East Afr. J. Biophys. Comput. Sci. (2026), Vol. 7, Issue. 1, 34-42  
ART ICLE  
Feeding Habits and Trace Metal  
Concentrations in Organs of the Nile Catfish,  
Synodontis schall (Bloch & Schneider) (Pisces:  
Mochokidae), in Lake Abaya, Ethiopia  
ARTICLE INFO  
Volume 7(1), 2026  
https:  
//dx.doi.org/10.4314/eajbcs.v7i1.4S  
Elias Dadebo 1�, Daniel WM-Bekele2, Abnet Woldesenbet 1�, Tamirat  
ARTICLE HISTORY  
Handago3, Tekleweyni Asayehegn1, Tiruken Aziz1, and Teshome Belay  
Received: 14 April, 2026  
4,∗  
Accepted: 26 May, 2026  
�
Published Online: 10 June, 2026  
1Department of Aquatic Sciences, Fisheries and Aquaculture, Hawassa University, P. O. Box 5, Hawassa,  
Ethiopia  
CITATION  
2Biology Department, Environmental Toxicology Program, Hawassa University, P. O. Box 5, Hawassa,  
Ethiopia  
Dadebo et.al (2026). Feeding Habits  
and Trace Metal Concentrations in  
Organs of the Nile Catfish, Synodontis  
schall (Bloch & Schneider) (Pisces:  
Mochokidae), in Lake Abaya, Ethiopia .  
East African Journal of Biophysical and  
Computational Sciences Volume 7(1),  
2026. .https://dx.doi.org/10.4314/eajbcs.  
v7i1.4S.34-42  
3Department of Biology, Wachemo University, PO Box 667, Hosaena, Ethiopia  
4Department of Animal Sciences, Dilla University, PO Box 419, Dilla, Ethiopia.  
∗Corresponding author: teshimeansc@gmail.com  
Abstract  
This study investigated the feeding habits and trace metal concentrations in different organs of the  
Nile catfish, Synodontis schall, in Lake Abaya, Ethiopia. Stomach content analysis was conducted  
using frequency of occurrence and volumetric analysis. The results of the study indicated that S. schall  
is an omnivore with polyphagous feeding habits; dominant food categories included phytoplankton,  
detritus, insects, zooplankton, and macrophytes. Seasonal shifts were observed: phytoplankton was  
the primary food source during the dry season, whereas zooplankton predominated during the wet  
season. Ontogenetic dietary shifts were also noted with juveniles consuming mainly phytoplankton and  
zooplankton, while adults mainly fed insects, detritus and phytoplankton. Trace metal analysis identified  
copper (Cu), cadmium (Cd), nickel (Ni), zinc (Zn), and manganese (Mn) in liver, kidney and muscle tissues,  
while lead (Pb) and cobalt (Co) were not detected. Metal concentrations in the liver were ranked as Cu >  
Zn > Mn > Ni > Cd, while in muscle and kidney tissues, the order was Zn > Cu > Mn > Ni > Cd. Significant  
difference (p < 0.05) in mean concentrations of Cu, Cd, and Zn were noted among tissues. All detected  
heavy metals were within the FAO and EU safety limits, suggesting that S. schall from Lake Abaya is safe  
for human consumption.  
OPEN ACCESS  
This work is licensed under the Creative  
Commons open access license (CC  
BY-NC 4.0).  
East African Journal of Biophysical and  
Computational Sciences (EAJBCS) is  
already indexed on known databases  
like AJOL, DOAJ, CABI ABSTRACTS and  
FAO AGRIS.  
Keywords: Feeding habits; Lake Abaya; Omnivory; S. schall; Trace metals  
River and its tributaries in the west, and in the Wabishebele River  
in the southeast (Golubtsov & Habteselassie, 2010; Golubtsov et al.,  
1995). Generally, S. schall is classified as an omnivore and benthic  
fish species, and its diet covers a wide spectrum of food ranging  
from plankton to invertebrates and plants (Lalèyè et al., 2006).  
This dietary flexibility, combined with a high tolerance for adverse  
environmental conditions, allows the species to remain abundant  
1 Introduction  
The genus Synodontis is widely distributed across African  
freshwaters ranging from the Nile basin, Chad, Niger, and much  
of the West African region (Paugy et al., 2003) (Cuvier, 1816,). In  
Ethiopia, the Nile catfish Synodontis schall (Bloch and Schneider,  
1801) is found in Lakes Abaya and Chamo in the south, the Baro  
Dadebo et.al (2026)  
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East Afr. J. Biophys. Comput. Sci. (2026), Vol. 7, Issue. 1, 34-42  
in most African fresh waters (Lowe-McConnell, 1987).  
