Effect of supplementary enzymes on the growth and feed utilisation of gilthead sea bream, Sparus aurata L.
A series of five experiments were carried out to determine the effect of supplementary enzymes on growth performance and feed utilisation of juvenile gilthead sea bream, Sparus aurata, fed diets in which soybean meal (S8M) partially replaced fish meal (FM). In the first of these experiments the addition of cocktails containing 1 g/kg low pH active protease and 1 glkg a-galactosidase or 1 glkg high pH active protease and ] glkg a.-galactosidase to a 320 glkg SBM, 260 glkg FM pressed diet were both found to significantly (P<0.05) improve performance of fish fed these diets compared to fish fed the unsupplemented diet and a 320 glkg FM, 220 glkg SBM diet. This improvement in performance was not obtained when fish were fed 440 glkg SBM, 230 glkg FM diets with the same enzyme combinations. In some parameters performance of fish decreased as the SBM level in the diets was increased. The significant improvements observed in Experiment 1, with addition of enzyme cocktails to the 320 glkg S8M diet, were not repeated in any of the subsequent experiments. The second experiment was aborted due to ahnormal feeding hehaviour of the fish. In the third experiment, in which the enzymes employed in Experiment 1 were used individually at 1 glkg in 320 glkg SBM diets, no significant differences in specific growth rate (SGR), food conversion ratio (FeR) or protein efficiency ratio (PER) were noted in comparison to fish fed the unsupplemented diet. This was also the case with fish fed diets to which the two enzyme cocktails had been added at enzyme inclusion levels of 0.5 glkg each. Although no significant differences were found, feeding the diet with low pH protease alone appeared to increase performance compared to fish fed the unsupplemented diet, and the results of fish fed diets with high pH protease alone or with a.-galactosidase alone indicated that there was a decrease in performance compared to fish fed the unsupplemented diet. In Experiment 4 fish fed 320 g/kg SBM diets with 0.5, 1.0 and 1.5 g/kg low pH protease showed similar SGRs, FCRs and PERs which appeared to show an improved performance (although not significantly so) compared to fish fed diets with 1.0 glkg agalactosidase used together with either 0.5 or 1.0 glkg low pH protease. In the fmal experiment fish were fed 320 g/kg SBM extruded diets to which 0, 0.33, 0.66, 1.00 and 1.33 glkg of low pH protease had been added. Although no significant differences in SGR, FCR or PER were obtained, fish fed the diets containing 0.66 and 1.33 glkg protease appeared to improve performance compared to fish fed any of the other diets or a diet containing 320 glkg FM and 220 glkg SBM. Fish fed the other 320 glkg SBM supplemented dietc; gave similar results. A histological study of the position of nuclei in hepatocytes and the presence of fat globules around hepatopancreatic tissue in liver samples taken from fish fed the various experimental diets failed to show any relationships with either SBM level or enzyme inclusion in the diet. A series of analyses on the distribution of activities of six enzymes in the digestive tract of sea bream indicated that relative activities differed from one enzyme to another and from one region to another. In an investigation into the variation of pH in various parts of the digestive tract after one or two feeds, it was observed that within the first 24 hours after feeding the pH in the stomach decreased to a minimum value of 2.5 and the pH in the rest of the intestine varied between 6.5 and 7.7. vi From a series of gastric evacuation trials which were performed, it was found that the time of day sea bream were fed a meal influenced the gastric evacuation rate, with fish fed in the afternoon taking longer to evacuate the meal than fish fed a similar meal in the morning. Doubling the size of a meal did not double the gastric evacuation time. Instead, the time to evacuate a given percentage of the larger meal only increased by 1.4 and 1.6 times in fish fed the pressed and extruded feeds respectively compared to fish fed the smaller meal. When the sea bream were fed multiple meals it was found that the evacuation rate of an earlier meal was increased by a subsequent meal. A series of trials investigating the distribution in consumption of a population of sea bream fed a single meal indicated that there was a wide variation in the amount of food consumed by each fish in the population and it was observed that even fish of the same size consumed very different quantities of food. Before any definite conclusions can be drawn regarding the use of the three enzymes tested in these experiments to improve growth and feed utilisation in FM-substituted diets, further investigations need to be carried out in an attempt to obtain more significant results. This thesis has shown that additional research into the mode of action of these enzymes is required as well as studies into how the digestive physiology of the sea bream may affect the use of these (and other) supplementary enzymes.