The history of grow-out diet evaluation at Elat was discussed and data evaluated. Recommendations for experimental modifications were proposed and experiments designed to achieve a more effective diet. Major testing to date has involved a study of protein levels (from 39–55%) and vitamin supplementation concentrations. Diets are all manufactured in pellet form at the Elat facility and are based on ingredients indigenous to Israel as used in general feed formulation mixtures in the country. Analytical services have been retained to check proximate analysis of various prepared diets. Aminoacid analysis and caloric value are not included, but will be incorporated into future determinations.
Good growth response with realistic food conversion rates have been achieved on a 45–48% protein diet. Conversion rates, based on a feeding at a 4% level, have been from 1.75:1 to 2,25:1, dependent on weight group of the fish. Conversion rates of under 3:1 are economically sound and those achieved at Elat certainly fall within this consideration. It is noted that after 3–4 gramme size, mortality in the fish is minimal. High survival rate in grow-out studies is further indication of the quality of the waterflow/culture facility and the general nutritional integrity of the diet. The latter, while not optimal (from aspects of cost and composition) is a sound template on which to formulate further dietary variations. Vests are planned for studies of aminoacid, carbohydrate and fat utilisation based on digestibility determinations.
Modifications of a basic grow-out diet being used have been proposed and will be included in the experiments planned for 1976. Essentially, these changes are based on the following considerations:
Fish meal levels in excess of 30% are, in the main, uneconomical)
Protein levels in ultimate least-cost grow-out diets should not exceed 40%. Tests of animal response to 30–40–50% protein levels on a modified grow-out diet are needed;
Carbohydrate level should be reduced, changing levels of soya flake from current levels of 45% to as low as 30%,
Feather meal and/or brewers yeast should be added at 3–5% levels to compensate for percentage of protein lost in soya flake reduction;
Wheat flour (at 9–10%) should be replaced with a high-gluten wheat protein to provide for increased protein, and especially better binding properties of the pelleted diet;
Soybean oil should be replaced with fish meal oil to reduce levels of saturated fatty acids in the diet and to provide for possible essential fatty acids needed by the fish;
Fish solubles, at a 3–5% level, should be added to provide for a flavour/feeding stimulus and possible fish growth factor(s) reported by other workers in fish solubles;
Concentrations of both mineral and vitamin premixes need to be evaluated further, especially the vitamin premix in terms of levels and cost increment added to the grow-out formulation. Both premixes will be based on data extrapolated from studies reported with other marine fish in order to establish physiologically-sound guidelines.
Further comments:
Ingredient cost analysis indicates economically valid ranges which lend themselves to further cost reduction. Primary emphasis now in grow-out diet development is on a nutritionally sound formulation and not on least-cost items. Cost reduction will be a logical follow-up in subsequent years and does not need emphasis at this stage of the programme;
Emphasis must be placed on achieving a more water-stable pellet. Primary means to achieve this objective will include:
Milling of ingredients, especially the soya flakes;
Use of steam-injection unit on pelletizer to obtain maximal gelatinization and carbohydrate/protein interaction, resulting in better binding properties;
Incorporation of feed ingredients with greater binding potential.
All these aspects should have urgent consideration in order to achieve an efficient feed producing capability.
The feed delivery system currently used (feeding to satiation) is effective. Some automation of the system is desirable and should be evaluated. Loss of substrate through inefficient pellet hardness is recognized and efforts are being made to eliminate this problem.
