Feeding pattern of the African big barb Labeobarbus intermedius (Rüppel, 1836) (Pisces: Cyprinidae) in Ribb Reservoir, Lake Tana Sub-basin, Ethiopia

Abstract

The purpose of this study was to investigate the feeding patten of Labeobarbus intermedius at Ribb Reservoir (Lake Tana Sub-basin, Ethiopia). A total of 498 specimens were collected using gillnets. Of these, 302 (60.6%) fish had different foods, while 196 (39.4%) were empty. The fish were dissected, their stomachs were removed, and stomach contents were preserved in a 5% formalin solution. Frequency of occurrence, volumetric method, index of preponderance, and geometric importance index were used to study the feeding pattern. Detritus was the primary food, accounting for 61.6% of the guts by volume. Mud, zooplankton, and phytoplankton were the second most important food items comprising 24.4%, 3.9%, and 2.8% by volume respectively. Feeding pattern differed with the size classes of fish. The smallest fish (<16.5 cm Fork length) consumed detritus, mud, and insects comprised 64.2%, 18.7%, and 7.3% by volume, respectively. The adult fish (>17.0 cm FL) majorly consumed detritus followed by mud. The importance of mud increases when the fish grows more except (>31.5 cm FL) size class. The proportions of food items of L. intermedius significantly varied with respect to the dry and wet seasons. Detritus and mud were mostly consumed in the dry season, contributing to 47.8% and 32.4% by volume, respectively. Detritus was the most preferred food item during the wet season, contributing 97.2% of the total guts and 75.6% by volume. Generally, L. intermedius feeding pattern depending on size classes and seasonal variations this may be linked to food availability in the reservoir.

Public Interest Statement

Lake Tana supports nine migratory spawner large cyprinid fishes. Of these, Labeobarbus intermedius migrates up to 60 km distance to reproduce. Ribb River is one of the breeding sites of this fish species. The African big barb Labeobarbus intermedius (family Cyprinidae) is the most commercially important fish species. Knowledge of their food and feeding profile is important to the establishment of aquaculture and for continuing fisheries management. The Ribb Reservoir is the basin with very important habitats of the endemic cyprinids fish fauna of the country. Knowledge of the diets and feeding habits of those fish species of the reservoir has not been systematically studied and explored. The necessity of conducting this study was to fill gaps in such biologically essential scientific information for the management and conservation of this dominant fish species in the reservoir.

Keywords:

• Feeding pattern
• frequency occurrence
• Labeobarbus intermedius
• Lake Tana sub-basin
• volumetric method

Reviewing Editor:

• Pedro González-Redondo, Associate Professor, University of Seville, Seville, Spain

Subjects:

• Agriculture & Environmental Sciences; Zoology; Marine & Aquatic Science

Introduction

Ethiopia is said to be the water tower in East Africa with many lakes (natural and manmade), rivers, and a chain of wetlands collectively provide the potential to capture fisheries (Awoke, 2015). The coverage of lakes, human-made waters, and flood plains are 13,637 km² while rivers cover about 8,065 km of the country (Tesfaye & Wolff, 2014). Ethiopia has a high fish production potential estimated at about 94,500 tons per year from lentic and lotic water bodies (Tesfaye & Wolff, 2014). The Ribb Reservoir is newly established and capture fishery is expected the livelihood of the community. Anteneh et al. (2023) documented that the actual fish production from the reservoirs of Ethiopia is estimated as 8,059 tons per year which is very low. L. intermedius (Pisces: Cyprinidae) (Ruppell, 1836) is a commonly found fish species in Kenya (Northern) part and the utmost parts of Ethiopian waters (Gebremedhin et al., 2019; Shewit et al., 2017; Awoke, 2015). This fish species is most commonly found in the Ethiopian Rift Valley system, Abay Basin, and Baro-Akobo Basin and is majorly found in the Lake Tana Basin (Awoke, 2015; Vijverberg et al., 2012; Sibbing & Nagelkerke, 2000).

