Title: Growth and population parameters of Nile tilapia, Oreochromis niloticus (L.) in the open waters of Lake Victoria, Kenya
Author: Nicolas Otieno Outa
Supervising Institution: EGU - Egerton University, Kenya
Since the reintroduction of Nile tilapia, Oreochrmis niloticus into Lake Naivasha in 2011 there has been no research carried out on aspects of it's biology and ecology. This makes it difficult to assess the success of it’s reintroduction. This study was carried out to address this knowledge gap for a period of four months within Lake Naivasha. The main focus of the study was on the diet, sex ratio, length-weight relationship and body condition of Nile tilapia. Fish samples were collected every two weeks from different parts of the lake by use of gill nets (50 mm to 150 mm mesh size) and beach seines (< 10 mm) between November 2013 and February 2014. Physico-chemical parameters (water temperature, conductivity, pH, dissolved oxygen and transparency) were measured insitu using HATCH multi probe meter electrodes (HQ40d) during each sampling session. Vertical hauls of zooplankton and phytoplankton samples were collected from the lake using 50μm and 25μm plankton nets respectively and identified under inverted and compound microscope with the aid of various identification keys. Stomach contents of all the sampled fish were analysed using point method where algae were found to contribute most to the diet of O. niloticus (56%). Fish parts contributed the least (2%) while zooplankton contributed more to the diet of the lower fish length classes than the upper length classes. O. niloticus selected green algae over diatoms and cyanobacteria. Nile tilapia fed more during the day with a peak at 12: 00 noon with more males sampled giving sex ratio of male to female of 2:1. A comparison of the length-weight relationship for males and females done by regression of log weight against log length showed that most of the fish had allometric growth (b <3) with males from Oserian and Hippo sampling stations point showing isometric growth (b =3). Length at first fish maturity for male and female fish obtained during this study was 22.5 cm and 23.8 cm respectively. The study showed that O. niloticus in Lake Naivasha was an omnivorous feeder and with good condition.