NUTRITIONAL EVALUATION OF ANIMAL BY-PRODUCTS FOR THE PARTIAL REPLACEMENT OF FISHMEAL IN DIETS FOR GILTHEAD SEA BREAM (SPARUS AURATA L.)
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As well as reducing the level of protein in feeds, the strategy of replacing fishmeal with alternative protein sources may be an effective approach towards reducing costs and offering more sustainable feeds for aquaculture. Within the framework of this study, four trials were conducted to evaluate the nutritional potential of selected animal by-products, namely: Poultry Meat Meal (PMM), steam Hydrolysed Feather Meal (HFM), Enzyme treated Feather Meal (EFM) and Spray Dried Haemoglobin (SDH), as pat1ial substitutes for fishmeal in the diet of gilthead sea bream. The research strategy employed followed a two phase scheme which consisted of determining ingredient restrictions in the first place and validating subsequent formulations on the biological performances of the fish in the second place. Ingredient restrictions were related to nutrient specification, digestibility and palatability, whereas biological performances of the fish were assessed in terms of growth response, feed utilization, nutrient assimilation, tissue integrity and composition as well as basic health status. Using a classical experimental design for the dete1mination of apparent digestibility coefficients (ADCs), 8 diets (made with the individual ingredients plus 3 blends) were tested in trial I. This trial demonstrated that protein and energy of PMM and SDH were highly digested by gilthead sea bream (80%) whilst protein ADC. for the feather meals were much lower (22-23%). It was moreover observed that processing feather meal with an enzymatic treatment did not yield any significant benefit over the standard steam hydrolysed method, and that combining feather meals with blood meal was clearly not advantageous. This preliminary investigation also yielded valuable numerical ADC for essential amino acids (EAA), revealing In some cases significant discrepancies with regard to the overall protein digestibility (e.g. isoleucine: 54%, and methionine: 60% in SDH). In the second trial, six iso-energetic/iso-nitrogenous diets were formulated on a protein digestibility basis to test various inclusion rates of PMM, SDH and EFM over a period of 9 weeks. In comparison to the fishmeal reference diet, results indicated that diet with a 25% replacement of fishmeal by PMM was effective in supporting the growth of gilthead sea bream (SGR: 1.78%) and conve1iing feed into body weight (FCR: 1.33) (P<0.05). Higher inclusion rates of PMM resulted in lower performance, but moderate inclusions of SDH and EFM were equally shown to be feasible without impairing fish productive values. These findings were further supported by histological and haematological assessments which provided evidence that such inclusions did not disrupt gut integrity, create anaemia conditions (P<0.05) or affect the physiological function of the liver. On the basis of trial 3 it was apparent that palatability of PMM could represent one of the main factors limiting the inclusion of this commodity in the diet for gilt head sea bream (daily feed intake/unit of time measured at 3.3g/min tor fishmeal and 2.6g/min for PMM). Finally, in accordance with the measurements of lipid inclusion in hepatocytes (trial 2) and the fatty acid analysis of the carcass (trial 4), it is believed that the high lipid content of PMM could represent further argument toward the limitation or dietary PMM incorporation as long as a high quality product is desired. From this study it is concluded that practical diets li)r gilthead sea bream would greatly benefit in terms of both nutrition and economics from adequate inclusions (considering specific ingredient restrictions) of animal by-products.
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