2.1. Fish Sampling and Laboratory Processes

Wels catfish samples were caught monthly from Sıddıklı Dam Lake between September 2015 and August 2016. Trammel and gill nets were used to capture fish. As a result, a total of 200 S. glanis samples were obtained. The total and standard lengths of the samples were measured with an accuracy of ± 1 mm, and their weights were weighed with an accuracy of ± 0.01 g.

The digestive system of wels catfish, from the esophagus to the anus, was cut with scissors and preserved in a 4% formaldehyde solution [19, 20, 21]. The samples were cleared of fat, mesentery fragments, and other foreign matter and tissue fragments [22].

The stomach samples that were ready for examination were weighed using a scale with a precision of ± 0.01 g, and the food types in the stomach contents were grouped to be identified. The weight values of each food group and empty stomachs were weighed with an accuracy of ± 0.01 g. Identification of macro-level organisms in the stomach content was made at the lowest possible taxonomic level using a binocular microscope. Various sources were used during these diagnoses [23, 24].

2.2. General Diet Analysis

2.3. Seasonal Analysis

The seasonal diet analyses were made separately Winter (December, January, and February), Spring (March, April, and May), Summer (June, July, and August) and Autumn (September, October, and November). To analyze the seasonal pattern, numerical percentage (N%), weight percentage (W%), frequency of occurrence (FO%), and index relative importance percentage (IRI%) were used. The overlap (similarity) in the food composition of wels catfish according to seasons was determined by Schoener’s Overlap Index (C_xy) [33]. Schoener’s Overlap Index value varies between 0 and 1. If this value is greater than 0.60, it means that the food of the two groups is similar, and if it is smaller, it means that there is no similarity between the food of the two groups [34].

2.4. Analysis by Age Groups

Stomach content analyzes of wels catfish were examined separately according to age. Age data was obtained from [35]. The ages of the samples ranged from 1 to 11 years, and there were samples for each age group. FI% and VI% were used to determine how the rates of full and empty stomachs changed according to age groups. Additionally, IRI% was used to determine the pattern of importance of food types present in the species’ diet with age.

2.5. Analysis by Length Groups

The total lengths of individuals ranged from 20.41 to 101.0 cm. In order to make the analyses according to length understandable, the samples were classified as small (20.41–47.0 cm), medium (47.1–74.0 cm), and large (74.1–101.0 cm) sized individuals. FI% and VI% were used to assess how the proportions of full and empty stomachs varied across different length groups. Additionally, IRI% was utilized to identify the pattern of importance of different food types in the species’ diet in relation to length. The similarity of food composition of S. glanis according to length groups was determined using Schoener’s Overlap Index (C_xy) [33].

2.6. Prey Selection

3.1. General Food Composition

The digestive tract contents of 200 wels catfish caught from Sıddıklı Dam Lake were examined. It was determined that 47% (N = 94) of the individuals examined had an empty stomach and 53% (N = 106) had a full stomach. Numerical percentage (N%), percentage weight (W%), frequency of occurrence (FO%), IRI and percentage value (IRI%) of food types are presented in Table 1.

A total of 18 food types belonging to fish, amphibian, mammal, mollusca, crustacean, and benthic invertebrate groups were detected in the stomach contents of wels catfish. The main food of the species is T. tinca (IRI% = 78.9) from fish groups. When the general food composition was examined, it was determined that the species showed piscivorous feeding characteristics. Additionally, no cannibalism was observed in the sample.

3.2. Seasonal Feeding Characteristics

Stomach FI, which is an indicator of feeding activity, varied between seasons. The average stomach FI reached its highest value (0.827) in the Winter. On the other hand, the lowest value (0.480) was determined in the Autumn. When empty stomach percentages are examined, the lowest percentage of empty stomach was recorded in Winter (14.3%), and the highest value was recorded in Summer (48.9%) (Figure 1).

It was determined that the most important food item for wels catfish in Spring is T. tinca (IRI% = 67.74). This was followed by fish remains (IRI% = 9.9%) and C. carpio (IRI% = 7.1) food types. Additionally, Ranidae (IRI% = 6.1), A. boyeri (IRI% = 5.5), unidentified fish (IRI% = 2.5), Isopoda (IRI% = 1.1), and Astacus spp. (IRI% = 0.4) are other foods consumed (Table 2).