Lake Abaya has a length of 79 km, a width of 29 km, and a surface  
area of 1,160 km2 (Baxter, 2002). It has a maximum depth of 13 m  
and is located at an elevation of 1,268 m (Baxter, 2002; Grove et al.,  
1975). Lake Abaya is a home to 21 different fish species that have  
economic and ecological roles (Golubtsov & Habteselassie, 2010).  
In Lake Abaya, S. schall is abundant in both littoral and pelagic  
environments, likely due to low predation and minimal fishing  
pressure (Dadebo et al., 2012) . While the species is among the  
most favored edible fishes in some African countries (Lalèyè et al.,  
2006), it currently holds low commercial importance in Lake Abaya.  
Although, it remains ecologically indispensable as a primary prey  
species for the commercially significant catfish, Bagrus docmac  
(Forsskål, 1775) (Anja & Mengistou, 2001). Previous studies across  
Africa have highlighted the species’ opportunistic feeding nature  
(Adeyemi, 2010; Akombo et al., 2014; Arame et al., 2021; Dadebo  
et al., 2012). Yongo et al. (2019) reviewed the feeding habits of  
some Synodontis species in African freshwaters, and reported that  
the genus feeds on a variety of food items, including vegetable  
materials, insects, mollusks, detritus, macrophytes, fish scales,  
and plankton. In Quémé River, the most frequent food items in  
the stomachs of S. schall were macrophytes, algae, crustaceans,  
rotifers, and mollusks (Lalèyè et al., 2006). Ofori-Danson (1992)  
reported that the frequent food items of S. schall in the Kpong  
head pond were benthic macroinvertebrates. Adeosun et al. (2017)  
indicated the importance of insects, rotifers, crustaceans, fish parts  
and phytoplankton in the diet of S. schall.  
Figure 1: Geographic location of Lake Abaya (Source: adopted from Shishitu, 2024)  
Beyond ecological dynamics, the health of fish populations is  
increasingly threatened by the accumulation of trace metals from  
natural and anthropogenic sources (Ali & Khan, 2018). Because fish  
occupy various trophic levels, they can accumulate toxic substances  
in vital organs and muscle tissues, posing risks not only to aquatic  
biota but also to human consumers through trophic transfer (Garai  
et al., 2021). Given the benthic feeding habits of S. schall, it is  
particularly susceptible to metals associated with lake sediments.  
2.2 Sampling and Measurements  
A total of 849 S. schall specimens were collected during the dry  
season (January to February, 2020) and the wet season (June to July,  
2020) (wet season). Sampling was conducted at both littoral and  
pelagic sites of the lake using a beach seine (25 m long and three  
meters wide with a mesh size of 0.6 cm) in the shallow littoral  
area and Nordic survey multi-mesh monofilament nylon gillnets  
(Appelberg et al., 1995) at the pelagic area of the lake.  
Despite its ecological importance, there is lack of information  
regarding the biology and ecology of S. schall in Ethiopia. To the  
knowledge of the researchers, there is no published data regarding  
the feeding habits and heavy metal load in the organs of S. schall  
specifically within Lake Abaya. Therefore, the aim of this study  
was to investigate the dietary patterns and concentrations of trace  
metals in different organs of this species. Such information is  
vital for future management of the fish stock for assessing the  
environmental health of the Lake Abaya ecosystem.  
The multi-mesh gillnets consisted of twelve randomly distributed  
panels of the mesh sizes 5, 6.25, 8, 10, 12.5, 15.5, 19.5, 25, 29, 35, 43,  
and 53 mm (bar mesh). Each panel was 2.5 m long, and hence the  
total length of each net was 30 m. The gillnets were set between  
three to five meters depths in the open water, about 1.5 km inward  
from the littoral sampling station. The gillnets were set early in the  
morning around 7.00 a.m. local time and pulled around 3.00 p.m.  
in the afternoon.  
2 Materials and Methods  
For each specimen, fork lengths (FL) and standard length (SL) were  
measured to the nearest mm using a measuring board. Total weight  
(TW) was measured to the nearest 0.1g using a SCALTEC digital  
balance (model 23565, USA). The stomach of each fish was split  
open, and the contents were collected and preserved in 5% formalin  
solution and transported to Hawassa University Fishery Laboratory  
for further analysis.  
2.1 Study Area  
Lake Abaya is the second largest lake in Ethiopia and  
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0
00  
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0
00  
geographically located between 5 55 9 and 6 35 30 N latitude,  
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0
00  
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0
00  
and 37 36 90 and 38 03 45 E longitude in the southern part of  
the Ethiopian Rift Valley, East of the Guge Mountains (Figure 1)  
(Shishitu, 2024) . The lake is fed on its northern shore by the  
Bilate River, which rises on the southern slope of mount Gurage  
(Golubtsov & Habteselassie, 2010). Other rivers that drain in to  
the lake include, Gelana, Milate, Gidabo, Harre, Baso, and Amesa.  
