THE FOOD AND FEEDING HABIT OF OREOCHROMIS NIILOTICUS L. (PISCES: CICHLIDAE) IN LAKE CRAMO, ETHIOPIA

Abstract

The food of Oreochromis niloticus in Lake Chamo, Ethiopia was
studied from the stomach contents of 449 adult (290-570 mID total length, TL) and 145 juvenile (61-115 mID TL) fish. O. niloticus was found to be essentially phytoplanktivores in Lake Chama, and the composition of the phytoplankton diet varied seasonally. The diet of both adult and juvenile fish consisted of 10 genera of blue greens whereas green algae and diatoms each contributed 8 genera. Blue greens as a group contributed the bulk of the diet of adult fish. The most frequently encountered genera from the blue greens were Anabaena (96-100), Lyngbya (85-100), Microcystis (81-100) and Oscillaloria (41-100). Cosmarium (79-100) from the green algae and Navicula (82-100)
from the diatoms were also frequently observed. In terms of percentage composition by number, the blue greens contributed over 60% of the total food ingested. Of these, more than 50 % was due to Anabaena, Lyngbya and Microcystis. Zooplankters occurred on rare occasions in the stomach contents of both adult and juvenile fish. The feeding
pattern of adult O. niloticus was observed to have a diel rhythm. O. niloticus in Lake Chamo is a continuous feeder during the day and peaked at 16 hr. Daily ingestion of phytoplankton was estimated to be about 3.7% of its wet body weight at an average water temperature of 26° C.