Labeobarbus intermedius is one of Ethiopian capture fisheries’ most commercially important fish species (Anteneh et al., 2012; Dejen et al., 2017). It is also edible fish from the Ribb Reservoir. Labeobarbus intermedius has a versatile feeding pattern and also characterized by a generalist as well as opportunistic omnivorous feeding behavior (Dadebo et al., 2013). Labeobarbus intermedius feeds on a variety of food items such as detritus, macrophytes, zooplankton, insects, molluscs, fish eggs, fish prey, benthic macroinvertebrates, nematodes, sand grain, mud, fish scale gastropods, plant seeds, and phytoplankton (Teshome et al., 2023; Bekele, 2020; Tesfahun & Temesgen, 2018; Nagelkerke et al., 2015; Engdaw, 2014; Dadebo et al., 2013; Deribe et al., 2011; Desta et al., 2006; De Graaf, 2003). Food is one of the most important biological factors that affect the lives of all organisms (Teshome et al., 2023). The diet composition of the fish varies considerably within a wide range of temporal, spatial conditions, and environmental factors (Teshome et al., 2023; Nagelkerke et al., 2015). The major factors that influence the diet composition of L. intermedius are fish size, maturity, environmental condition, season (water level), depth, latitude and habitat types (Dadebo et al., 2013; Anteneh et al., 2012). In aquatic systems, where the water level in lakes and reservoirs have been known to fluctuate, the quality and abundance of the food items vary significantly over a time (Tesfahun & Temesgen, 2018). A particular characteristic of L. intermedius is that the increase in size during their ontogeny is correlated with changes in food quality and quantity in aquatic systems and growth varies according to the food availability in the environment (Wakjira, 2013). The ontogenetic dietary shift of fish is due to the productivity of the water and habitat differences (Tesfahun & Temesgen, 2018; Teshome et al., 2023). The seasonal variation in the diet of the fish is due to the opportunistic habits of the fish which has the ability to shift from one food item to another food item. Besides, the seasonal variation of the food and feeding habits of fish is also due to the seasonal production difference of the prey in the water bodies (Engdaw, 2014). The study of feeding pattern of freshwater fish species is a subject of continuous research (Tesfahun & Temesgen, 2018).

Because it is important for the continuous of an effective management program on capture fishery and fish culture (Shalloof & Khalifa, 2009). In addition to this, studies on food and feeding habits of fish empower the credentials of the trophic relations found in the aquatic ecology, understanding diet composition, structure, and stability of food webs in the ecosystem (Otieno et al., 2014). Furthermore, information on food and feeding habits of fish are also important for fisheries management and aquaculture establishment (Adeyemi et al., 2009). Therefore, understanding its food and feeding habit is important for effective fish culture in a controlled condition. In the Ribb Reservoir, a few studies such as the effect of the dam on fish migration, zooplankton assessment, fish diversity assessment, and food and feeding habits of the Oreochromis niloticus (Alebachew et al., 2022; Tesfahun & Alebachew, 2023; Mequanent et al., 2021, 2022) have been studied, there has so far been no study on the feeding pattern of L. intermedius, despite its importance to the commercial fishery of the area. Such area-specific information on the feeding pattern of this commercially and ecologically important species is important for management to sustain fish resources. Therefore, the current study was conducted to study the feeding pattern of the African large cyprinid L. intermedius (Rüppel, 1836) from Ribb Reservoir, South Gondar, Ethiopia.

