Nutritional regulation of lipid deposition in blackspot seabream (Pagellus bogaraveo)

Find new finfish candidate species is one of the strategies that producers may follow to maximise their sustainability and profitability, ensuring a continuous expansion of the Mediterranean aquaculture industry. The nutritional and economical importance of blackspot seabream (Pagellus bogaraveo) in Mediterranean market, associated with the eminent reductions in its capture, requires urgent nutritional studies for the consolidation of this species farming. The reduced growth rates associated to the high propensity of this species to deposit fat (18-21% wet weight), even at low dietary lipid level incorporation (12%), prompt us to test two high protein (50 or 60%)/ low lipid (6 or 10%) dietary level as a first attempt to maximize this species growth and reduce its excessive fat accumulation (Chapter 2). Both, 60P/6L or 50P/10L dietary P/L tested, resulted in high lipid retentions, confirming the sustained propensity of blackspot seabream to deposit fat even at low dietary lipid levels, and suggested the conversion of nutrients other than lipid into body fat. Hence, in Chapter 3, the lipogenic potential of carbohydrate type on blackspot seabream was evaluate using two isolipidic (10%) and isonitrogenous (45%) diets containing either crude starch (CS) or gelatinized starch (GS). Starch type did not cause any significant effect on blackspot seabream body composition or lipogenic pathways, whereas a reduction of 10% dietary protein level (45 to 35%) markedly depressed hepatic activities of lipogenic enzymes. Besides nitrogen level, the work presented and discussed in Chapter 4, demonstrated that blackspot seabream lipid metabolism also depends on dietary nitrogen nature, which explain at some extent the high lipid retentions found when fish meal (FM) is partially replaced by plant protein (PP) sources. The present Thesis also demonstrates the role of the dietary dispensable amino acids (DAA) content on blackspot seabream lipid metabolism, with Ala+Ser playing a preponderant role on species lipogenic pathways regulation. Once the consideration of blackspot seabream by aquaculture industry passes through the feasibility of FM replacement by plant feedstuffs, besides the dietary IAA/DAA balance also the absolute amounts of DAA should be taken into account. Overall, different dietary P/L ratios and protein level have a relevant role on viscera lipid content regulation, while PS and AA nature has mainly affected liver lipid content through lipogenic pathways. In fact, dietary protein (level and nature) play a major role on blackspot seabream lipid metabolism and can contributed, in three months, to increases up to 2% on body lipid contents. The knowledge obtained with this Thesis permit us to suggest that a diet with a dietary P/L of 45P/10L combined with a feed distribution by self-feeders appears to be the most adequate for optimizing growth and reducing body lipid content in blackspot seabream.