Green algae as protein source for Oreochromis niloticus and Tilapia zillii
The potential of the unicellular green algae Chlorella vulgaris and Scenedesmus obliquus and the filamentous green algae Cladophora glomerata and Hydrodictyon reticulatum as protein sources in Oreochromis niloticus and Tilapia zillii diets was investigated. When O. niloticus and T. zillii were fed with fresh C. vulgaris and S. obliquus, a high percentage of the ingested algae was found to be undigested. Heat treatment of the algae at 40°, 60°, 80° and 100°C produced increased growth and protein utilizations in the fishes compared to those fed the untreated algae. Feeding C. vulgaris treated at 100°C for 30 minutes and S. obliquus treated at 100°C for 15 minutes was found to have produced the best growth responses in O. niloticus and T. zillii. C. glomerata meal and H. reticulatum meal were each fed separately as fishmeal substitutes in pelleted rations formulated to contain 30% protein with varying proportions of this supplied by the fishmeal and the algal meal. A diet containing 25% protein supplied by the algal meal alone was also fed. When 5% of the fishmeal protein was replaced with algal protein (both C. glomerata and H. reticulatum) and fed to T. zillii, the growth and protein utilization values recorded were superior to those obtained for the control 30% fishmeal protein diet. Higher levels of algal protein substitution were, however, found to produce poorer growth and protein utilization values in both fish species. Diets containing only algal protein (both C. glomerata and H. reticulatum) produced the poorest growth responses in both fish species. Hydrodictyon reticulatum was found to be limiting in methionine and histidine. Supplementation of these essential amino acids produced improved growth in both O. niloticus and T. zillii. It was concluded from these studies that the green algae evaluated may be suitable partial dietary protein sources for tilapias.