Materials and methods

Study area descriptions

Ribb Dam was built in the middle of two districts, Farta and Ebinat, the purpose for irrigation practices (Mequanent et al., 2022; Bezabih, 2021) (Figure 1). The Ribb reservoir is located in the northeast of Lake Tana Basin. 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 reservoir is situated at 11°59'0" N to 12°2'0"N Latitude and 38°0'0"E to 38°2'59"E Longitude (Figure 1). The Ribb Basin is experiencing a higher rate of irrigation, particularly in the Ribb Reservoir, which increases at an alarming rate from time to time (Mekuanent et al., 2021). The climate condition of the reservoir is categorized from May to July as the rainy season. The monthly rainfall varies from 65 mm in May to 411 mm in July. At the Ribb Reservoir, the mean and the minimum precipitation are about 1,400 mm and 1,200 mm annum-1 respectively. The weather condition of Ribb Reservoir is also categorized as Woina Dega (Ezezew, 2019). The annual temperature ranges from 22.5 °C- 26.2 °C (Mequanent et al., 2022). Blue-green algae like Microcystis are dominant on the littoral side of the reservoir (Tesfahun & Alebachew, 2023). Oreochromis niloticus, Clarias gariepinus, and Labeobarbus species were found in the reservoir (Mequanent et al., 2022). Fourteen species of zooplankton were inhabited in the reservoir.

Figure 1. Study area map shows sampling sites at the Ribb Reservoir, Ethiopia.

Fish sampling, and morphometric measurements

The data collection was performed once per month. Four hundred ninety-eight fish samples were collected during the dry and the wet seasons from February to April 2021 and June to July 2021, respectively. Single filament gillnets were applied with a mesh size of 4, 5, 6, 8, 10, and 12 cm and 25 m in length. Gillnets were set in the evening time (7:00 p.m.) and collected the catch of the following day (7:30 a.m.). Hereafter, the morphometric measurements such as total length (TL), fork length (FL), and standard length (SL) were measured by means of a measuring board of the nearest 0.1 cm. The weight of the fish (TW) was also measured via a sensitive balance with a sensitivity of 0.1 g.

Extraction of gut contents

After morphometric measurements, the fish were longitudinally (at the ventral side) dissected with a dissecting kit. Then, the whole gut contents of L. intermedius were removed via applied pressure to extract the contents of the guts. The gut contents were kept in a 5% formalin solution for further analysis. Lastly, all gut samples were labeled as (date of sampling, measurement of length and weight, habitats of sampling, fish species, etc.) and transported to the Debre Tabor University for the gut content analysis.

Gut content analysis

During the laboratory study, the volume of the food items were measured using a measuring cylinder. Following this, the preserved foods were placed on the Petridish and identified (Hyslop, 1980). So, the microscopic prey types were examined via the XSZ-70DN dissection microscope model and the ST-30-2L stereo microscope (100–400 magnification), and the largest prey was determined without the aid of the microscope. The identified food types were grouped to the lowest taxonomic level by using descriptions, keys, and literature (Vuuren et al., 2008; Carling et al., 2004). The food items were determined by relative measures of prey quantities (RMPQ) (Hyslop, 1980). The relative importance of each prey type in the total food content of the gut was studied based on percentage frequency (%Q) and percentage contribution (%V) (Assis, 1996).

Frequency of occurrence

The number of gut specimens contain one or more food types is stated in percentages of all food-contained guts. The role of each food item category in the total stomach contents was analyzed by the percentage frequency of occurrence (%Q) (Bowen, 1983; Hyslop, 1980).

The frequency of occurrence was estimated as: (1) $$\mathrm{\%}\mathrm{\text{Oi}}=\frac{\mathrm{\text{ni}}}{\mathrm{n}}\mathrm{x}100$$(1)

where %Oi = frequency of occurrence of the i food item in the sample; ni = number of guts in which the i item is found; n = total number of food contained guts in the sample (Bowen, 1983).

Volumetric analysis

Food types identified in the gut contents were categorized into different taxonomic classes. Then, water displaced in each category of foods is measured in partially filled graduated cylinders and represents a percentage of total stomach volume (Bowen, 1983).

The volumetric analysis method (%Vi) was estimated as: (2) $$\text{\%\hspace{0.33em}Vi\hspace{0.33em}}=\frac{\text{Volume \hspace{0.33em}of\hspace{0.33em} particular\hspace{0.33em} food\hspace{0.33em} item}\hspace{0.17em}\text{\hspace{0.33em}present\hspace{0.33em} in}\hspace{0.17em}\text{\hspace{0.33em}all \hspace{0.33em}samples}}{\text{Volume \hspace{0.33em}of\hspace{0.33em} all\hspace{0.33em} food}\hspace{0.17em}\text{\hspace{0.33em}items\hspace{0.33em} in}\hspace{0.17em}\text{\hspace{0.33em}all \hspace{0.33em}samples}}\text{\hspace{0.33em}x\hspace{0.33em}}100$$(2)

where %Vi is the percentage of volume.