The only outflow of the lake is through the lower reaches of Kulfo  
River directly below an alluvial pan at an elevation of 1,190 m.  
Arba Minch town lies on its southwestern shore and the southern  
shores are part of the Nechisar National Park (Teffera et al., 2019).  
2.3 Stomach Content Analysis  
The stomach contents of each specimen were examined visually  
to identify macroscopic food items, whereas  
a
dissecting  
microscope (Leica, MS5, magnification- 40x) and a compound  
microscope (Leica DME, magnification- 1000x) were used to  
identify microscopic food items. Quantitative analysis of the diet  
Dadebo et.al (2026)  
35  
 
East Afr. J. Biophys. Comput. Sci. (2026), Vol. 7, Issue. 1, 34-42  
was conducted using frequency of occurrence and volumetric  
methods of analyses.  
then fully dried in a laboratory oven at 175°C for three hours and  
processed separately except for Pb and Cd. For Pb and Cd 60°C  
were considered. A solution of aqua regia (3:1 hydrochloric to nitric  
acid) was prepared as per Nwani et al. (2010). One gram of dried  
muscle and 0.5 grams of liver and kidney were added to a 100 ml  
flask with 10 ml of aqua regia and refluxed overnight to dissolve  
organic materials and release trace metals, following Muinde et al.  
(2013) method. After refluxing, samples were digested at 60°C for  
three hours to enhance reaction kinetics. Each sample was digested  
in triplicates and diluted to a final volume in a 50 ml volumetric  
flask and filtered with the attached monochromater filter.  
Frequency of occurrence (%FO): the number of stomach samples  
containing one or more individuals of each food category was  
expressed as a percentage of all stomachs containing food (Hyslop,  
1980). In volumetric analysis (%V), the food items found in each  
stomach were sorted into different food categories, and the water  
displaced by the group of volume of items in each category was  
measured (Bowen, 1996). The relative importance of each category  
was then expressed as a percentage of the total volume of food  
categories.  
2.7 Determination of Heavy Metals  
2.4 Ontogenetic Dietary Shift and Dietary  
Overlap  
The digested fish organ and muscle were analyzed for Cd,  
Co, Cu, Mn, Ni, Pb, and Zn using flame atomic absorption  
spectrometry (FAAS) with a dual background correction system  
(BUCK SCIENTIFIC, Model 210VGP, USA). An air-acetylene flame  
was employed, utilizing aqueous calibration standards from  
stock solutions of the metals. Three standard solutions and  
a blank solution, made from the acid used in digestion, were  
prepared to minimize errors and avoid overestimating heavy  
To assess ontogenetic dietary shifts, specimens were categorized  
into five size classes (Class I: 5- 9.9 cm; Class II: 10-14.9 cm; Class  
III: 15-19.9 cm; Class IV: 20-24.9 cm; Class V: 25- 29.9 cm). The  
total volume of food items in each size class was determined and  
volumetric contribution of each category of food items was then  
expressed as a percentage of total volume of food consumed in each  
size class. The dietary overlap between different size-classes was  
calculated as percentage overlap using the Schoener Diet Overlap  
Index (SDOI) (Schoener, 1970; Wallace Jr, 1981) based on Eq.(1) :  
metal concentrations due to contamination.  
The trace metal  
concentrations in the organs were calculated by subtracting the  
levels in the stock solution from those measured in the acid. Each  
sample was aspirated into the FAAS for direct readings, and the  
blank was created by combining all reagents in a 50 ml volumetric  
flask and diluting with deionized water. Finally, the FAAS was  
adjusted with the following detection limit capacity of the element  
as Cd (0.03 mg/kg), Co (0.02 mg/kg), Cu (0.005 mg/kg), Mn (0.03  
mg/kg), Ni (0.02 mg/kg), Pb (0.03 mg/kg), and Zn (0.005 mg/kg),  
respectively.  
ꢀ
ꢁ
n
i=1  
α = 100[l − 0.5]  
|Pxi − Pyi|  
(1)  
where α is percentage overlap SDOI, between size group x and y,  
Pxi and Pyi are proportions of food category (type) i used by size  
group x and y, and n is the total number of food categories. Diet  
overlap in the index is generally considered to be strong dietary  
similarity and overlap when α value exceeds 0.60 (Mathur, 1977;  
Zaret & Rand, 1971).  
2.8 Statistical Analysis  
The chi-square test was employed to compare the variations of the  
frequency of occurrence of the different food categories during the  
dry and wet seasons (Worms and Touati, 2017). For volumetric  
data, the Mann-Whitney U test was used to assess seasonal  
differences (Worms & Touati, 2017). This non-parametric test was  
used because the data violated the assumption of homogeneity  
of variance required for parametric test. For the comparisons of  
ontogenetic dietary overlap between different size classes, their  
schooner dietary overlap index was considered depending on the  
benchmark (0.6). The concentration of considered trace metals  
from the muscle, kidney, and liver of S. schall was compared using  
one-way analysis of variance with the aid of SPSS v20 software at a  
95% confidence interval.  
