Food and feeding habits of the Nile tilapia Oreochromis niloticus (Linnaeus, 1758) from Ribb reservoir, Lake Tana sub-basin, Ethiopia

Abstract

This study aimed to examine the food and feeding habits of Oreochromis niloticus at the Ribb reservoir for sustainable management. A total of 512 specimens were collected using gillnets of different mesh sizes. Of these, 348 (67.9 %) fish had different foods, while 164 (32.1%) fish had empty stomachs. Mud was the main food item that accounted for 89.1% of estimated stomachs and 55.1% of the total volume in the diet. Detritus and phytoplankton were the second most important food items contributing 25.1% and 14.1% of the total volume respectively. Prey items differed depending on the fish size (ANOVA, p < 0.05). The smallest-sized fish preferred detritus, zooplankton, and mud while the food items such as mud and phytoplankton were preferred by the larger-sized fish species from the Ribb Reservoir. The seasonal variations in food composition were observed (ANOVA, p < 0.05). The volumetric contribution of foods such as mud and detritus were higher in the dry season and contributed in 61.2% and 19.2% respectively. During the wet season, phytoplankton and detritus were the most important food items and their corresponding volumetric contributions were 52.3% and 42.1% of the total volume in the stomach O. niloticus Generally, O. niloticus exhibited omnivorous feeding habits in their diet from the newly established Ribb Reservoir. It is possible to conclude that food and feeding habits of O. niloticus influenced by seasons, and fish sizes from Ribb Reservoir. Therefore, water buffer zone management is needed to improve the food and feeding habits of this fish for the better sustainable utilization.

Keywords:

• Feeding habits
• omnivorous
• Oreochromis niloticus
• Ribb Reservoir
• Tana sub-basin

PUBLIC INTEREST STATEMENT

Oreochromis niloticus Nile tilapia is one of the most commercially important fish species in Ethiopia. A study on the food and feeding habits of O. niloticus is useful to understand about established Ribb Reservoir ecosystem for managing the reservoir’s fish resources and understanding the feeding behavior of the reservoir’s fish to use the fish for aquaculture. The food and feeding habits of this fish species is impacted by the expansion of irrigation practices such as high sedimentation load, poor land use practice, lack of vegetation cover and the construction of dams and weirs within the rivers. However, this fish resource is not well utilized due to the scientific knowledge gap. The Ribb Reservoir is the newly established. Therefore, water buffer zone management is needed to improve the food and feeding habits of this fish species for the better sustainable utilization.

1. Introduction

Nile tilapia Oreochromis niloticus (Linnaeus, 1758) is the most commercially preferable fish species in Ethiopia (Temesgen et al., 2022). It covers more than 50% of the total catch in Ethiopian water bodies (G. Tesfaye & Wolff, 2014). It is also the leading commercially accessible fish species in the Ribb Reservoir (Alebachew et al., 2022; Mequanent et al., 2022). This fish species is broadly found in all Ethiopian major drainage systems such as in the rift valley, Abay, Awash, Baro-Akobo, Omo-Gibe, Tekeze, Ethiopian highland lakes and rivers, and Wabishebele-Genale basins (Awoke, 2015).