The frequency of occurrence (%Qi) reflects evidence of the proportion of fish guts that includes specific prey (Hyslop, 1980). The mean volume of each food type is measured visually compared to all the food types existing in the guts.

Gut content analysis parameters such as frequency of occurrence (%Qi) and volumetric percentage (%Vi, ml) were used to determine the index of food preponderance (PIi) and geometric importance of index (GIIi).

To measure the relative importance of each food item the index of preponderance (IoPi) was calculated as (Tomojiri et al., 2019). (3) $$\mathrm{\text{IoPi}}=\left(\mathrm{\%}\mathrm{\text{Vi}}\right)\left(\mathrm{\%}\mathrm{\text{Oi}}\right)$$(3)

where Qi is the frequency of occurrence of species i and %Vi is the percent composition by volume of species i. To understand comparisons among species, IoPi was converted into percent IoPi (%IoPi). Besides, to evaluate the relative importance of food items and species-level dietary changes, the Geometric Index of Importance (GIIi) (Assis, 1996) was measured as: (4) $$\mathrm{\text{GIIi}}=\frac{(\sum \mathrm{\text{RMPQi}})}{\surd \mathrm{n}}$$(4)

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

Data analysis

Gut content analysis and feeding profiles in relation to season and size class of L. intermedius in the Ribb Reservoir were studied using the percentage of volumetric contribution, frequency occurrence, preponderance importance index (IoP), and geometric importance of index (GIIi) of the five size classes (<16.5, 17.0–21.5, 22.0–26.0, 26.2–31.0 and >31.5 cm FL), and seasons (the dry and the wet) of the year (Matunguru et al., 2023; Tomojiri et al., 2019). Frequency of occurrence (%O), volumetric contributions (%V), and other descriptive statistics were analyzed by Microsoft Excel 2016. SPSS version 20 was used to investigate the diet composition and feeding habits variations within the seasons and the size classes of the L. intermedius via a Chi-square test (χ²) at a 95% confidence level.

Results

Feeding pattern of Labeobarbus intermedius

A total of 498 samples of L. intermedius were collected in this study. From these samples, 302 (60.6%) guts had different foods, while 196 (39.4%) were empty guts. The gut content of L. intermedius consisted of diversified prey types such as mud, detritus, zooplankton, phytoplankton, insects, ostracods, sand particles, and macrophytes (Table 1). Detritus (%O = 98.7 and %V = 61.6) was the most ingested food type followed by mud (%O = 79.8 and %V = 24.4), zooplankton, and phytoplankton in the gut contents of L. intermedius. However, insects, sand particles, ostracods, and macrophytes were occasionally consumed in the fish diet. The percentage of the geometric importance index (%GIIi) showed that detritus was the major prey type ingested by the fish (Figure 2 and Table 1). In the same way, based on this index, mud, zooplankton, and phytoplankton, were the second most significant prey types in L. intermedius diet, while sand grains, macrophytes, ostracods, and insects were irregularly consumed.

Figure 2. The mean relative geometric index importance (%GIIi) in the diet of L. intermedius (n = 302) from Ribb Reservoir (DT- detritus; MD-mud, PT–Phytoplankton; ZP–Zooplankton; DT– detritus; SAG-sand grain; and OC-ostracod. Dotted vertical lines separate the different grades of favorite food items).

Table 1. Proportions of different food items in the diet of L. intermedius (n = 302) in Ribb Reservoir, Ethiopia.

