Food is a complex organic matter that is rich in energy needed for movement and reproduction; it is also a material for growth and replacement of worn-out tissues and for the welfare or wellbeing of every living organism [1]. All fish need energy, which must be obtained from their food sources for growth, movement, and reproduction [2, 3]. Understanding the food and feeding habits of fish is useful to all scientists who are concerned with any aspect of fisheries. The study of the dietary habits of fish based on stomach content analysis is widely used in fish biology and ecology to indicate the position of the species within a food web and to provide information on the contribution of different prey items to the diet [4]. It also helps in understanding food consumption, feeding and assimilation rate, catabolism, habitat, segregation [5], defining predator-prey relationships, estimation of trophic level, and the creation of trophic models as a tool for understanding complex ecosystems [6]. Fisheries ecologists have conducted studies on fisheries ecology to evaluate the dietary composition and the food habit of a fish species in order to provide useful information for fisheries management, aquaculture valuation, and habitat protection. Based on fish habits, fish can be classified as herbivores, carnivores, omnivores, parasites, scavengers, and detritivores [7].

Fish are sources of food for human beings and other animals (consumption) rich in protein and vitamins, especially vitamin A (retinol). Statistics have shown that fish account for more than forty percent of the protein diet of two-thirds of the global population [8]. It is unfortunate that the protein requirement of most African countries still grossly outweighs its supply. The silver catfish, Chrysichthys nigrodigitatus (Lecepede, 1803) commonly known as “okpo ocha” by Omambala of Anambra State, is among the dominant African commercial fishes of high economic value and widely serves as food for human consumption in west Africa [9]. In its natural habitat, the fish can grow to an asymptomatic length of 50 cm total length (TL) and exhibit a sexual dimorphism where the males, when fully grown, usually have a broader head, which they use to dig out their breeding nest in their native habitat [10]. In general, the species show a gray/blue/silvery color except for the ventral surface, which is white. The fins are grayish-pink, the adipose fin is black, and the lips and the barbels are pink. Chrysichthys nigrodigitatus exhibits a pointed snout slightly longer than or equal to the width of the mouth, and the pre-maxillary tooth plate width makes up 20–30% of the head length. They possess large mouths and eyes and relatively small barbels, which usually relate to the habitat where they reside, being clear water where large barbels for feeling for food are not needed, hence the large eyes for hunting prey [11].

Chrysichthys species have been found to be a typical example of fish without a strict feeding habit. It is regarded as an omnivore because of its ability to use just any material present in its environment (Royle). Chrysichthys nigrodigitatus is a prominent number of the Claroteids and occurs abundantly in some Anambra State Rivers (Otuocha River), Akwa Ibom State waters (Imo River, Utaewa), Qua Iboe River (Ibeno), and Cross River (Itu) in Nigeria and several West African countries [12]. In Anambra State in particular and Nigeria in general, Chrysichthys nigrodigitatus is a highly valued food fish, a source of income for the artisanal fishers, and contributes to the internally generated revenue of the state as patronage comes from other states [13]. The estuarine water of the Otuocha River in Anambra State, Nigeria, is one of the major hydrographic features in Anambra State. It is characterized by two seasons: dry and wet seasons [14]. The natural habitats offer a great diversity of organisms that are used as food by fish, which differ in sizes (microscopic and macroscopic) and taxonomy groups. The anthropogenic activities going on within the Otuocha aquatic ecosystem, such as oil exploration and exploitation, fishing activities, and other agricultural and industrial activities, might cause changes in the availability and abundance of aquatic flora and fauna that serve as food to fish, including C. nigrodigitatus [15]. Hence this research on the food and feeding habits of C. nigrodigitatus becomes imperative as information from the biology and food studies of this species can be used during species selection in fish culture. This is particularly useful in polyculture because proper selection of fish with different feeding habits will prevent or significantly reduce competition during culture. Information on the biochemical composition and energy levels of the ingested food and its absorption in the alimentary canal provides baseline data useful in artificial feed formulation for fish during their culture. The aim of the study is to investigate the food and feeding habits of Chrysichthys nigrodigitatus in the Otuocha Fishing River.