Fish species need a balanced diet to perform growth, reproduction, and other physiological functions as described by Temesgen et al. (2022). The scientific studies on fish food and feeding habits provide successful management of both capture fisheries and aquaculture, recognize the trophic relationships available in water ecosystems, finding the diet structure, composition, and stability of food webs (Otieno et al., 2014; Tesfahun & Temesgen, 2018). In natural water bodies, plankton (phytoplankton and zooplankton), macrophytes, aquatic insects and their larvae and pupae, nematodes (round and flatworms), and sediment are the primary food items of O. niloticus (Hussian et al., 2019). The presence of quality foods in water bodies governs the fish’s health, growth performance, and fecundity potential (Gebru, 2020). Wagaw et al. (2022) also confirmed that the condition factor and length-weight relationship of fish are determined by the prey availability in the waterbodies. Of course, several water environmental factors such as physicochemical and biological water quality parameters, food availability, and fish stock can impact the food selection and feeding habits of fish species (Temesgen et al., 2022). Besides, the prey size and age differences of the fish can also influence their food preference habits (Tomojiri et al., 2019). Based on feeding habits, fishes rely on food items that can fit into their mouth with respect to the size of their stomach. Otieno et al. (2014) and Wakil et al. (2014) reported that when fishes increase in weight or length, their stomach also develops well and their digestive system becomes more advanced. Several authors have studied the food and feeding habits of Nile tilapia with respect to the seasonal variation and size differences in Ethiopian water bodies (A. Tesfaye et al., 2020; Engdaw et al., 2013; Gebru, 2020; Habib & Adugna, 2021; Negassa & Prabu, 2008; Teame et al., 2016; Teferi et al., 2000; Temesgen et al., 2022; Tesfahun & Temesgen, 2018; Wagaw et al., 2022; Wakjira, 2013). Very few studies were carried out on aspects of fish diversity and abundance (Alebachew et al., 2022; Mequanent et al., 2022) and zooplankton diversity assessment (Mequanent et al., 2022) from Ribb reservoir. However, there is no study carried out with reference to the food and feeding habits of Oreochromis niloticus Nile tilapia from the Ribb Reservoir Lake Tana Bain, Ethiopia. This study is useful to understand about established Ribb Reservoir ecosystem for managing the reservoir’s fish resources and understanding the feeding behavior of the reservoir’s fish to use the fish for aquaculture. Therefore, the present study aims to fill the gap by investigating seasonal and fish size effects on the food and feeding habits of the fish in Ribb Reservoir.

2. Materials and methods

2.1. The study area

Ribb Dam has newly formed primarily for irrigation purposes (Mequanent et al., 2022) (Figure 1). Accordingly, the construction of this reservoir was completed in 2018. The dam is formed when the damming of Ribb River in the South Gondar Zone Lake Tana sub-basin. The Ribb River is about 130 km long and has an average drainage area of 1790 km², with an average annual drainage area of 14.6331 m³s⁻¹. The dam site has a drainage system of approximately 685 km². Ribb Reservoir is also located between 392,174,64 E, 1,330,225,76 N, and 390,813,45 E, 1,330,018 N at altitudes ranging from 1880 to 1,970 m. At the Ribb Reservoir, the mean and the minimum precipitation are about 1,400 mm and 1,200 mm annum⁻¹ respectively. The maximum and minimum temperatures are about 30 °C and 11.5 °C respectively (Bezabih, 2021).

Figure 1. The map indicates the locations of the Ribb Reservoir sampling sites in Ethiopia.

The amount of sunlight is reduced to 6.0 h in July and 6.5 h in August (Bezabih, 2021). The reservoir is dominated by blue-green algae Microcystis sp. around the littoral side (personal observation). Zooplankton species such as Cladocera (Daphnia magna, Daphnia pulex, Diaptomus pallidus, Daphnia longispina, Bosmina longirostris, and Bosmina coregoni), copepods (Thermodiaptomus galebi, Mesocyclops aequatorialis similis, Cyclops strenuous and Rybocyclops dussarti) and Rotifers (Asplanchna brightwelli, Brachionus calyciflorus, Brachionus plicatilis and Brachionus angularis) were found in the reservoir (Mequanent et al., 2022). Among the fish population Cyprinidae (Labeobarbus nedgia, Labeobarbus intermedius, Labeobarbus brevicephalus, Labeobarbus megastoma, Labeobarbus crassibarbis, Labeobarbus platydorsus, Labeobarbus macrophtalmus, Labeobarbus beso, Labeobarbus surkis), Cichlidae (Oreochromis niloticus) and Clariidae (Clarias gariepinus) are found in the reservoir (Mequanent et al., 2022). However, a recent finding reported also the existence of three more fish species C. gariepinus, L. intermedius and O. niloticus (Alebachew et al., 2022).