In Summer, T. tinca (IRI% = 75.84) constituted the main food item of the species. A. boyeri (IRI% = 14.3) was determined as the second important food type. Additionally, fish remains IRI% = 2.62), Astacus spp. IRI% = 1.94), Ranidae IRI% = 1.57), Potamon spp. IRI% = 1.46), unidentified fish IRI% = 0.73), Dipter larva IRI% = 0.3), Squalius seyhanensis IRI% = 0.29), C. carpio IRI% = 0.23), Gammarus spp. IRI% = 0.11), A. escherichii IRI% = 0.11), Dipter pupa IRI% = 0.08), Odonata IRI% = 0.032), Bivalvia IRI% = 0.03), and S. glanis (IRI% = 0.028) consumed (Table 2).

T. tinca (IRI% = 83.9) was determined as the most consumed food type in the Autumn season. This was followed by fish remains (IRI% = 8.4) and unidentified fish (IRI% = 6.4) groups. In addition, it was determined that the importance of Rodentia (IRI% = 1.1) and C. carpio (IRI% = 0.2) foods in the diet was quite low (Table 2).

T. tinca (IRI% = 63.0) constituted the main food item of wels catfish in Winter. Fish remains (IRI% = 15.6) were identified as the second important nutrient. In addition, Rodentia (IRI% = 10.7), unidentified fish (IRI% = 8.4) and C. carpio (IRI% = 2.3) were consumed at low rates (Table 2).

According to Schoener overlap index (C_xy) values, it was determined that the diets of wels catfish were similar in all seasons except Summer and Winter (C_xy = 0.596251). The highest similarity was detected in the Autumn and Winter diets (C_xy = 0.936877) (Table 3).

3.3. Feeding Features According to Age Groups

It has been determined that the stomach FI varies between age groups. It was determined that the average stomach FI value was highest (3.8) in 1-year-old fish and lowest (0.23) in 3-year-old fish. When empty stomach percentages are examined, the lowest empty stomach percentage was found in 10-year-old fish (0.0%), and the highest empty stomach percentage was found in 3-year-old fish (75%). Both stomach FI and empty stomach percentage generally tended to decrease with advancing age (Figure 2).

When the stomach contents are examined in terms of age groups, the main food of 1-, 2-, and 3-year-old wels catfish is A. boyeri (IRI%>75). In the sample, the primary food of individuals aged 4, 5, 6, 7, 9, 10, and 11 years was T. tinca (4th age IRI% = 91.3, 5th age IRI% = 80.4, 6th age IRI% = 85.9, 7th age IRI% = 47.2, 9th year IRI% = 81.1, 10th year IRI% = 80.2, 11th year IRI% = 53.8). However, in 8-year-old fish, the IRI% value of T. tinca decreased to 28.1 and the IRI% value of Rodentia increased to 26.8, and these two species are considered the main food in this age group (Table 4).

In general, T. tinca was consumed extensively by fish aged 4 years and above but was not consumed by individuals aged 1, 2 and 3 years. Depending on the increase in age, a decrease in the importance of Odonata, Bivalvia, Isopoda, Gammarus spp., dipter pupae, and dipter larvae in the diet was determined. It has also been determined that wels catfish feed entirely on fish and vertebrates after the age of 4. Finally, the importance of C. carpio in the diet, which has a higher body size than other forage fish, increased at the ages of 10 and 11 (Table 4).

3.4. Feeding Features According to Length Groups

Stomach FI varied between length groups. It was determined that the average stomach FI was at the highest level (0.82) in small-sized (<47.1 cm) individuals, while it was at the lowest level (0.64) in medium-sized (47.1–74.0 cm) individuals. The lowest value of empty stomach percentage (44%) was found in individuals with large length (>74.1 cm), while the highest value (54%) was found in the small length group (<47.1 cm). The most intense feeding activity was detected in samples from the small-size group (Figure 3).

When stomach contents were examined according to size groups, A. boyeri (IRI% = 73.3) constituted the most important food of small-sized wels catfish. The secondary important food type was T. Tinca with an IRI value of 15.7%. The main food type of wels catfish in both medium and large size groups was determined as T. tinca (medium size, IRI% = 83.8; large size, IRI% = 68.2) (Table 5).

