Nutritional studies in a number of fish species of potential to the aquaculture industry in Malta

Abstract

Four growth trials were performed on three species of fish which are of potential to the aquaculture industry in Malta. In the first investigation a feeding trial was conducted with 60g gilthead sea bream, Sparus aurata, over a period of sixteen weeks, including two weeks of acclimation. The fish were fed four different fish meal-based diets. The fish meals used were Chilean, Danish, Danish Aqua and Danish Low Temperature. Each was prepared by two forms of pellet manufacture, steam pressing and extrusion, giving a total of eight different feeds. A pressed Standard commercial diet was given as the ninth feed. The aim of the experiment was to determine which combination of fish meal and manufacturing process gave the best performance, and the overall performance of the different manufacturing processes. The growth of the fish was taken weekly over the experimental period, and fish samples for carcass proximate analysis, determination of hepatosomatic index and liver colour score were taken along with feed samples. An amino acid profile of the feeds was determined, and the determination of apparent protein digestibility of the feeds was also attempted. Using these data a number of nutritional parameters were calculated for each of the feeds. At the end of the experiment, fish fed the extruded feed were found to have performed better than the corresponding pressed feed in all the nutritional parameters studied, whatever the fish meal used and in some cases significantly so. Best performance overall was found in the fish fed the extruded Danish meal feed, followed closely by the extruded Danish LT meal feed. Of the pressed feeds the Danish Aqua meal feed gave the best performance, followed closely by the Standard feed and the Danish LT meal feed. The Chilean feed gave an inferior performance whatever the manufacturing method used. Protein sparing was found in the fish fed the extruded feed compared to the fish fed the corresponding pressed feed, and a higher carcass lipid content in the fish fed the extruded feeds was also found. No significant differences were found between treatments in hepatosomatic index and liver colour scores. Protein digestibilities were slightly higher in the extruded feeds. A cost analysis was performed for each of the feeds in a hypothetical 510 tonne/year farm, using the values of specific growth rate and food conversion ratio obtained in the experiment. The extruded feeds all gave a larger calculated profit than did the pressed feeds, the extruded Danish meal feed giving the best profit of all. Some pellet quality parameters were also studied, and a clear difference between the two types of pellets formed by steam pressing and extrusion was found. In the second experiment, 1.3 and 30g Sparus aurata L were used. In this trial the fish were fed four commercial bream feeds to satiation. The best growth was given by feed C in the case of the small fish and feed A in the larger fish with specific growth rates of 4.76 and 2.09%/day respectively. Feed A gave the best performance as regards feed conversion ratio, feed efficiency and protein efficiency ratio in both sizes of fish. The values obtained for these parameters in the small fish fed feed A were 1.33, 75% and 1.43 respectively, while those obtained in the big fish fed feed A were 1.73, 58% and 1.20 respectively. In both sizes of fish, the fish which consumed the largest amount of feed, feed D in the case of the small size fish and feed C in the case of the large fish., gave the most inferior performance as regards feed conversion ratio, feed efficiency and protein efficiency ratio. Feed A is clearly the better feed to give to the larger size fish and would also be recommended for the smaller fish. The third study investigated two species of Tilapia grown in three different salinity conditions. This trial was set up so as to investigate the performance and survival of these two species present in Malta with a view to assessing of their potential for culture on an industrial scale. The two species were the Oreochromis spilurus (Gunther) and a red Tilapia hybrid, Oreochromis mossambicus x Oreochromis niloticus. In all the salinities studied, 1, 18 and 37ppt, the performance of the Red Tilapia was superior to that of the 0. spilurus. Best growth was obtained by the Red Tilapia grown in brackish water followed by the Red Tilapia grown in fresh water, with specific growth rates of 3.08 and 2.96%/day respectively. In food conversion ratio, feed efficiency and protein efficiency ratio, the best performance was given by the Red Tilapia grown in fresh water, with values of 1.22, 81.97% and 2.02 respectively. Mortalities were highest in the Red Tilapia grown in sea water, with 25% mortalities over the experimental period. These fish gave the lowest performance of the Red Tilapia, but still gave a superior performance to the 0. spilurus in the growth parameters studied, except in survival. Of the 0. spilurus, the best growth was given by the fish grown in brackish water, followed by those in fresh water, with specific growth rates of 1.96 and 1.64%/day respectively. In all other parameters the O. spilurus grown in fresh water gave the most inferior performance. The fish grown in the other two salinities gave similar results, with the best values of food conversion ratio and feed efficiency being 2.17 and 46. 08% given by the fish grown in brackish water and a protein efficiency ratio of 1.17 given by the fish grown in sea water. While growth of O. spilurus in sea water is possible with an acceptable growth and food conversion ratio, Red Tilapia is best grown in fresh or brackish water. A recycling brackish water system is recommended for Red Tilapia where fresh or brackish water are in short supply. The fourth growth study involved a preliminary study with 56g Seriola dumerili, Risso 1810, to observe the potential of this species for fish fanning in Malta using formulated pellets. The fish were weaned off a moist feed to dry pellets only. The overall growth, which was monitored by sample weighings, was from 56g to 750g in a period of 358 days. The overall food conversion ratio and specific growth rate over this period were 2.11 and 0. 72%/day respectively. The importance of temperature on the growth of the amberjacks was very evident in the growth curve obtained. Mortalities during the growth trial itself were negligible, and the fish themselves were not adversely affected by handling. The potential of this species using dried pellets looks very promising, and prospects can only get better when further studies are performed and feed is specialised further for this fish. The opportunity was also taken to perfo1m various biometries on the fish, especially the weight to length relationships. Details about the digestive tract and associated organs were also recorded.