الملخص الإنجليزي
Protein digestibility, feeding and growth studies in Nile tilapia were carried out using diets based on six different fish meals prepared from fisheries by-catch and processing waste. The fish meals were selected on the basis of their protein and ash content. Higher grade meals were characterized with >65% crude protein and <16% ash. There were no significant differences between the apparent protein digestibilities values determined using standard in vivo methods for each meal. The meals were highly digestible (range 89.9 – 92.5%) and their high ash content (up to 24%) did not influence the apparent uptake of protein.
In a comparative in vitro study, enzymes were extracted from the proximal intestine of tilapia in phosphate buffer (10mm) and separated by centrifugation and dialysis. Chymotrypsin was assayed against an N-Succinyl-LAla-LAla-LPro-LPhe pNitroanilide (SAPNA) substrate to determine activity level. The required enzyme solution volume was calculated and added to fish meal samples which were stirred and incubated at 25°C for 18 hr. Samples of 1 ml were collected cooled and centrifuged. 0.1% trinitrobenzensulphonic acid (TNBS) was added to each sample which was then incubated for an hour at 60°C in water bath. After cooling, absorbance was recorded at 420 nm. The TNBS measurements were transformed into D,L Alanine equivalents from a D,L Alanine standard curve. This in vitro test detected the variations in the bio availability of the protein from fish meal of various qualities. For example it revealed significantly larger amounts of enzyme-released free amino acids from a high quality commercial fish meal (FM 1) than from fish meals prepared from by-catch and processing waste.
For growth and feed utilization studies five iso-nitrogenous and iso-caloric feeds were formulated, containing fish meal as the major protein ingredient. The diets were fed to triplicate groups (each 30) of juvenile tilapia for 9 weeks and various measures taken of growth and feed efficiency. Few significant differences were found in the specific growth rates (SGR) or food conversion rates (FCR) of fish fed high or low quality meals. One diet containing by-catch meal, prepared from mixed catfish species (Arius spp.), gave significantly higher SGR and lower FCR than the remaining diets, including a control diet containing a commercial anchovy diet. This meal contained the highest levels of essential amino acids. No significant differences were seen in the moisture, protein, ash or phosphorus content of whole fish at the beginning and end of the feeding experiment.
This study demonstrated the potential for the production of fish meal from the fisheries by-catch and fish processing wastes in Oman. The fish meals had high nutritional value and were highly digestible by fish.