Benthic invertebrates were consumed mostly by small-sized wels catfish. However, they are also consumed, to a small extent, by medium-sized wels catfish. In addition, the type of food consumed in small-sized wels catfish was less diverse compared to other size groups. Common carp have never been consumed by small-sized individuals, and their importance in the diet increases as the size increases. Food diversity in medium-sized wels catfish increased and all fish species present in the Sıddıklı dam lake were included in the food composition. Ranidae group food species are included in the diet of medium-sized wels catfish and have increased their importance in the diet of large-sized wels catfish. Astacus spp. was consumed more by small-sized wels catfish and had no effect on the diet in the medium and large-sized wels catfish. Large-sized wels catfish have a completely vertebrate diet and mostly fish are consumed as a food type (Table 5).

According to Schoener overlap index (C_xy) values, similarity was detected only in the diets of medium- and large-sized wels catfish (C_xy = 0.8534). As a result of other pairwise comparisons between length groups, no similarity (C < 0.60) was determined in terms of food consumed (Table 6).

3.5. Food Preference of Wels Catfish

When Pearre’s Selectivity index values were examined in small-sized (20.1–47.0 cm) samples, A. boyeri (V_a = 0.39518) was the most preferred food type by individuals in this length group, and the selectivity index value was statistically significant (x² = 31.2336, p  < 0.001). Similarly, T. tinca (V_a = 0.062060975) was selected as positive, but the selectivity index value was not statistically significant (x² = 0.77031, p  > 0.05). No other fish species were consumed as food by individuals of the small-size group. For this reason, the selectivity index values of other species were accepted as 0, and it was concluded that they were not selected in any way (Figure 4).

When the selectivity index values in individuals in the medium size (47.1–74.0 cm) group were examined, T. tinca, whose density was relatively low in the lake, was positively selected and this selectivity was determined to be statistically significant (V_a = 0.63564, x² = 80.8073, p  < 0.001). On the other hand, C. carpio (V_a = −0.21325, x² = 9.09487, p  < 0.005), A. boyeri (V_a = −0.28563, x² = 16.3165, p  < 0.001), S. glanis (V_a = −0.232837732, x² = 10.84268185, p  < 0.001), and S. seyhanensis (V_a = −0.14694, x² = 4.311816, p  < 0.025) species were selected negatively, and these selections are also statistically significant. Similarly, A. escherichii (V_a = −0.01221, x² = 0.02982, p  > 0.05) was negatively selected by wels catfish, but it was determined that this situation did not have any statistical significance (Figure 5).

Large-sized (74.1–101.0 cm) wels catfish positively selected the T. tinca (V_a = 0.666495, x² = 88.4307, p  < 0.001) species, which they consume as their main food, and this trend was statistically significant. A. boyeri (V_a = −0.35267, x² = 24.8749, p  < 0.001), the most abundant species of the Sıddıklı dam lake, was significantly negatively selected by S. glanis individuals. Similarly, C. carpio (V_a = −0.10258, x² = 2.10444, p  > 0.05) was also selected as negative, but this trend was not statistically significant (Figure 6).

4.1. Seasonal Feeding Characteristics

As a result of examining the empty stomach rate according to seasons, it was found that it was highest in Summer (48.9%) and lowest in Winter (14.3%). It is thought that the high percentage of empty stomach in Summer is due to the rapid digestion of food as a result of the increase in metabolic rate in parallel with the increase in temperature. An increase in the empty stomach rate started from the Spring season and reached its highest level in the Summer season. During these seasons, which include the breeding period of the species, feeding may have slowed down due to reproductive activities. It is thought that the decrease in the empty stomach rate in Winter is due to the acceleration of feeding activities in the prebreeding period and the length of the dark period in this season, as it is a nocturnal hunting species. Long night hours increase the probability of wels catfish, which are nocturnal hunters, to find food and therefore cause the empty stomach rate to be low. Like the findings of this study, Alp [40] found the highest percentage of empty stomachs in the study he conducted in Menzelet Dam Lake in the Spring season, which is the breeding season of the species.

There are differences in the food types of wels catfish depending on the seasons. However, the main food type did not differ between seasons. There is richness in terms of food diversity in the Spring and Summer seasons. However, diversity decreased significantly in the Winter and Autumn seasons. As a matter of fact, Doğan Bora and Gül [38] reported in their study in Hirfanlı Dam Lake that the food diversity of wels catfish reached its highest value in the Summer season, whereas there was a narrowing in the food range in the Autumn and Winter seasons. In his research conducted in Menzelet Dam Lake, Alp [40] reported that the food diversity in the stomach contents of wels catfish was more intense in the Spring and Summer seasons compared to the Autumn and Winter seasons. The findings of this study are compatible with studies in different habitats in terms of seasonal feeding density. Since the density of the foods consumed by the species in habitats varies seasonally, it is normal to see food diversity in their stomach contents.