2.2. Fish sampling and morphometric measurements

Fish specimen collection was conducted in three sampling sites S1, S2 and S3 from February-April 2021 and from June and October 2021 during the dry and the wet season respectively. The sampling sites were selected based on the abundance of fish availability. Gillnets were used with a mesh size of 3.5, 5, 6, 8, and 10 cm and 25 m in length. Gillnets were placed in the evening afternoon (5:00 p.m.) and checked the following early morning (6:00 a.m.). Consequently, the morphometric measurements such as total length (TL), fork length (FL), and standard length (TL) were measured using a measuring board of the nearest 0.1 cm. The total weights of fish (TW) were measured using a sensitive balance of the nearest 0.1 g.

2.3. Fish stomach sampling and preservation

After the morphometric measurements, the fish were dissected by a dissection kit. Following this, the full stomachs of O. niloticus individuals were isolated and pressed to extract the stomach contents. Furthermore, the stomachs were dissected the stomach contents were removed and preserved in a 5% formalin solution for more study. Finally, all specimens were labeled (date of sampling, length and weight measurements, location of sampling, fish species, etc.) and transported to the zoological sciences laboratory at Debre Tabor University for additional analysis.

2.4. Food items examination method

During laboratory analysis, the stomach contents were placed into an Agar plate and investigated (Hyslop, 1980). Accordingly, the smallest food items were analyzed under a dissecting microscope model XSZ- 70DN and stereo microscope (ST-30-2 L) (100× to 400× magnification), and the larger food items were identified without the aid of a microscope. The identified food items were categorized to the lowest taxonomic level by using descriptions, keys, and literature (Carling et al., 2004; Vuuren et al., 2008). Fish stomach content was analyzed by relative measures of prey quantity (RMPQ) (Hyslop, 1980). The role of each food item category in the total stomach contents was analyzed by the percentage frequency of occurrence (%Q) and percentage contribution volume (%V) (Assis, 1996).

The frequency of occurrence was computed as:

$\mathrm{\%}0=\frac{\mathrm{J}\mathrm{i}}{\mathrm{p}\mathrm{i}}\times 100$

where Ji, is the number of fish containing food items and p is the number of fish with food in their stomach. The volumetric technique (%Vi) was also computed as:

$\mathrm{\%}\mathrm{V}\mathrm{i}=\frac{\mathrm{N}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{p}\mathrm{o}\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{s}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{o}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{t}\mathrm{o}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{i}}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{p}\mathrm{o}\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{s}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{o}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{t}\mathrm{o}\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}\times 100$

where %Vi is the percentage volume of the prey component i.

The frequency of occurrence (%Qi) provides evidence of the proportion of fish stomachs comprising a specific prey item (Hyslop, 1980). The volume role of each food item is visually measured relative to all of the food items existing in the stomach.

The stomach analysis parameters such as frequency of occurrence (%Qi) and volumetric contribution (%Vi, ml) were used to estimate the index of food preponderance (PIi) and geometric importance of index (GIIi). To determine the importance of each food item the index of preponderance (PIi) (Tomojiri et al., 2019) was calculated as:

$\mathrm{P}\mathrm{I}\mathrm{i}=\frac{\mathrm{Q}\mathrm{i}\mathrm{x}\mathrm{\%}\mathrm{V}\mathrm{i}}{\sum \left(\mathrm{Q}\mathrm{i}+\mathrm{\%}\mathrm{V}\mathrm{i}\right)}\times 100$

Where %Vi is the percent composition by volume of species i and Qi is the frequency of occurrence of species i. To simplify assessments amongst species, PIi was transformed into percent PIi (%PIi). Besides, to estimate the relative importance of food items and species-level dietary differences, Geometric Index of Importance (GIIi) (Assis, 1996) was computed as:

$\mathrm{G}\mathrm{I}\mathrm{I}\mathrm{i}=\frac{\left(\sum \mathrm{R}\mathrm{M}\mathrm{P}\mathrm{Q}\mathrm{i}\right)}{\sqrt{\mathrm{n}}}$

Where RMPQi = as a percentage of total occurrences (percentage of volume and frequency of occurrence) and n = total number of RMPQ. The index of GIIi ranges from 0–1 (1–100%), with values close to 0 representing feeding specialization whereas values near to 1.0 indicating generalization (Hurlbert, 1978).

