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 mar ket value of cod, it is regarded as an alternative species with high potent ial for commercial farming. Therefore, one of the priorities for developmen t of a profitable commercial activity is formulation of cost-effective diet s. 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 o f cod liver oil for human health purposes. A 16-week growth trial was conducted with juvenile cod (233 g, average tota l weight) using three replicates per treatment. Length measurements were pe rformed every 4 weeks and samples of muscle and liver were taken for total lipid, lipid classes and fatty acid determination, proximate analysis and l iver histology. The effect of the diets was evaluated based on growth (SGR and GF3), feeding performance (FCE, PER), condition, hepatosomatic index an d biochemical composition. The tested diets induced good performance results, with a protein sparing e ffect by lipid and a more efficient use of protein in diets with a lower pr otein 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, Th e muscle and particularly the liver FA profile, reflected that of the diets . The FA profile of farmed cod liver oil showed promising properties, despi te having a slightly higher content of saturated fatty acids and linoleic a cid, compared to "natural" cod liver oil. (C) 2001 Elsevier Science B.V. Al l rights reserved.