Food items	%Oi	%Vi	%IoP	%GIIi
Zooplankton	100	3.9	4.5	39.2
Cladocera	64.2	2.7	2.0	25.3
Copepod	19.2	1.2	0.3	7.7
Rotifer	2.0	0.06	0.01	0.8
Phytoplankton	98.3	2.8	3.2	38.2
Blue-green algae	67.9	1.3	1.0	26.1
Green algae	52.9	0.4	0.3	20.1
Euglenoids	57.9	1.1	0.7	22.3
Detritus	98.7	61.6	69.5	60.5
Insects	17.5	1.5	0.3	7.2
Ostracods	3.3	1.0	0.03	1.6
Sand particles	20.9	1.2	0.3	8.3
Macrophytes	4.7	0.3	0.02	1.9
Mud	79.8	24.4	22.3	39.3

Percentage frequency of occurrence (%Qi), percentage of volumetric contribution (%Vi), percentage index of preponderance (%IoP) and Geometric Index of Importance (GIIi).

Zooplankton (Rotifers, Copepods, and Cladocera) jointly occurred in 100% of the total food consumed and comprised 3.9% of the total volume in the total guts. Phytoplankton (blue-green algae, green algae, and euglenoids) collectively contributed 98.3% of the total guts and accounted for 2.8% of the total volume. The preponderance index (%IoP) of prey types such as detritus, mud, zooplankton, and phytoplankton were 69.5, 22.3, 4.5, and 3.2 respectively. Detritus (%GIIi = 60.5%) was the main preferred food type in the gut of the fish. Mud (%GIIi = 39.3%), phytoplankton (%GIIi = 38.2%), and zooplankton (%GIIi = 39.2%) showed the second preferred food types in the diet. In contrast, the other food items such as insects (%PI = 0.3%; %GIIi = 7.2%), ostracods (%PI = 0.03%; %GIIi = 1.6%), sand particles (%PI = 0.3%; %GIIi = 8.3%), and macrophytes (%PI = 0.02%; %GIIi = 1.9%) contributed less in the diet of L. intermedius (Table 1 and Figure 2).

Feeding pattern with respect to the size class

In this study, similar food types were studied in the guts of all size classes. However, the proportions of different food items were significantly varied in relation to the size classes (χ², p < 0.05). Detritus was the principal significant food type followed by mud and insects for the size class below 16.5 cm (FL) (Figure 3). Their equivalent volumetric contributions were 64.2%, 18.7%, and 7.3% of the total volume in the gut compositions of this size class respectively. However, food items such as phytoplankton (%V = 4.6) and zooplankton (%V = 4.9) had a minor contribution to the diet of the fish of this size class.

Figure 3. The size class variation in the diet of L. intermedius in Ribb Reservoir, Ethiopia.

Detritus was the most frequently consumed food type for the size class of 17.0–21.5 cm (FL). It constituted 68.7% of the total volume of the guts. In addition, mud (%V = 17.9) was the next preferred food item in this size class. While, remaining food items such as zooplankton (%V = 5.2), phytoplankton (%V = 3.1), and insects (%V = 1.0) had minor volumetric importance in the diet of the fish.

Detritus was the most important food item for the size class ranging between 22.0 and 26.0 cm (FL). It accounted for 47.1% of the total volume in the diet of this size class. Mud was the second significant food item in this size class and its volumetric contributions belong to 39.1% of the total volume. However, food items including phytoplankton (%V = 2.6) and zooplankton (%V = 4.3) had less importance to the diet of the fish this size class.

Detritus was the most important food item for the size class of 26.2–31.0 cm (FL). It constituted 55.8% of the total volume of the diet. Mud was the next consumed food item in this size class and its volumetric contribution was 33.0%. However, other food items such as zooplankton (%V = 3.1) and phytoplankton (%V = 3.2) contributed less importance to the diet of the fish this size class.

Detritus was by far the most ingested food type for the size class of >31.5 cm (FL). It comprised 71.9% of the total volume of diets. Mud (%V = 9.6) was a moderately ingested food item in this size class. However, other food items such as zooplankton (%V = 3.0), phytoplankton (%V = 3.1), and insects (%V = 2.4) were consumed in the smallest amount in the diet of the fish in this size class. In this study, detritus was the most preferred food type followed by mud in the guts of L. intermedius in all size classes. However, their relative proportion is significantly varied among the size classes (χ², p < 0.05) (Figure 3).