Study Area

Otuocha, originally known as Otu Okenku Di Nmba Aguleri, is a town east of the Niger River and headquarters of the Anambra East Local Government Area of Anambra State, Nigeria. It lies between latitudes 6°10′ and 7°20 ′ north and longitude 7°40′ east of the River Niger. The River is a major tributary to the River Niger and flows into the Niger River from the eastern part of Nigeria. It is marked by a rainy season (April-September/October) and a dry season (October/November-March). From December to February, the basin is influenced by the harmatan, but its effect is not marked. The vegetation in the basin is guinea savanna, but the lentic water bodies are often fringed with macrophytes like Pteocarpus spp., Dalbergia spp., Jussiaea spp., Vossia cuspidate, Pennisetum spp., Cybodon spp., and in some areas, Raphia hookeri.

Figure 1: Map of Anambra River Basin with Arrow Showing Sampling Location.

Sample Collection

The species of fish used is Chrysichthys nigrodigitatus. The fish specimens were procured from fishermen at a landing site in the Otuocha axis of the Omambala River, Anambra State.

Collection and Identification of Fish Samples

Collection of samples was done after capture. Samples were collected in transparent plastic containers, and formaldehyde of 5% (formalin) was added to maintain the freshness of the fish and to avoid them from going into the rigor mortis period at the commercial landing station of the fishermen. Various sizes of live Chrysichthys nigrodigitatus ranging from small to large were sorted out and used for the study. The study lasted for a period of four months (February to May, 2023) and sample collection was done on a monthly basis.

Fish Measurement

Morphometric parameters were measured on each sample. Morphometric parameters were measured on each sample with the aid of a meter rule and weighing balance. These parameters were measured in centimeters (cm) and grams (g) and recorded on data collection sheets.

Stomach Content Analysis

The fish were dissected and the stomach contents were emptied into Petri dishes, to which 10% saline water was added to disperse the contents. The food items were sorted into categories and identified to species level and were possible using identification keys with the help of a laboratory scientist and microscope. Stomach contents were analyzed using two (2) methods: frequency of occurrence and numerical methods.

Frequency of Occurrence

The number of stomachs in which each food item occurred was sorted out and expressed as a percentage of the total number of fish stomachs examined.

Fi = 100 ni/n

Where:

Fi: frequency of occurrence of the food in the sample

ni: number of stomachs in which the i item is found

n: number of stomachs with food in the sample.

Analysis of Stomach Fullness

To get percentage of frequency (%F), the formula was used:

% Frequency = No. of stomach content in a particular food item/Total no. of stomach examined × 100/1

For full stomach: 10/16 × 100/1 = 62.5%

For ¾ stomach: 2/16 × 100/1 = 12.5%

For ½ stomach: 3/16 × 100/1 = 18.75%

For an empty stomach: 1/16 x 100/1 = 6.25%

Numerical Method

The number of individuals of each food item was counted in each stomach; sum up to give the total of each kind, then the grand total of items was calculated and expressed as a percentage of the overall items found in each stomach; this was done according to the method described by Crisp et al. [16].

Statistical Analysis

The data collected during this experiment were presented in tables, frequency, and percentage. It was further subjected to correlation analysis using the SPSS computer package (version 20), and groups were considered significant at p < 0.05.

Table 1 shows the stomach fullness of Chrysichthys nigrodigitatus. Out of 16 samples of Chrysichthys nigrodigitatus that were sampled, 10 had a full stomach, 2 had 3/4 stomach full, 3 had 1/2 stomach full, and 1 had an empty stomach. Table 2 shows the diet of C. nigrodigitatus in the Otuocha axis of the Omambala River during the sampling months. The results revealed that diatoms (phytoplankton) and sandy particles constituted a large proportion of the diets, while molluscs and fish particles constituted the lowest in all sampling weeks. Table 3 shows the total composition of food items of C. nigrodigitatus in Otuocha Axis of Omambala River. The results indicated that diatoms (phytoplankton) had the highest composition (27.69%), which was closely followed by sand particles (22.69%), while molluscs and ants constituted the lowest (3.46%) composition of the sampled food items.

Table 1: Analysis of Stomach Fullness of C. nigrodigitatus in Otuocha Axis of Omambala River.

Table 2: Diet of C. nigrodigitatus in Otuocha Axis of Omambala River.

Table 3: Total Composition of Food Items of C. nigrodigitatus in Otuocha Axis of Omambala River.

