Studies on Diel Feeding and Digestibility of Cyanobacteria

Abstract

Beira Lake is eutrophic and covered with a bloom of Cyanobacteria. Hybrids of Oreochromis niloticus and Oreochromis mossambicus dominate the lake. Present study deals with feeding cycle and digestibility of the bloom by Oreochromis species. Volume method and the point method were used to determine feeding intensity of the hybrids. Stomach pH and Chloropltyll-a content in stomach and hindgut were determined at different hours of the day. Four to five diumals were conducted in the field and in the laboratory. Results indicated that, oreochromis hybrids commenced their feeding at the 6th hour and stomachs were filled up to 25%. At the 12th hour, majority of fish were fed from 50% to 75% and on the 18th hour up to full stomach level. Feeding decreased from 75% to 50% at the onest of resting phase at the 24th hour. Results were similar under laboratory conditions. At the onset offeeding pH was 7-8 in the stomach. Ch-a concentration in the hindgut was high, and food passed undigested. At the 12th hour, pH decreased from 3-4 and reached 1-2 at the 18th hour indicating onset of digestion. From 18th to 24th hour, active digestion occurred.