Seasonal variations in the feeding pattern of Labeobarbus intermedius

The seasonal variations in the guts of L. intermedius were shown in (Table 2) from Ribb Reservoir. From the present study, the results showed that notable seasonal differences in the diet of the fish were noted (χ², p < 0.05). During the dry season, detritus (%IoP = 51.8 and %GIIi = 52.2) was the most significant food item contributing to 98.3% of the total guts, and accounting for 47.8% of the total volume. Mud (%IoP = 34.9 and %GIIi = 46.5), insects (%IoP = 6.4 and %GIIi = 29.2), and zooplankton (%IoP = 4.5 and GIIi%=32.9) were secondary food items in the diet of the fish. They occurred in 97.8%, 73.9% and 87.3% of the total guts respectively and their volumetric contributions were 32.4%, 7.9%, and 4.7% by volume respectively (Table 2). However, food types such as phytoplankton (%V = 1.2) (%IoP = 1.0 and %GIIi = 27.3), sand particles (%V = 2.3) (%IoP = 0.8 and %GIIi = 11.8), ostracod (%V = 0.5) (%IoP = 0.1 and %GIIi = 6.3), and macrophyte (%V = 1.7) (%IoP = 0.4 and %GIIi = 8.4) had less contribution to the diet of the fish.

Table 2. The mean relative importance of food items in the diet of L. intermedius (n = 229) during the dry season in Ribb Reservoir, Ethiopia.

Food items	%Oi		%Vi		%IoP		%GIIi
	Dry	Wet	Dry	Wet	Dry	Wet	Dry	Wet
Detritus	98.3	97.2	47.8	75.6	51.8	92.9	52.2	72.0
Zooplankton	87.3	13.7	4.7	1.8	4.5	0.3	32.9	6.5
Cladocera	83.9	–	3.6	–	3.3	–	31.3	–
Copepod	20.4	13.1	1.4	1.8	0.3	0.3	7.8	6.2
Rotifer	1.7	–	0.08	–	0.00	–	0.6	–
Phytoplankton	75.2	24.6	1.2	1.3	1.0	0.4	27.3	10.8
Blue-green algae	34.8	6.8	0.6	0.2	0.25	0.01	12.6	2.9
Green algae	30.4	–	0.3	–	0.1	–	11.0	–
Euglenoids	10	17.8	0.3	1.1	0.03	0.2	3.7	7.9
Mud	97.8	13.7	32.4	1.1	34.9	0.2	46.5	6.2
Insect	73.9	41.4	7.9	11.4	6.4	6.0	29.2	22.0
Ostracods	17	–	0.5	–	0.1	–	6.3	–
Sand particles	30.4	–	2.3	–	0.8	–	11.7	–
Macrophytes	21.7	5.4	1.7	3.7	0.4	0.3	8.4	3.8

Percentage frequency of occurrence (%Qi), percentage of volumetric contribution (%Vi), percentage index of preponderance (%IoP) and Geometric Index of Importance (GIIi).

During the wet season, detritus was by far the most significant food category in the guts of L. intermedius. It occurred for 97.2% of the total guts and its corresponding volumetric contribution was 75.6% of the total volume. Insects (%O = 41.4 and %V = 11.4) were also the second important prey type in the diet of the fish. Moreover, the %IoP and %GIIi indices also revealed that detritus (%IoP = 92.9 and %GIIi = 72.0) was by far the most preferred food item in the diet during the wet season. Insects (%IoP = 6.0 and %GIIi = 22.0) were the second most important food items in the diet of the fish. However, zooplankton (%IoP = 0.3 and %GIIi = 6.5), phytoplankton (%IoP = 0.4 and %GIIi = 10.4), macrophyte (%IoP = 0.3 and %GIIi = 3.8), and mud (%IoP = 0.2 and %GIIi = 6.2) and their respective volumetric contributions were 1.8%, 1.3%, 3.7%, and 1.1% respectivelyhave less contribution to the diet of the fish during the wet season (Table 2).

