The combination of palm and rapeseed oils emerges as a good dietary alternative for optimal growth and balanced lipid accumulation in juvenile gilthead sea bream reared at an elevated temperature

Abstract

The aquaculture sector needs to develop new tools to optimally increase production for feeding, while facing future environmental changing conditions. One of the major improvements in this industry has been reducing the usage of fishmeal and fish oil (FO) in diet formulations to promote a more sustainable production. In this line and considering global warming, in the present study, juvenile gilthead sea bream were fed a diet with a 60% of FO substitution by vegetable oils (VO) containing mainly palm oil (P) (80% of the VO content) and maintained at 21 or 28 ºC. Furthermore, in the condition of elevated temperature, fish were fed with two other diets with the same level of FO substitution and containing either 80% of the VO content of rapeseed oil (R) or an equilibrated combination of both VO (named PR). High temperature induced a significant increase in growth, but a reduction in the hepatosomatic and viscerosomatic indexes, and circulating triglycerides. Otherwise, significant differences were not found among the fish fed the three distinct diets at 28 ºC for any of the somatic or plasma parameters analyzed. In the adipose tissue, the higher number of adipocytes of smaller size in fish maintained at 28 ºC compared to fish at 21 ºC, suggested an activation of adipogenesis despite the gene expression of peroxisome proliferator-activated receptor gamma (pparg) was significantly decreased. Similarly, lipid droplets of smaller area and downregulation of the fat translocase cd36 and the fatty acid binding protein fabp1 were found in the liver of fish reared at 28 ºC compared to those at 21 ºC. Concerning differences among dietary groups, fish fed R diet showed in the liver a more extensive surface area occupied by lipid droplets of bigger size, while contrarily, fish fed PR diet had the smallest area filled with smaller lipid droplets. In adipose tissue, PR diet-fed fish had also adipocytes within the smaller size range. These histological data, together with the significant upregulation of several transcription factors (i.e. ppars) and genes related to lipid transport in the two tissues, indicated that the PR diet could be the most equilibrated for gilthead sea bream, to grow at a high rearing temperature while avoiding excessive fat accumulation. Thus, this study highlights the need of fine-tuning the substitution of FO by VO in the elaboration of feeds for this species to assure proper growth and physiological status in a future global warming scenario.