Food and Feeding Ecology of Tilapia, Oreochromis Niloticus L. And Effects of Diet on the Lipid Quality of Fish in Some Lakes in Ethiopia
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Date
1998-12
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Addis Ababa University
Abstract
Oreochromis niloticlls L. is commercially the most important fish in Ethiopia. Despite its
importance, little information is available on the biology and ecology of this fish in most
water bodies in Ethiopia. This thesis is a contribution to understanding the feeding
ecology and importance of diet on the lipid quality of fish with emphasis on O. niioticlIs
in Lakes Chamo and Langeno. The natural food of O. niloticlIs were studied, based on
monthly samples collected, during December 1995 and December 1996 from both lakes.
The diet of Chamo fish was mainly composed of phytoplankton. Cyanobacteria,
both filamentous (e.g. Anabaena, Anabaenopsis, Raphidiopsis, Oscillatoria) and coccoid
forms (e.g. ChrococclIs and Microcystis), were found to be the most important food
items. Diatoms, mainly Navicula, and Nitzschia and green algae (e.g. Cosmarium and
Scenedesmus) also constituted the diet. Macrophytes were found in abundance during the
rainy season while food of animal origin was only encountered occasionally. Significant
seasonal variations (ANaYA p<O.OOI) were observed in the levels of total organic matter
(TOM) (431.9-742.8 mg g-l d.w.), carbohydrates (162.2-281.4 mg g-l d.w.), proteins
(163.5-287.8 mg g-l d.w.), and lipids (66.3-131.1 mg g-l d.w.) of the food, peaking
during January to April. TOM (37.9%), carbohydrates (39.9%), proteins (42%), and
lipids (34.6%) were assimilated nearly equally well. However, the extent of assimilation
of these nutrients varied significantly between months (AI "lOY A, P<O.OOOI). The quality
of the food, expressed as the ratio of digestible protein to digestible energy (P:E), (12.9-
25.9 mg KJ-l), was sufficient for growth throughout the year.
In Lake Langeno the diet of O. niioticlIs was composed of a mixture of algal-based
detritus, macrophyte scraps, phytoplankton and silt. Inorganic silt, macrophytes and
detritus dominated more in the diet during the wet season than in the dry season. Total
organic matter (TOM) in the diet was generally low (348-521 mg g-l d.w.) which could
be due to a high proportion of inorganic silt in the diet in all seasons. The levels of
Carbohydrates (111-198 mg g-l d.w.), proteins (124--231 mg g-l d.w.), and lipids (39-
76 mg g-l d.w.) varied significantly (ANaYA P<O.OOOI) between months and declined
during the wet season, coinciding with the onset of the rainy period. Protein (41 %) was
assimilated more than total organic matter (32.7%), carbohydrate (31.7%) or lipids
(29.8%). The quality of the food expressed as P:E ratio (18.6-28.8 mg KJ-l) ranged
from inadequate to that which can support growth.
Generally, the levels of all chemical nutrients and the assimilation efficiency of
most nutrients were higher in Chamo fish than Langeno fish. Thus, the relatively better
condition and higher growth rate of O. niioticlIs in Lake Chamo could partly be explained
by both higher quality and abundance of food in the lake. Moreover, the high water temperature of Lake Chamo (>25°C) all year round promotes feeding rate and
assimilation efficiency of the diet.
Total lipids (1.7-21 % of d.w.) and the fatty acid (1.6-9.3% d.w.) contents of O.
niloticus dorsal muscle tissue varied significantly between samples taken from five lakes
in Ethiopia. Most fish from Lakes Haiq and Chamo contained higher levels of fat (2::10%
d.w.) compared to fish from Lakes Ziway, Langeno and Awassa which contained ~5%
d.w. fat. The 003/006 ratios ranged from 1.3-7.6 and O. niloticus from L. Haiq showed
the highest ratios, 5.1-7.6, indicating that the fat was of high nutritional quality. The
reason for this distinction likely mirrors the varied composition of the diet available to the
fish in the various lakes. The extent of variation was more pronounce in the herbivore O.
niloticus (17.2-208.2 mg g-! d.w.) than the omnivore Em'bus sp.(24.9-94.9 mg g-!
d.w.) or the carnivore Clarias gariepinus (18.7-90.8 mg g-! d.w.). Irrespective of the
species, the most important FA were palmitic acid (16:0), docosahexaenoic acid
(22:6003), stearic acid (18:0) oleic acid (18:1009) and arachidonic acid (20:4006).
Aquaria feeding studies have also shown that the FA composition of the fish can be
influenced to some extent by the fatty acid content of the diet. Water temperature appears
to show little influence on the FA content of this therrnophillic fish. Rather it promotes
feeding rate and conversion efficiency of the diet. In general, the success of O. nilotiClis
in most lakes in Ethiopia is very likely attributed to the plasticity of the feeding habit of
the fish to utilise available food sources in water bodies.
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Biology