2.5. Food and feeding habits in relation to season and fish size

Diet composition with respect to temporal and size class differences of O. niloticus in the Ribb Reservoir was studied via the percent volumetric contribution, frequency of occurrence, index of food preponderance (PIi), and geometric importance of index (GIIi) among the five size classes (<20, 20–25, 25.1–29.8, 30–34 and>35 cm TL) of fish and seasons (dry and wet) of the year (Tomojiri et al., 2019).

3. Data analysis

Microsoft Excel was used for the descriptive statistics. One-way ANOVA was also used to analyze the food and feeding habit variations between the seasons and among the fish size classes (SPSS version 20).

4. Results

4.1. Diet composition and feeding habits of O. niloticus

During this study, 512 specimens of Oreochromis niloticus were collected. Of these, 348 specimen (67.9%) had different food, while 164 (32.1%) had empty stomachs. The diet of O. niloticus is consisted of various prey types such as mud, plant detritus, phytoplankton, zooplankton, sand particles, ostracods, and macrophytes (Table 1). Among these, mud, phytoplankton, zooplankton, and detritus constitute the majority of foods such as consumed. Other foods Sand particles, ostracods, and macrophytes were contributed less to the diet of the fish. The percentage of the geometric index (%GIIi) also shows that mud is the primary prey type consumed (Figure 2). According to this index, phytoplankton, detritus, and zooplankton were the second most important preys in O. niloticus diet, while sand grains, ostracods, and macrophytes are rarely consumed.

Figure 2. The percentage contribution of geometric index importance (%GIIi) in the diet of O. niloticus (n=348) from Ribb Reservoir.

Note: PKT–Phytoplankton; ZPK–Zooplankton; DET— detritus; SAG-sand grain; OST-ostracod and MAC-macrophyte. Dotted vertical lines separate the different grades of favorite food items.

Table 1. The proportions of food items in the diet of O. niloticus (n = 348) in Ribb Reservoir, Ethiopia

Food items	Oi	%Oi	Vi	%Vi	IP	%IP	%GIIi
Mud	311	89.1	513.1	55.1	4913.1	52.7	54.4
Detritus	231	66.2	233.66	25.1	1663.62	17.8	34.5
Phytoplankton	285	81.7	131.1	14.1	1154.9	12.4	36.2
Blue-green algae	280	80.3	112.4	12.1	975.6	10.5	-
Green algae	104	29.8	14.5	1.6	50.68	0.5	-
Euglenoids	62	17.8	4.2	0.5	9.8	0.1	-
Zooplankton	200	57.3	47.2	5.1	295.6	3.2	23.6
Cladocera	222	63.6	29.2	3.1	198.3	2.1	-
Copepod	82	23.5	10.7	1.1	26.9	0.3	-
Rotifer	72	20.63	7.3	0.8	18.5	0.2	-
Sand particles	65	18.6	6.7	0.72	14.4	0.2	7.3
Ostracods	32	9.2	1.6	0.2	1.9	0.02	3.5
Macrophyte	12	3.5	4.1	0.4	1.5	0.01	1.5

Note: NB: Frequency of occurrence (Qi), volumetric contribution (Vi), index of preponderance (IP), percentage frequency of occurrence (%Qi), percentage of volumetric contribution (%Vi), percentage index of preponderance (%IP) and percentage Geometric Index of Importance (%GIIi).

Mud accounted for 89.1% of total food consumption and comprising for 55.1% of the total volume in the diet. Detritus accounted for 66.2% of the total food occurrence in the diet and comprised 25.1% of the total volume. Phytoplankton (blue-green algae, green algae, and euglenoids) collectively consisted of 81.7% and are estimated about 14.1% of the total volume of food items. Zooplankton (Rotifers, Copepods, and Cladocera) accounted for 57.3% of total food consumption and comprising for 5.1% of the total volume in the diet. According to the %IP index, mud, detritus, and phytoplankton accounted for approximately 52.7%, 17.8 and 12.4 of the diet respectively. The index of preponderance (%IP = 3.2%) and geometric importance index (%GIIi = 23.6%) exhibited that zooplankton was the second preferred food type. Whereas, other food items such as sand particles, ostracods and macrophytes had less contribution in the diet of O. niloticus (Table 1 and Figure 2).