2.5 Fish Samples Collection and  
Preservation for Determination of  
Heavy Metals  
A total of 20 S. schall specimens were collected from Lake Abaya  
for heavy metal analysis using gill nets and a beach seine. For  
each specimen, fork length (FL) and total weight (TW) of each  
fish were recorded to the nearest 0.1 cm and 0.1 g, respectively.  
Fish dissection for muscle, kidney, and liver samples followed the  
EMERGE protocol, ensuring proper handling (Gupta & Mullins,  
2010). The separated organs and muscles were quickly wrapped  
in aluminum foil and then placed in plastic bags. These bags  
were subsequently stored in an icebox and transported to the deep  
freezer at Hawassa University Laboratory. Finally, the samples  
were preserved at a temperature of -20 °C in the deep freeze.  
3 Results  
3.1 Diet Composition  
2.6 Sample Preparation for Atomic  
Absorption Spectroscopy (AAS)  
Analysis  
From a total of 849 S. schall specimens examined, 751 (88.5%)  
contained food, while 98 (11.5%) had empty stomachs. The  
sampled fish ranged in size from 5.4 to 33.0 cm in fork length  
and weighed between 2.6 and 566 g in total weight. The diet of  
S. schall in Lake Abaya was diverse, consisting of phytoplankton,  
Muscle, liver, and kidney tissues were mechanically crushed with  
a stainless steel knife and partially air-dried overnight. They were  
Dadebo et.al (2026)  
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East Afr. J. Biophys. Comput. Sci. (2026), Vol. 7, Issue. 1, 34-42  
zooplankton, insects, macrophytes, detritus, ostracods, nematodes,  
hydracarina and fish scales at different proportion (Table 1).  
Of these, the frequency of occurrences of detritus, insects,  
macrophytes, zooplankton and phytoplankton were identified as  
major food categories, while ostracods, nematodes, fish scales  
and Hydracarina were found to be of minor importance (Table  
1). Volumetrically, the contributions of Phytoplankton (31.20%),  
detritus (20.10%), insects (16.80%), zooplankton (13.40%) and  
macrophyte (12.80%) were dominant as a diet of S. schall. While, the  
volumetric contributions of other identified food categories were  
negligible (Table 1).  
Table 1: Diet Compositions of S. schall (n = 751) in Lake Abaya, Ethiopia  
Frequency of  
occurrences  
Volumetric  
contribution  
Food type  
FO  
%FO  
VC(ML)  
%VC  
Phytoplankton  
Blue green algae  
Green algae  
Diatoms  
352  
236  
57  
133  
3
46.90  
31.40  
7.60  
170  
154.70  
87.23  
8.72  
58.51  
0.20  
31.20  
17.60  
1.80  
11.80  
0.04  
Euglinoids  
0.40  
Zooplankton  
Cladocera  
411  
343  
87  
54.70  
45.70  
11.60  
4.70  
66.42  
58.35  
6.11  
13.40  
11.80  
1.20  
Calanoid copepods  
Cyclopoid copepods  
Rotifera  
35  
1.76  
0.40  
5
0.70  
0.19  
0.04  
Insects  
484  
354  
101  
80  
59  
23  
8
64.40  
47.10  
13.40  
10.70  
7.90  
83.24  
50.90  
8.70  
12.01  
4.35  
1.32  
1.52  
4.12  
0.12  
16.80  
10.30  
1.80  
2.40  
0.90  
0.30  
0.30  
0.80  
0.02  
Diptera  
Ephemeroptera  
Plecoptera  
Coleoptera  
Hymenoptera  
Tricoptera  
Anisoptera  
Hemiptera  
3.10  
1.10  
7
0.90  
8
1.10  
Macrophytes  
Detritus  
Ostracods  
Hydracarina  
Nematodes  
Fish scales  
431  
490  
203  
10  
57.40  
65.20  
27.00  
1.30  
12.40  
1.90  
63.54  
99.77  
22.26  
0.23  
12.80  
20.10  
4.50  
0.05  
0.68  
0.60  
93  
3.38  
14  
3.02  
3.2 Seasonal Variation in the Diet  
Composition  
the occurrences of zooplanktons (70.50%), detritus (68.20%), and  
macrophytes (58.80%) were the three most ingested food items of S.  
schall. The remaining food categories in both dry and wet seasons  
were negligible (Table 2). Volumetrically, during dry season the  
contribution of phytoplankton (41.20%), detritus (19.60%), insects  
(17.40%), and macrophytes (13.10%) were the major food categories  
of S. schall. On the other hand, during wet season, zooplankton  
(26.40%), detritus (21.00%), phytoplanktons (18.33%) were the  
dominant food categories of S. schall. The contributions of other  
food categories were relatively low (Table 2).  
