Protein/lipid ratios in extruded diets for Atlantic cod (Gadus morhua L.): effects on growth, feed utilisation, muscle composition and liver histology

Atlantic cod is a species with a high biological potential for aquaculture. Nowadays, as a result of the decline of natural stocks and increase in market value of cod, it is regarded as an alternative species with high potential for commercial farming. Therefore, one of the priorities for development of a profitable commercial activity is formulation of cost-effective diets. The present work was developed with the aim of testing four extruded diets differing in the levels of protein (48% or 58%) and lipid (12% and 16%). A second objective was to verify whether the producers could profit from the lipid-rich livers characteristic of farmed cod, exploiting it as a source of cod liver oil for human health purposes. A 16-week growth trial was conducted with juvenile cod (233 g, average total weight) using three replicates per treatment. Length measurements were performed every 4 weeks and samples of muscle and liver were taken for total lipid, lipid classes and fatty acid determination, proximate analysis and liver histology. The effect of the diets was evaluated based on growth (SGR and GF3), feeding performance (FCE, PER), condition, hepatosomatic index and biochemical composition. The tested diets induced good performance results, with a protein sparing effect by lipid and a more efficient use of protein in diets with a lower protein content being clear at 12 weeks but not so obvious after 16 weeks of culture. It is the authors' opinion that the 48/16 diet appeared to be the best compromise between growth, feed utilisation and cost. The diets with a higher lipid level induced a higher HSI and lipid content in the liver. The muscle and particularly the liver FA profiles reflected that of the diets. The FA profile of farmed cod liver oil showed promising properties, despite having a slightly higher content of saturated fatty acids and linoleic acid, compared to "natural" cod liver oil.