4.2. Diet variation in respect to size

In O. niloticus the diet of the different sizes was significantly different (ANOVA, p < 0.05). Detritus was the main food item for the size class under 20.0 cm TL (Figure 3) and contributed 40.0% of the total volume in the diet of this size class. Zooplankton and mud were the second most important food items in this size class and their corresponding volumetric contributions were 11.2% and 8.1% respectively. However, the remaining food items such as phytoplankton and sand grains had minor contribution in the diet of this size class. Mud was the main food for size categories between 20.0–25.0 cm (TL). It contributed 60.6% of the total volume of the diets. Besides, detritus, phytoplankton, and zooplankton were the next most preferred food items in this size class and their conforming volumetric contributions were 25.9%, 7.0%, and 5.6% respectively. Whereas, other food items such as ostracods, macrophytes, and sand grains had a minor role in the diet of the fish.

Figure 3. The size-based proportion of food items in the diet of O. niloticus in Ribb Reservoir, Ethiopia.

Mud was the main food item for the size class between 25.1–29.8 cm (TL). It contributed 42.3% of the total volume in the diet of this size class. In addition, detritus, phytoplankton, and zooplankton were the following most favored food items in this size class and their compliant volumetric contributions were 26.1%, 27.0%, and 5.0% respectively. But, other food items such as ostracods, macrophytes, and sand grains had less contribution in the diet of this size class.

Mud was also the main food item for the size class between 30.0–34.0 cm (TL). It contributed 67.1% of the total volume in the diets. Detritus, phytoplankton, and zooplankton were the next most preferred food items in this size class and their conforming volumetric contributions were 11.8%, 15.0%, and 3.4% respectively. Whereas, other food items such as ostracods, macrophytes, and sand grains had a minor role in the diet of this size class.

Mud was the main food item for the size class above 35 cm (TL). It contributed 64.3% of the total volume in the diets. Besides, detritus, phytoplankton, and zooplankton were the next most preferred food items in this size class and their conforming volumetric contributions were 22.4%, 9.0%, and 4.3% respectively. Whereas, other food items such as ostracods, macrophytes, and sand grains had a minor role in the diet of the fish. The volume of the mud and phytoplankton has shown an increasing trend with the size of the fish increases. Whereas the volumetric contribution of detritus and zooplankton showed decreasing trend as the fish size increases (ANOVA, p < 0.05 (Figure 3).

4.3. Temporal Variation in the diet of O. niloticus

The temporal contributions of different foods to O. niloticus stomach contents are shown in Tables 2 and 3. In the Ribb Reservoir, results revealed seasonal changes in fish’s diet composition (ANOVA, p < 0.05). Mud (%IP = 69.6 and %GIIi = 67.0) was the most important food item occurred in 99.7% of the total stomachs and contributed 61.2% of the total volume of food items during the dry season. Detritus (%IP = 13.7 and %GIIi = 34.0), phytoplankton (%IP = 4.6 and %GIIi = 43.1), and zooplankton (%IP = 5.8 and %GIIi = 42.3) occurred in 62.5%, 99.3%, and 96.1% of the total stomach contents and their corresponding volumetric contributions were 19.2%, 4.1% and 5.3% of the total volume in the diet of fish during the dry season (Table 2).

Table 2. Food items in the diet of O. niloticus during the dry season in Ribb Reservoir, Ethiopia

Food items	Oi	%Oi	Vi	%Vi	IP	%IP	%GIIi
Mud	308	99.7	509.4	61.2	6101.1	69.6	67.0
Detritus	193	62.5	159.6	19.2	1200	13.7	34.0
Phytoplankton	300	99.3	33.6	4.1	407.1	4.6	43.1
Blue-green algae	241	77.9	26.8	3.2	249.3	2.8	-
Green algae	81	26.2	2.6	0.3	7.9	0.1	-
Euglenoids	62	20.1	4.2	0.5	10.1	0.1	-
Zooplankton	290	96.1	43.8	5.3	509.3	5.8	42.3
Cladocera	209	67.6	26.5	3.2	216.3	2.5	-
Copepod	78	25.2	10.1	1.2	30.2	0.3	-
Rotifera	70	22.7	7.2	0.9	20.4	0.2	-
Sand particles	64	20.7	6.3	0.8	16.6	0.2	8.7
Ostracods	32	10.4	1.6	0.2	2.1	0.02	4.4

Note: NB: Frequency of occurrence (Qi), volumetric contribution (Vi), index of preponderance (IP), percentage frequency of occurrence (%Qi), percentage of volumetric contribution (%Vi), percentage index of preponderance (%IP) and percentage Geometric Index of Importance (%GIIi).

