الملخص الإنجليزي
The costs of feeding fish represent between 30-60 % of the variable cost of intensive or semi-intensive fish culture. This is largely due to the high costs of fish meal needed to meet the protein requirements of some omnivorous and most carnivorous fish species. Considerable research is currently directed towards reducing the cost of feeds by replacing fish meal with alternative sources of protein. Fish silage is one possible alternative protein source for use in aquafeeds. Fish silage is a liquefied product which results from the process of hydrolysis when whole fish or processing wastes are ground and mixed with an acid or with a bacterial culture. The manufacture of fish silage is possible in regions where by-catch, surplus catches or fish processing wastes are available. In this study, sardines were chosen as the raw material for silage production since they are regionally-abundant, are harvested in quantities which exceed demand for consumption and consequently have low cost During this study the preparation of sardine silage was examined as a potential method for preparation of a stable, high protein feed ingredient. Dried sardine silage was evaluated through digestibility studies as a replacement for fish meal in practical diets for the tilapia, Oreochromis aureus.
A sardine silage was prepared by adding 1.5% v/v of a mixture of propionic and formic acids (1:1) to minced raw sardine An anti-oxidant ethoxyquin (200 ppm) was also added. The sardine silage was left outdoors at ambient temperatures during the liquefaction period and then sun-dried separately or after mixing with wheat bran. The resulting acid silage was stable, with pH in the range 4-4.5. After three days of ensiling, 60 % of the total nitrogen was measured as non-protein nitroger a rapid process of protein hydrolysis and breakdown to peptides and free amino acids. These changes were reflected in measurements of viscosity taken during liquefaction. Temperature during the ensiling period was 29-40 °C. Amino acid analysis of dried sardine silage revealed that the silage met the known qualitative and quantitative requirements of essential amino acids of fish. Fatty acid analysis and a high iodine value (271) confirmed the highly unsaturated nature of the fatty acids in sardine oil. From the results of fatty acid analysis, it was evident that dried sardine silage is a valuable source of essential and non-essential fatty acids for fish. The moisture content of co-dried sardine silage after 72 hr of drying was 10.6 %
compared to 16.0 % of the sardine silage. Mixing of the silage with a cereal, wheat bran, increased the rate of solar drying. The digestibility study showed that sardine silage was highly digestible for protein and gross energy with no significant differences measured between the digestibility of sardine silage and a high quality (72 % crude protein) commercial fish meal. Apparent digestibility coefficient (ADC % for protein and gross energy of sardine silage were 95.15 % and 89.85 % compared to those of fish meal, 95.82 % and 92.23 % respectively
This study demonstrated the potential for the production of a sun-dried sardine silage under ambient conditions of temperature and humidity in Oman. The dried silage had comparable chemical properties with a high-protein fish meal and was highly digestible by fish.