The result indicates that out of the 16 samples of Chrysichthys nigrodigitatus that were sampled with weights ranging from 308g to 31g, 10 had a full stomach, 2 had ¾ stomach full, 3 had ½ stomach full, and 1 had an empty stomach. The total length of the fish varied from 40.50 cm to 14.00 cm, while the standard length ranges from 36.20 cm to 11.50 cm. Some major items were found in the stomach of Chrysichthys nigrodigitatus, which include algae, diatoms (phytoplankton), zooplanktons, and fish parts; while annelids, crustaceans, ant parts, sand, and unidentified materials were minor items. The feeding habit of any fish is often reflected in its general morphology. The general morphology of C. nigrodigitatus suggests that it is an efficient bottom feeder, although stomach content may prove otherwise. The variety of food items contained in the stomachs of fish often reflects the ability of fish to obtain food from different locations. Chrysichthys nigrodigitatus in the lower Omambala River feeds on a variety of food items of which algae, diatoms, zooplanktons, fish parts, and insect parts were of primary importance and other food items that probably serve as supplementary are mollusc, and mud(sand) indicating that C. nigrodigitatus is an omnivorous feeder. Similar results have been obtained for C. nigrodigitatus by Udoh and Ekpo [17]. This is an indication of flexibility in trophic level, which gives the fish an ecological advantage to feed effectively on different categories of diet based on the availability of food items. Similar observations had earlier been recorded by George and Atakpa [18]. Moreover, Yem [19], who studied the food habit of the catfish C. nigrodigitatus in Kainji Lake, Nigeria, reported that the species ecological advantage enables it to switch from one food category to another in response to fluctuation in abundance and also enables species to utilize a number of different food items effectively.

The composition of food consumed by C. nigrodigitatus showed that algae were consistently included and dominant in the diet. Similar results have been obtained by Lawal et al. [20], who studied the morphometry and diet of C. nigrodigitatus in Epe Lagoon, Nigeria, and concluded that the quantity of algae consumed by C. nigrodigitatus is not surprising due to the fact that algae contribute largely to the diet of most fish. It should, however, be noted that algae in the tropics may occur at any period of the year depending on the favorability of the physical and chemical conditions of the particular ecosystem. The occurrence of fish parts in the diet of C. nigrodigitatus showed its carnivorous nature. This agrees with Ayoola and Abotti [21] that the dietary components for Gymnarchus niloticus, a typical carnivorous fish, are whole fish and fish parts. This is so because the intestines of carnivorous fish have evolved for processing a highly digestible, nutrient-dense diet that is high in protein and low in carbohydrate. The occurrence of fish parts, mollusks, and insects in the diet of C. nigrodigitatus implies that the species is an opportunistic feeder in the water column.

The inclusion of mud in the diet of C. nigrodigitatus may be attributed to accidental ingestion along with other food items. Similarly, mud/sand particles serve as food for some fish species [22]. This is because it contains amino acids and other products of decay, which together with saprophytic bacteria and other protozoan microorganisms constitute a rich source of crude protein, as discussed by Welcome [23]. The high percentage inclusion of unidentified mass in the diet of C. nigrodigitatus could be as a result of posthumus digestion of food items from the time of capture to the time of landing of the species. The larger size classes of C. nigrodigitatus consumed a larger variety of food items than any other small food items. Food items that constituted a major compound for the larger size class were present in the environment, whereas other food items preferred by the smaller size class were constantly fluctuating in abundance throughout the study period. Similar observations have been recorded by Nwani et al. [24] for Campylomormyrus tamandua in Anambra River, Nigeria. This is so because as fish increase in individual size, the size of their diet items also increases.

The results of the present study have clearly shown that the most important categories of food items consumed by Chrysichthys nigrodigitatus in the Otuocha axis of the Omambala River were diatoms, algae, zooplanktons, and small fishes. Foods of minor importance were annelids, crustaceans, ant parts, sand, and unidentified materials. The gut content analysis suggested that C. nigrodigitatus is an omnivorous fish in its feeding habits. The silver catfish is omnivorous, as it feeds on a variety of food items, from algae to animals’ materials. Unidentified mass formed a greater portion of the stomach fullness. They relied on the same food items at the juvenile and adult stages, which were dominant throughout the study period. This made them live in relatively good condition. This information is useful in the formation and production of diet for the culture of this species. In order to uphold the United Nations Chapter (1992) that all species and habitats should be safeguarded to the extent that is technically, economically, and politically feasible, the information obtained is important parameters for conservation of the population dynamics, which could promote the species in the wild.

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