Table 3. Food items in the diet of O. niloticus during the wet season in Ribb Reservoir, Ethiopia

Food items	Oi	%Oi	Vi	%Vi	IP	%IP	%GIIi
Phytoplankton	38	95	92.3	52.3	4990.5	36.2	67.0
Blue-green algae	39	97.5	85.5	48.6	4740.5	34.4	-
Green algae	23	57.5	6.8	3.9	225.3	1.6	-
Detritus	38	95	74	42.1	4001.5	29.1	62.3
Mud	3	7.5	4	2.3	20.5	0.2	4.5
Zooplankton	15	37.5	3.43	0.4	16.3	0.1	17.1
Cladocera	13	32.5	2.7	0.02	0.7	0.01	-
Copepod	4	10	0.6	0.34	3.4	0.03	-
Rotifer	2	5	0.1	0.06	0.3	0.01	-
Macrophyte	10	2.5	4	2.3	6	0.04	2.2

Note: NB: Frequency of occurrence (Qi), volumetric contribution (Vi), index of preponderance (IP), percentage frequency of occurrence (%Qi), percentage of volumetric contribution (%Vi), percentage index of preponderance (%IP) and percentage Geometric Index of Importance (%GIIi).

During the wet season, phytoplankton and detritus were the most important food items in the stomach of O. niloticus. They occurred in 95.0% and 95.0% of the total stomachs and their corresponding volumetric contributions were 52.3% and 42.1% of the total volume respectively. The %IP and %GIIi indexes were also showed phytoplankton (%IP = 36.2 and %GIIi = 67.0), and detritus (%IP = 29.1 and

%GIIi = 62.3) were dominant food items in the diet during the wet season. However, mud, zooplankton, ostracods and macrophytes had less contribution in the diet of the fish (Table 3).

5. Discussion

In this study, 348 (67.9%) specimens had a variety of food items in their stomach while 164 (32.1%) fish samples possessed empty foods in their stomach. Most empty stomachs are probably related to the digestive process after the fish are caught. Likewise, inappropriate sampling of fish also leads to an accelerated digestion process (Wagaw et al., 2022). The authors confirmed that the a great proportion of fishes with empty stomachs were recorded in various Ethiopian water bodies when fishes were sampled with gillnets (Engdaw et al., 2013; Temesgen et al., 2022; Wagaw et al., 2022). This may be because the food in their stomachs may be vomited or assimilated as the fish fight to escape from the gill net during the catch. In this study, seven different food items including mud, detritus, phytoplankton, zooplankton, sand particles, ostracods, and macrophytes were recovered from the stomachs of O. niloticus. In Ethiopian water systems, many other studies also revealed that O. niloticus contained a variety of food types in their stomach (A. Tesfaye et al., 2020; Assefa & Getahun, 2015; Engdaw et al., 2013; Habib & Adugna, 2021; Teame et al., 2016; Teferi et al., 2000; Temesgen et al., 2022; Tesfahun & Temesgen, 2018; Wagaw et al., 2022; Wakjira, 2013). In the current study, mud (sediment) was the primarily preferred food item in all stomachs of O. niloticus in Ribb Reservoir. Whereas detritus, phytoplankton, and zooplankton were the second most important prey in the diet. In the reservoir, the fish exhibited omnivorous feeding habits. The omnivorous feeding habits of O. niloticus were also reported from Ero Reservoir, Nigeria by Oso et al. (2006). However, other findings revealed that phytoplankton is the major food item in the diet of this fish species. For instance, studies conducted from lakes Langeno (Temesgen et al., 2022), Shalla (Wagaw et al., 2022), Ziway (A. Tesfaye et al., 2020), Koka (Engdaw et al., 2013), Tekeze and Hashange (Teame et al., 2016) investigated the dominance of phytoplankton in the diet of O. niloticus as the major food item. Different fish feeding habits may be food due to different sources in different places. The availability of foods also correlated with nutrient access and the optimization of physicochemical characteristics of water bodies. In the current study, mud was found excessively in the stomachs of the fish. This is probably due to anthropogenic impacts, such as high sediment loads, extreme irrigation activities, and uncontrolled land use around the buffer zone of Ribb Reservoir. The other reason might be due to the fluctuation of water level when the water resource is needed for irrigation purposes and water abstraction.

