranged from 0.65 to 2.3%, and no significant differences were found at a given feeding level between dietary treatments. At the highest feeding level, no signific ant differences in the viscerosomatic, hepatosomatic and mesenteric fat ind ex were observed, although the daily fat gain was always higher in fish fed the 17% lipid diet. Under restricted feeding conditions, no significant di fferences in daily N gain were found. However, in fish fed to visual satiet y, N gain was significantly lower in fish fed the lipid enriched- diet. Thu s, when considering protein retention, a reliable protein sparing effect of dietary lipid was only found under restricted feeding conditions, when a f eed gain ratio near to 1 was found with both diets at the intermediate rati on size (1.75%). Plasma growth hormone (GH) levels decreased with the incre ase of ration size. Besides, under fixed feeding levels, plasma GH levels w ere lower in fish fed the high lipid diet. An opposite trend was found in f ish fed to satiety. After 8 days of fasting, fish previously fed the 17% li pid diet to satiety also exhibited a more pronounced hypersomatotropism and hypoglycemia, linked to increased loss of body fat. As a practical consequ ence, when high fat diets are used, these findings show the need to optimiz e ration size to avoid unwanted adiposity and impaired growth performance i n gilthead sea bream. (C) 1999 Elsevier Science B.V. All rights reserved.