Discussion

Gut contents and feeding profiles of Labeobarbus intermedius

In this study, L. intermedius was consumed several food types such as mud, detritus, zooplankton, phytoplankton, insects, ostracods, sand particles, and macrophytes. Of these aforementioned food types, detritus (%O = 98.7 and %V = 61.6) was the most ingested food type followed by mud (%O = 79.8 and %V = 24.4), zooplankton, and phytoplankton in the gut contents of L. intermedius. But, insects, sand particles, ostracods, and macrophytes were rarely investigated in the diet of the fish. Mud was mainly fed by Labeobarbus forskalii from the upper Blue Nile River (Teshome et al., 2023). It was also reported that detritus is the main food source of L. intermedius in Ethiopian Lakes in Tana (Engdaw, 2014; De Graaf, 2003), Koka (Dadebo et al., 2013), in the upper Blue Nile River (Teshome et al., 2023), Tekeze Reservoir (Gebru, 2020) and in Ethiopian water bodies (Tesfahun & Temesgen, 2018). The ingestion of detritus as the main food item in the diet of fish is an indicator of the reduction of high-quality foods. Because detritus has low nutritional quality (Bowen, 1979). From this study, the contribution of insects was low. However, insects were primarily ingested by Labeobarbus nedgia in the upper Blue Nile River (Teshome et al., 2023). This might be due to a lack of aquatic macrophytes because they serve as surface film assemblage of benthic macroinvertebrates. In contrast to the present study, insects, gastropods, fish prey, and fish scales were as primarily consumed in the diet of L. intermedius in Lake Hawassa (Bekele, 2020; Desta et al., 2006). Mud (%GIIi = 39.3%), was the second important food type in the diet of the fish. This may be due to human impacts such as high sediment loads, high irrigation activities, and uncontrolled use of land around the Ribb reservoir buffer area (Tesfahun & Alebachew, 2023). Additionally, the reason may be variations in physicochemical water quality parameters, water levels fluctuate when water resources are needed for irrigation and water extraction purposes (Tesfahun & Temesgen, 2018). The food and feeding habits of L. intermedius rely on seasonal differences, prey availability, habitat variation, and fish size classes (Teshome et al., 2023; Tesfahun & Temesgen, 2018; Desta et al., 2006; Admassu & Dadebo, 1997).

Feeding pattern with respect to the size class

Fish prefer the type of prey that fits into their mouths and the type of prey that their stomach can digest (Otieno et al., 2014). As fish grow, the stomach enlarges and their digestive system develops (Wakil et al., 2014). From this study, L. intermedius did not show considerable ontogenetic dietary shift, where small and sized fish consumed similar food types. This may be due to lack of small sized fish (fingerlings) in the specimen during data collection. However, the proportions of different food items were significantly varied in relation to the size classes (χ², p < 0.05). Detritus was the most important food type followed by mud and insects for the size class < 16.5 cm (FL). In agreement to this study, detritus and insects were the most important food items for the smallest fish in Lake Koka (Dadebo et al., 2013). In contrast to this study, the smallest fish mainly fed on phytoplankton followed by detritus in the upper Blue Nile River (Teshome et al., 2023). Furthermore, L. intermedius did not show ontogenetic dietary shifts from Lakes Hawassa (Bekele, 2020) and Tana (Engdaw, 2014). Similar preferability of this fish shows can be due to the development of similar morphological and physiological features and the feeding of similar foods. Fish change their feeding behavior from one food item to other with the high-energy demands as they grow (Dadebo, 2009). The growing energy demand of the fish cannot be met by feeding only on a particular food item. This enables them to shift their feeding behavior from consuming a particular food item to generalist behavior (Bekele, 2020). The ontogenetic diet shift of the fish also correlate with the differences in morphology and physiological conditions, the type of water, and the habitat of the fish (Engdaw, 2014). For instance, the smallest fishes inhabiting in the littoral region and feeds invertebrates and zooplankton (Dadebo, 2009). As their size increases, they swim in open water and feed large food items once their morphological and physiological features developed.