In this study, the phytoplankton blue-green algae were the second most important food type. Similar findings were reported in Ethiopian water bodies via the dominance of blue-green algae in Lake Ziway Hayq (Assefa & Getahun, 2015), and Koka Reservoir (Engdaw et al., 2013).

In contrast, the dominance of diatoms in the diet of fish was reported from Lakes Langeno (Temesgen et al., 2022) and Shalla (Wagaw et al., 2022). These differences may be linked to changes in the environment and biological factors of the lake, which influence the food type in the diet of this fish (Temesgen et al., 2022).

It is known that O. niloticus feeds on various food items (Tomojiri et al., 2019). Its diet may vary depending on size (Temesgen et al., 2022; Wagaw et al., 2022). In this study, O. niloticus showed a difference in the food habits in respect to size. The most preferred food items were mud and phytoplankton in the larger-size class, while the smallest-sized fish showed a preference on detritus, zooplankton, and mud. This indicates that the smallest fishes (<20 cm TL) size class of O. niloticus are omnivorous in their feeding habits. Other researchers reported the omnivorous feeding habits of the juveniles (<10 cm TL) and fingerlings (10–15 cm TL) of O. niloticus (Temesgen et al., 2022; Wagaw et al., 2022). However, (A. Tesfaye et al., 2020; Engdaw et al., 2013; Teferi et al., 2000; Tesfahun & Temesgen, 2018) reported a preference for invertebrates (zooplankton and insects) in the diet of the smallest fish. The difference in O. niloticus food habits is due to energy demand, the development of fish morphology, and physiological features during growth (Njiru et al., 2004). In addition, the life history of O. niloticus depends on the habitat used and their feeding habits vary greatly (Temesgen et al., 2022; Wagaw et al., 2022).

In fish-feeding research, it is always necessary to consider the seasonal effects. Considering that the seasonal variation of biological and non-biological factors alters the structure of the food availability throughout the year, fish often exhibit seasonal food changes (Wagaw et al., 2022).

The current study showed significant seasonal variations in the diet of O. niloticus from Ribb Reservoir. Many researchers also pointed out a seasonal variation of food items in the diet composition of O. niloticus in the Ethiopian water bodies (A. Tesfaye et al., 2020; Assefa & Getahun, 2015; Engdaw et al., 2013; Habib & Adugna, 2021; Teame et al., 2016; Teferi et al., 2000; Temesgen et al., 2022; Tesfahun & Temesgen, 2018; Wagaw et al., 2022; Wakjira, 2013). Mud (sediment) followed by detritus and phytoplankton was the most important food item during the dry season. However, other authors reported the importance of phytoplankton in the diet of the fish in Ethiopian water bodies during the dry season (A. Tesfaye et al., 2020; Assefa & Getahun, 2015; Engdaw et al., 2013; Habib & Adugna, 2021; Teame et al., 2016; Teferi et al., 2000; Temesgen et al., 2022; Tesfahun & Temesgen, 2018; Wagaw et al., 2022; Wakjira, 2013). Moreover, Assefa and Getahun (2015) confirmed that phytoplankton is the most preferred food item during the dry season. The availability of prey items is different with respect to Ethiopian waters due to the physicochemical water parameters such as DO, pH, Temperature, salinity, nitrates, phosphate, NH3, turbidity, etc and pollution these factors influence the availability of prey items (Temesgen et al., 2022).