Significant seasonal variations were observed in the diet of S.  
schall in Lake Abaya (Table 2). The frequency of occurrence of  
phytoplankton and zooplankton significantly varied during the  
dry (64.80%) and wet (29.30%) seasons (x2 test, p < 0.05; Table  
2). The occurrences of insects (68.8%), phytoplanktons (64.8%),  
detritus (62.20%), and macrophytes (53.3%) were the major food  
items of S. schall during dry season. While, during wet season,  
Dadebo et.al (2026)  
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East Afr. J. Biophys. Comput. Sci. (2026), Vol. 7, Issue. 1, 34-42  
Table 2: Diet Compositions of S. schall during the Dry (n = 304) and Wet (n = 447) Seasons in Lake Abaya.  
Frequency of occurrence (%) Volumetric contribution (%)  
Food items  
Dry  
Wet  
Dry  
Wet  
Phytoplankton  
Blue green algae  
Green algae  
Diatoms  
64.80  
48.40  
13.20  
17.80  
-
29.30  
12.80  
3.10  
18.30  
-
41.20  
28.60  
2.80  
9.80  
-
18.33  
3.60  
0.40  
14.30  
-
Euglinoids  
Zooplankton  
Cladocera  
Copepoda  
Rotifera  
28.60  
10.50  
22.40  
-
70.50  
69.60  
7.60  
3.00  
0.80  
2.20  
-
26.40  
25.40  
0.90  
0.90  
0.20  
Insects  
68.80  
61.50  
10.90  
2.30  
3.00  
5.90  
-
59.50  
37.40  
15.40  
15.90  
11.20  
1.10  
1.10  
-
17.41  
13.00  
1.70  
0.50  
0.30  
0.40  
-
15.94  
6.80  
2.80  
3.80  
1.70  
0.10  
0.70  
-
Diptera  
Ephemeroptera  
Plecoptera  
Coleoptera  
Hymenoptera  
Tricoptera  
Anisoptera  
Hemiptera  
2.30  
0.70  
1.50  
0.010  
0.90  
0.040  
Macrophytes  
Detritus  
Ostracods  
Hydracarina  
Nematodes  
Fish scales  
53.60  
62.20  
203  
-
4.30  
3.00  
58.80  
68.20  
25.50  
2.20  
17.90  
1.10  
13.10  
19.60  
4.60  
-
0.10  
1.00  
12.5  
21.00  
4.30  
0.05  
0.580  
0.90  
3.3 Ontogenetic Diet Shift and Dietary  
Overlap  
classes I and II (α = 52.0%), I and III (α = 58.8%), and II and IV (α  
= 55.0%), suggesting a higher degree of dietary partitioning among  
these groups.  
The diet of S. schall exhibited distinct changes across the five size  
classes (Figure 1). S. schall in size class 5-9.9 cm FL widely relied  
on phytoplankton (60.3%) and zooplankton (17.4%) compared to  
the contributions of other identified food categories (Figure 1).  
When S. schall attained 10-14.9 cm FL size class, the importance  
of detritus, insects, macrophytes and ostracods increased while  
the contributions of phytoplankton and zooplankton decreased  
(Figure 2). As the fish grew to the 15-19.9 cm FL size range,  
it relied mainly on phytoplankton (30.6%), detritus (19.8%),  
insects (17.4%), macrophytes (12.4%) and ostracods (4.1%). When  
the fish further grew to 20-24.9 cm FL size range, it mainly  
consumed phytoplankton (25.6%), detritus (21.5%), insects (18.2%),  
macrophytes (11.6%) and ostracods (4.1%) (Figure 2). The major  
food categories of the largest size class (25-29.9 cm FL) were  
phytoplankton (45.5%), macrophytes (17.4%), detritus (16.5%) and  
insects (16.1%) by volume (Figure 2). Other food categories, namely  
fish scales and zooplankton had negligible role and unimportant in  
the diet of the largest size class.  
The Schoener Diet Overlap Index (SDOI) indicated significant diet  
similarity (> 60%) between several size classes, with the highest  
overlap observed between classes III and IV (α = 93.3%) and II  
and III α = 81.8%). Other significant overlaps were recorded for  
combinations III and V (α = 79.9%), IV and V (α = 77.8%), II and V  
(α = 69.0%), I and IV (α α = 66.9%), and I and V (α = 65.1%) (Table  
3). In contrast, diet overlap was not biologically significant for size  
Figure 2: Mean volume of food items consumed by different size class of S. schall  
sampled from Lake Abaya.  
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East Afr. J. Biophys. Comput. Sci. (2026), Vol. 7, Issue. 1, 34-42  
Table 3: Schoener Diet Overlap Index (SDOI) in five size classes of S. schall from Lake  
and Mn - were detected in all tissue types at levels exceeding the  
analytical detection limits (see the detection limit from material  
and method at sub section of determination of heavy metals).  
