LENGTH-WEIGHT RELATIONSHIP, NATURAL FOOD AND FEEDING SELECTIVITY OF NILE TILAPIA, Oreochromis niloticus L. IN FERTILIZED EARTHEN PONDS

Nile tilapia, Oreochromis niloticus (L.) was stocked in fertilized earthen ponds and received both organic and inorganic fertilizers. Fish samples were collected for growth determination and were categorized into different sizes, i.e. <25g, 25-50g, 50-75g, 75-100g and 100-125g. Every two weeks, fish samples were collected and individual weight and length were measured. Samples for stomach content analysis were taken. The obtained results showed that the L ratio was approximately similar in all fish sizes and ranged from 2.9 to 3.4. The composition percentage of food items in stomach contents of Nile tilapia was ranked as phytoplankton>detritus>zooplankton at all fish sizes. Detritus consisted mainly of scraps of macrophytes and mud, and its contribution to gut content decreased by increasing fish size, while phytoplankton contribution increased. Zooplankton did not exceed 1 % of total stomach’s contents. The main algal species found in fish stomach belonged to Cyanobacteria, Chlorophyceae, Bacillariophyceae and Euglenophyceae. Bacillariophyceae represented the main phytoplanktonic division at small fish size (<75 g/fish), while Chlorophyceae is the dominant group at large fish size (>75 g/fish). Results revealed that Nile tilapia could select Cyanobacteria and Euglenophyceae at all sizes. Chlorophyceae and Bacillariophyceae were eaten with slight selectivity at large sizes