The volumetric contribution of phytoplankton followed by detritus was higher in the diet of the fish during the wet season. A similar finding was investigated from Gilgel Gibe I Reservoir as the main food item of phytoplankton in the diet of the fish during the wet season (Wakjira, 2013).

In addition to phytoplankton, the results of the study showed that detritus contributed more during wet seasons. This is due to floods that bring parts of dead plants and animals into the lake and partially decompose. The contribution of detritus in the diet of O. niloticus was also reported in Ethiopian water bodies (A. Tesfaye et al., 2020; Assefa & Getahun, 2015; Engdaw et al., 2013; Habib & Adugna, 2021; Teame et al., 2016; Teferi et al., 2000; Temesgen et al., 2022; Tesfahun & Temesgen, 2018; Wagaw et al., 2022; Wakjira, 2013). The biomass of phytoplankton was higher in O. niloticus diet in the wet season and this might be associated with flooding, which contributes to the wet season to high phytoplankton populations by transporting nutrients from catchments, which supports the growth of phytoplankton and thus the productivity of zooplankton (Temesgen et al., 2022).

6. Conclusions

This study demonstrated the preference of mud as the main food source, followed by detritus, phytoplankton, and zooplankton. The smallest fish mostly fed on detritus followed by zooplankton and mud while the largest fish fed mainly on mud followed by phytoplankton and detritus. The prey items such as mud followed by detritus, phytoplankton, and zooplankton were most preferred food types during the dry season. Whereas phytoplankton and detritus were the most important food items in the diet O. niloticus in the wet season. Generally, O. niloticus exhibited omnivorous feeding habits in their diet from the newly established Ribb Reservoir. It is possible to conclude that the food and feeding habits of fish are influenced by seasons, and the size of fish reservoir. Therefore, water buffer zone management is needed to improve the food and feeding habits of this fish species for the better sustainable utilization.

Availability of data

Upon a reasonable request, all the data sets used in this manuscript are available in the corresponding author.

Author contribution

Agumassie Tesfahun and Sale Alebachew: Conceptualization, Methodology, Software: Sale Alebachew and Agumassie Tesfahun. Data curation, writing-original draft preparation. Agumassie Tesfahun Visualization and Investigation. Sale Alebachew, Supervision.: Agumassie Tesfahun Software, Validation.: Sale Alebachew and Agumassie Tesfahun: Writing- Reviewing and Editing,

Statement of ethical approval

Ethics approval and consent to participate permits for this research work issued by the Ethiopian Wildlife Conservation Authority (EWCA), Debre Tabor University, and Farta District Livestock and Fisheries Office. Fish sampling was conducted with respect to the Ethiopian Wildlife Conservation Authority Proclamation number 41/1993.

Acknowledgments

We are very grateful to thank the Guna Tana Integrated Field Research Development Centre and College of Natural and Computational Sciences Debre Tabor University for providing a car allowance. We also acknowledge the fishermen of Ribb Reservoir for the collection of fish samples. Furthermore, we are very grateful to all those who made the effort to obtain data for us.

Disclosure statement

The authors declare that there is no conflict of interest.

Additional information

Funding

There is no direct fund for this research

Notes on contributors

Agumassie Tesfahun

Agumassie Tesfahun) has received his MSc Degree in Fisheries, Limnology, and Aquatic-eco-toxicology from Hawassa University, Ethiopia. At the moment, he is the Assistant Professor at the Department of Biology, Debre Tabor University, Ethiopia. Moreover, he has been working on fisheries biology of the most commercially important fish species from Ribb Reservoir Tana basin, Ethiopia, and aquaculture establishment (earthen pond system) from Fogera District, South Gondar, Ethiopia.

Sale Alebachew

Sale Alebachew) has awarded his MSc Degree in Animal production from Debre Markos University, Ethiopia. Now, he is a lecturer and researcher at the Department of Animal Sciences, Debre Tabor University, Ethiopia. Moreover, he has been working on fisheries biology of the most commercially important fish species from Ribb Reservoir Tana basin, Ethiopia, and aquaculture establishment (earthen pond system) from Fogera District, South Gondar, Ethiopia.