Conversely, Pb and Co were found at levels below the detection  
threshold of the equipment, as detailed in Table 4. The distribution  
of metals in the liver was ranked as Cu > Zn > Mn >Ni > Cd, while  
in the muscle and kidney tissues, the order was Zn > Cu > Mn > Ni  
> Cd.  
Abaya, Ethiopia  
Size class  
SDOI (%)  
I and II  
52.0  
I and III  
I and IV  
I and V  
58.8  
65.1*  
66.9*  
81.8*  
55.0  
69.0*  
93.3*  
79.6*  
77.8*  
II and III  
II and IV  
II and V  
III and IV  
III and V  
IV and V  
Mean concentrations exhibited significant tissue specific variations  
(Table 3). The mean concentration of Cu was notably higher  
in the liver (3.847±0.341 mg/kg DW) compared to the kidney  
(1.211±0.168 mg/kg DW) and muscle (0.944±0.028 mg/kg DW).  
Similarly, Zn levels were elevated in the liver (1.85±0.153 mg/kg  
DW) relative to the kidney (1.38±0.179 mg/kg DW) and muscle  
(1.26±0.169 mg/kg DW). Conversely, more Cd concentration was  
detected in the kidney (0.03±0.006 mg/kg DW), followed by the  
liver (0.017±0.002 mg/kg DW) and muscle (0.011±0.00). The mean  
concentrations of Cu in muscle (0.94), kidney (1.21), and liver (3.85)  
exhibited significant differences (p < 0.05). Likewise, Cd levels in  
muscle (0.01), kidney (0.03), and liver (0.02) also showed significant  
variation (p < 0.05). The mean Zn concentrations in muscle (1.26),  
kidney (1.38), and liver (1.85) indicated significant differences (p <  
0.05). In contrast, the concentrations of Ni and Mn did not show  
significant differences (p > 0.05) across the three tissues.  
*indicated SDOI value showed strong dietary overlap between considered size classes  
3.4 Concentration of Some Heavy Metals in  
Muscle, Liver and Kidney  
The concentrations of seven heavy metals in liver, muscle, and  
kidney tissues of S. schall from Lake Abaya are summarized in Table  
3. The findings indicated that five heavy metals - Cu, Cd, Ni, Zn,  
Table 4: Mean concentrations of trace metals in muscle, kidney and liver (mean concentration in mg/kg dry weight ± standard error) of S. schall in Lake Abaya.  
Element  
Muscle  
Kidney  
Liver  
Cu  
Cd  
Ni  
0.94 ± 0.028a  
0.01 ± 0.000a  
0.05 ± 0.006a  
1.26 ± 0.169a  
0.07 ± 0.008a  
ND  
1.21 ± 0.168a  
0.03 ± 0.006b  
0.05 ± 0.007a  
1.38 ± 0.179a  
0.09 ± 0.011a  
ND  
3.85 ± 0.341b  
0.02 ± 0.002a  
0.04 ± 0.006a  
1.85 ± 0.153b  
0.09 ± 0.009a  
ND  
Zn  
Mn  
Pb  
Co  
ND  
ND  
ND  
Note: Superscript represented by different letters indicate significant difference (p < 0.05), ND–Not detected.  
3.5 Discussion  
gastropods, and fish scales. Moreover, various other workers  
studying the food and feeding habits of S. schall in different African  
water bodies have indicated the polyphagus nature of the species  
(Adeyemi, 2010; Akombo et al., 2014; Arame et al., 2021) .  
The results of the present study indicated that S. schall feeds on  
various food items including phytoplankton, zooplankton, insects,  
detritus, macrophytes, ostracods, nematodes, fish scales and  
Hydracarina in Lake Abaya (Table 1). From the various food items,  
phytoplankton, detritus, insects, zooplankton and macrophytes  
were the major food items while ostracods, nematodes, fish scales  
and Hydracarina were of minor importance. Various authors  
studied the feeding habits of S. schall and reported its polyphagous  
nature. Hickley and Bailey (1987) studying S. schall in the Sudd  
Swamps of River Nile (Sudan) have pointed out the importance of  
detritus, benthic algae, macrophytes, benthic crustaceans, insects  
and fish scales in its diet. Ofori-Danson (1992) working on the  
ecology of some Synodontis species in Kpong Head-pond (Ghana)  
have reported the dominant food items of S. schall as detritus,  
insects, oligochaets, nematodes and Hirudinae. Dadebo et al.  
(2012) reported a similarly diverse diet for S. schall in Lake Chamo,  
including zooplankton, plant materials, insects, fish fry, fish eggs,  
The high frequency and substantial volumetric contributions of  
both plant materials and macro- invertebrates in the stomachs of  
S. schall were a good evidence for its omnivorous feeding habits in  
Lake Abaya. Various authors have also reported the omnivorous  
feeding habits of S. schall in different African inland water bodies.  