Detritus was by far the most important food type for the larger size class of >31.5 cm (FL). It comprised 71.9% of the total volume of diet. A comparable study was conducted from Lake Koka that detritus was the main food source for larger-sized fish (Dadebo et al., 2013). However, in the upper Blue Nile River the larger fish species mainly fed on phytoplankton (Teshome et al., 2023). The ontogenetic dietary shift may be due to the productivity of the water and habitat differences of the fish.

Seasonal variations in the feeding pattern of Labeobarbus intermedius

From the present study, the results showed that notable seasonal differences in the proportion of food items the fish were noted (χ², p < 0.05). Throughout the dry season, detritus (%IoP = 51.8 and %GIIi = 52.2) was the most significant food item contributing to 98.3% of the total guts, and accounted for 47.8% of the total volume. In the same way, Lake Koka (Dadebo et al., 2013) documented that detritus followed by insects were the main food item in the diet of the fish during the dry season. However, phytoplankton was reported as the main food item during the dry season from the upper Blue Nile River (Teshome et al., 2023). This seasonal difference in the diet of L. intermedius may be caused by the variation in the physicochemical parameters of the reservoir. Additionally, this may be due to the high turbidity of the reservoir may be due to the water abstraction and water release for irrigation purposes (personal observation). The seasonal variation in the diet of the fish may be due to the opportunistic habits of the fish which has the ability of shifting from one food item to another food item. Besides, the seasonal variation of the food and feeding habits of L. intermedius also may be due to the seasonal production difference of the preys in the reservoir. Furthermore, the feeding habits of fish correlates with the availability of preys in the reservoir.

During the wet season, detritus was by far the most important food item in the guts of L. intermedius. It occurred in97.2% of the total guts and its corresponding volumetric contribution was 75.6%. Insects (%O = 41.4 and %V = 11.4) were also the second important prey type in the diet of the fish. The importance of detritus and insects as major foods were also reported in Gilgel Gibe Reservoir (Wakjira, 2013), Lake Koka (Dadebo et al., 2013), and upper Blue Nile River (Teshome et al., 2023). Similarly, L. nedgia primarily fed insects in the upper Blue Nile River (Teshome et al., 2023). In contrast to this study, L. forskalii mainly consumed phytoplankton followed by mud from the upper Blue Nile River (Teshome et al., 2023). During the wet season, detritus was far the most important food item, the source of detritus could be probably floods that introduced fragments of plant materials in the lake. Overall, the current results showed that seasonality affects L. intermedius’ proportion of food and prey types in the Ribb reservoir.

Conclusions

This result indicated a preference for detritus as the main food, followed by mud, phytoplankton, and zooplankton. Detritus followed by mud was the most important food item of all sizes, but its proportions were different. Smaller fish and larger fish were fed similar food types. Prey species such as detritus and zooplankton and phytoplankton were the most preferred foods in the dry season. While insects and detritus were the most important food in L. intermedius’ diet during the wet season. L. intermedius revealed omnivorous feeding pattern in their diet from the Ribb Reservoir. Generally, feeding pattern of fish are affected by habitat differences, seasons, and the size class of fish in the reservoir.

Author’s contribution

Agumassie Tesfahun: Conceptualization, Methodology, Software: 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.

Acknowledgments

The authors highly acknowledged the research directorate and research vice president’s office of Debre Tabor University for their financial support. We are also very thankful to the Guna Tana Integrated Field Research Development Centre Debre Tabor University for helping with the financial provision and vehicle support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Availability of data

All the data sets used in this manuscript are accessible to the corresponding author via a reasonable request.

Additional information

Notes on contributors

Agumassie Tesfahun

Agumassie Tesfahun pursued his MSc Degree in Fisheries, Limnology, and Aquatic-eco-toxicology from Hawassa University, Ethiopia. Now, he is the Assistant Professor at the Department of Biology, Debre Tabor University, Ethiopia. In addition, he has been working in community service and research 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 was received his MSc Degree in Animal Production from Debre Markos University, Ethiopia. Nowadays, 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.