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°C) with gilthead sea bream fingerlings fed under restricted (1–1.75%) and unrestricted (2.5–3%) conditions with two practical diets (55% protein–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 significant differences in the viscerosomatic, hepatosomatic and mesenteric fat index were observed, although the daily fat gain was always higher in fish fed the 17% lipid diet. Under restricted feeding conditions, no significant differences in daily N gain were found. However, in fish fed to visual satiety, N gain was significantly lower in fish fed the lipid enriched- diet. Thus, when considering protein retention, a reliable protein sparing effect of dietary lipid was only found under restricted feeding conditions, where a feed gain ratio near to 1 was found with both diets at the intermediate ration size (1.75%). Plasma growth hormone (GH) levels decreased with the increase of ration size. Besides, under fixed feeding levels, plasma GH levels were lower in fish fed the high lipid diet. An opposite trend was found in fish fed to satiety. After 8 days of fasting, fish previously fed the 17% lipid diet to satiety also exhibited a more pronounced hypersomatotropism and hypoglycemia, linked to increased loss of body fat. As a practical consequence, when high fat diets are used, these findings show the need to optimize ration size to avoid unwanted adiposity and impaired growth performance in gilthead sea bream.