Baras and Laleye (2003) reported the omnivorous feeding habit of  
S. schall with a strong tendency to predation. Willoughby (1974)  
described S. schall as an omnivorous species feeding on insect larvae  
and nymph, fish eggs and detritus. Dadebo et al. (2012) also  
reported the omnivorous feeding habits of S. schall in Lake Chamo.  
Seasonal variations in the diet of S. schall were observed during  
the present study (Table 2). During the dry season, the diet was  
dominated by phytoplankton, detritus, insects and macrophytes  
Dadebo et.al (2026)  
39  
 
East Afr. J. Biophys. Comput. Sci. (2026), Vol. 7, Issue. 1, 34-42  
(Table 2). The reason for the abundance of phytoplankton during  
the dry season might be attributed to higher light penetration  
and reduced water turbulence, which favor autotrophic production  
(Drakare et al., 2002). Detritus was the second important food  
of S. schall in the dry season (Table 2). The contribution of  
insects was considerable during the dry season (17.4%). Among  
insects, Diptera (Chironomidae larvae) was the most important  
contributing more than 70% by volume of all insect groups. This  
high contribution of Diptera was probably due to the ease of capture  
and also their ability to flourish in wide range of environmental  
conditions (Drakare et al., 2002). Ofori-Danson (1992) also reported  
the importance of Diptera and other insects in the diet of S, schall  
in the Kpong Head-pond in Ghana.  
depending on food availability and their size. According to Lalèyè  
et al. (2006), large size S. schall browse on benthic deposit as can  
be seen from the presence of detritus and mud in the stomachs  
of large fish. The same authors also noted the importance of  
fish scales in the diet of S. schall as its size increases. Similarly,  
Dadebo et al. (2012) reported that fish scales become important  
food items in large size S. schall in the neighboring Lake Chamo.  
Bishai and GideirI (1965) found a significant difference between  
the diets of large and small S. schall in Khartoum. Several other  
investigators also demonstrated that S. schall showed an ontogenetic  
diet shift as a result of the change in habitat use in different  
water bodies (Araoye, 1999; Dadebo et al., 2012; Ofori-Danson,  
1992). The other probable factor for such dietary variations across  
size classes might be aligned with the habitat that they survive.  
Juvenile S. schall hide themselves from the risk of predators (Baras  
& Laleye, 2003). Similar to the present finding, Araoye (1999)  
and Dadebo et al. (2012) reported that, fry and fingerlings of S.  
schall were usually restricted to the flooded littoral zone of the lake  
where they feed mainly on zooplankton, insect larvae and other  
macro-invertebrates.  
Macrophytes were ingested in considerable quantities during the  
dry season. It is probable that the fish might ingest part of  
macrophytes incidentally as they pursue their prey in the littoral  
region where the prey normally seek refuge from predators  
(Thomaz et al., 2025). More focused study is needed to determine  
the importance of macrophytes to the nourishment of the species  
by comparing the nutritive values of the plant fragments in the  
fore and hind guts of the fish (Thomaz et al., 2025). During the  
wet season, the contributions of zooplanktons were dominated and  
widely represented by Daphnia (Table 2). The reason for this was  
probably due to seasonal reproductive cycle of the cladocerans  
population, which often peak during rainy season in tropical lakes  
(Choedchim et al., 2017). Detritus was also an important food  
item during the wet season. The source of this food category  
could be the floods that introduce different plant materials into the  
lake and plant leaves falling into the lake and undergoing partial  
decomposition. Araoye (1999) reported that the contribution of  
plant materials and detritus in the diet of S. schall during the wet  
season was high, and such food items were dispersed along the  
surface water column at this period due to floods and overturn.  
The concentrations of the five heavy metals detected were generally  
higher in the kidney and liver compared to muscle tissue (Table 5).  
For example, Cu levels were elevated in the liver relative to both  
the kidney and muscle tissues, consistent with findings of Gerenfes  
et al. (2019) for Enteromious species in Lake Chamo, Ethiopia and  
Shahid et al. (2016) for Cyprinus carpio. This distribution can be  
explained by the liver’s function in detoxification and synthesis of  
copper-binding metallothioneins, highlighting its role as a crucial  
bio-indicator for evaluating Cu levels in aquatic ecosystems (Javed  
& Usmani, 2013). In terms of Cd levels, S. schall from Lake Abaya  
exhibited a muscle tissue concentration of 0.01 mg/kg, which is  
higher than that of bream (0.009 mg/kg) and mandarin fish (0.0009  
mg/kg) from Poyang Lake (Wei et al., 2014). Additionally, Cd  
concentrations ranging from 0.001 to 0.009 mg/kg were identified  
in eleven fish species from Rio de Janeiro State, Brazil (Medeiros  
et al., 2012), but the finding in this study is lower than the 0.03 to 1.57  
mg/kg detected in fish from the Pearl River Delta (Cheung et al.,  
2008). All observed concentrations of the detected heavy metals fall  
below the limits set by the EU (2001), TFC (2002) and FAO (1983)  
guideline for human consumption.  
