Growth Performance of Flathead and Thinlip Grey Mullet Fry in Green Water as Affected by Dietary Protein Level During Secondary Rearing

Abstract

Thinlip mullet rearing experiment: Thinlip mullet fry grew heavier when fed on the 25% - 30% crude protein diets (6.6-7.4 gm /fingerling). The 18% - crude protein treatment produced slightly lower harvest weight (5.9 gm /fingerling) compared to those of higher protein diets at lower feeding costs. The 18% - crude protein diet was an appropriate substitute for the 30 % - crude protein diet in terms of economic efficiency. However, the 30% - crude Protein diet superseded all other treatments in terms of condition factor and growth performance (P<0.05). When searching for alternative sources for nutrition of thinlip mullet fry during secondary rearing, the results indicated that best daily weight gain and economic efficiency were obtained when feeding thinlip mullet fry with 18% - crude protein diet, followed by the use of 25% - crud protein diet, when compared to the use of 30% - crude protein diet. The present results showed that the use of ground corn for mullet fry nutrition in green water tanks resulted in significant reduction in weight gain by 50% (p<0.05) with slight reduction in condition factor. Flathead mullet rearing experiment: Mean individual weights of fingerlings at harvest after 45 days of rearing were significantly higher (p<0.05) for fry stocked in the 25% - crude protein treatment (7.5 gm / fingerling) compared to those reared on the corn treatment (5.9 gm. / fingerling) or the 35% - crude protein treatment (6.9 gm / fingerling). The addition of corn to fertilized fry tanks was most efficient in growing mullet fry to larger size. Therefore, for rearing of grey mullet fry in concrete tanks, it is recommended to fertilize water, when low quality single ingredient diet (such as ground corn) is used to promote growth conditions in green water system. Good results were obtained when ground corn and the 25% crude protein diet were used. Consequently, the use of corn can be recommended as suitable diet for the rearing of flathead mullet fry and fingerling in fertilized tanks. The least feed costs (6.58 L.E/ kg) with moderate growth (4.0%) per day and survival (65.0%) were obtained when ground corn was used in green water tanks compared to that of the 25 % crude protein treatment. Key words: flathead mullet , rearing, thinlip mullet, growth performance, protein level, green water.