From the results of the present study, it was evident that S.  
schall showed a clear ontogenetic dietary shift during its life cycle  
(Figure 1). Smaller individuals relied widely on phytoplankton  
and zooplankton, whereas larger fish incorporated more insects,  
detritus, and macrophytes. Bishai and GideirI (1965) reported that  
some members of the genus Synodontis switch from benthic feeding  
to surface feeding or vice versa by using ventrally positioned mouth  
Table 5: Comparisons of Concentration of Trace Metals in Fish Muscle Relative to the Standards (mg/kg dry weight).  
Parameter (guidelines)  
Cu  
Zn  
Mn  
Cd  
0.01  
–
–
–
Ni  
0.05  
–
–
–
References  
Present study in fish muscle  
0.94  
1.26  
0.07  
FAO  
EU  
30  
4
50  
–
30  
–
FAO (1983)  
EU (2002)  
TFC (2002)  
Turkish Food Codex  
20  
50  
20  
Mn were found in all three tissues, while Pb and Co were absent.  
In the liver, the concentration ranking was Cu > Zn > Mn > Ni >  
Cd, whereas in muscle and kidney tissues, it was Zn > Cu > Mn  
> Ni > Cd. From the present study, S. schall is an omnivorous  
in its feeding strategy. Overall, heavy metal concentrations were  
generally higher in the kidney and liver than in muscle. The level  
in muscle showed below FAO and EU maximum acceptable limits  
for human consumption. The present result suggested conducting  
4 Conclusion  
This study has clearly shown that S. schall in Lake Abaya ingests  
a wide range of plant based and animal origin of food categories.  
However, the diet composition of S. schall was different based on  
their size classes and season of sampling. With the exception of  
some size classes, strong dietary overlap was seen across different  
size classes. Trace metals analysis revealed that Cu, Cd, Ni, Zn, and  
Dadebo et.al (2026)  
40  
 
East Afr. J. Biophys. Comput. Sci. (2026), Vol. 7, Issue. 1, 34-42  
further heavy metal analysis is required based on simultaneous  
study including water quality analysis, sediment analysis, and the  
interaction between feeding ecology with trace metal concentration  
for further comparisons.  
River in Northern Benin. International Journal of Aquatic  
Biology, 9, 41–54. https://doi.org/10.22034/ijab.v9i1.973  
Araoye, P. (1999). Spatio-temporal distribution of the fish  
Synodontis schall (Teleostei: Mochokidae) in Asa lake,  
Ilorin, Nigeria. Revista De Biología Tropical, 47, 1061–1066.  
https://doi.org/10.15517/rbt.v47i4.19311  
Baras, E., & Laleye, P. (2003). Ecology and behavior of catfish. In  
G. Arratia, B. G. Kapoor, M. Chardon, & R. Diogo (Eds.),  
Catfish. Science Publishers.  
Baxter, R. (2002). In: Ethiopian Rift Valley Lakes. In C. Tudorancea  
& W. D. Taylor (Eds.). Backhuys Publishers.  
Conflict of Interest  
None declared.  
Funding  
Bishai, H., & GideirI, Y. (1965). Studies on the biology of the genus  
Synodontis at Khartoum: II. Food and feeding habits.  
Hydrobiologia, 26, 98–113. https : / / doi . org / 10 . 1007 /  
BF00731551  
This research was supported by NORAD project  
Acknowledgements  
Bowen, S. H. (1996). Quantitative description of the diet. In Fisheries  
techniques (2nd, pp. 513–532). American Fisheries Society.  
Cheung, K., Leung, H., & Wong, M. H. (2008). Metal concentrations  
of common freshwater and marine fish from the Pearl  
River Delta, South China. Archives of Environmental  
Contamination and Toxicology, 54, 705–715. https : / / doi .  
org/10.1007/s00244-007-9064-7  
The authors are grateful to the Department of Aquatic Sciences,  
Fisheries and Aquaculture for providing laboratory facilities and  
the logistics for the field trips. Dr. Andargachew Gedebo, NORAD  
project coordinator and Hawassa University are acknowledged  
for providing a vehicle for the field trips. We thank fisherman  
Asaminew Matte for his assistance during sample collection.  
Choedchim, W., Van-Damme, K., & Maiphae, S. (2017). Spatial and  
temporal variation of Cladocera in a tropical shallow lake.  
International Journal of Limnology, 53, 233–252. https://doi.  
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