Özdemir [59] stated that with the decrease in water temperature, species need less feeding. In this case, the decrease in food diversity may be related to this phenomenon, as the species that make up the food of wels catfish will also engage in less foraging activities. Also, fish species breeding, etc. Migration in different seasons for many reasons also contributes to this situation. Many species present in the Spring and Summer diet are not consumed in other seasons. In particular, freshwater crayfish were not consumed in the Winter and Autumn diets. Similarly, Czarnecki et al. [54] reported in their study in Lake Goreckie that crayfish species were consumed abundantly in the Summer and Spring diets of wels catfish but were not included in the Winter and Autumn diets.

Rodentia consumed in Autumn and Winter seasons was not consumed in other seasons. This situation can be explained by the decrease in the abundance of fish species in the Autumn and Winter seasons and the wels catfish experiencing food shortages and therefore turning to food types that are not available in other seasons.

4.2. Feeding Features According to Age Groups

According to age groups, T. tinca was the main food for wels catfish aged 4 and above, while A. boyeri was consumed as the main food for 1, 2 and 3 year olds. The importance of benthic invertebrates in the diet decreased with increasing age. It has been determined that wels catfish only prey on fish, crustaceans, and other vertebrates after the age of 4. In addition, C. carpio has taken its place in the food groups since the age of 5. Tanyolaç and Karabatak [60] reported that wels catfish predated carp fish in Lake Mogan starting from the age of 5. It is thought that this is due to the fact that carp fish have higher body heights than other forage fish.

4.3. Feeding Features According to Length Groups

Fish constituted the most consumed food type in the diet of individuals belonging to the small, medium, and large-size groups. Benthic invertebrates were consumed only by small- and medium-sized individuals. As the size of wels catfish increased, the body height of the food intake also increased. So much so that carp fish, which were never included in the diet of small-sized individuals, partially appeared in medium-sized individuals, and their importance in the diet of large-sized individuals increased. Wysujack and Wysujack and Mehner [39] reported that as the size of wels catfish in Lake Feldberger Haussee increases, the size of food consumed also increases, and that the importance of Perca fluviatilis individuals in the diet, which have a larger body length compared to other fish. Carol et al. [8] reported that wels catfish up to 30 cm in Flix and Riba-roja reservoirs feed mostly on benthic invertebrates and plant materials, and then show an ontogenetic feeding feature, with freshwater crayfish as their main food. The same researchers also stated that the ontogenetic change was in fish in the Susqueda and Sau reservoirs. Pavlović et al. [51] determined that benthic invertebrates were consumed in individuals smaller than 60 cm in length, whereas they were not consumed at all in individuals larger than 60 cm, and the importance of fish in the diet increased even more. Didenko and Gurbyk [58] reported that as the size of wels catfish increases in the Kaniv reservoir, the importance of fish in the diet increases and the importance of invertebrates decreases. In addition, the same researchers reported that as the size of wels catfish increased, the size of the fish consumed also increased. Looking at the existing literature, it is seen that there are ontogenetic changes in the diet of wels catfish, and as the size of wels catfish increases, the size of the forage fish they feed on also increases. These two main results coincide with the findings of this study. On the other hand, the faunistic features of the habitats and the abundance of fish affect the main food consumed and ontogenetic nutritional change. For this reason, it is thought that it will not be possible to make comparisons between habitats based on these criteria.

4.4. Food Preference of Wels Catfish

According to Pearre’s selectivity index values, small-sized fish preferred A. boyeri, while medium and large-sized fish preferred T. tinca. While medium-sized individuals avoided consuming C. carpio, A. boyeri, S. glanis, and S. seyhanensis species, they positively selected T. tinca species. It is thought that the negative selection of carp fish is due to their body height or the presence of spiny rays on their dorsal fins. In large-sized wels catfish, A. boyeri and C. carpio species were negatively selected, whereas T. tinca was positively selected and consumed. Negative selection of carp fish is not statistically significant. In this case, it can be said that the increase in the length of wels catfish increases the selectivity as well as the importance of species in the diet, especially those with relatively large body lengths, such as carp. In their experimental study, Adámek et al. [61] found that wels catfish showed negative selection towards Rutilus rutilus and Pseudorasbora parva species, slightly avoided consuming Carassius auratus gibelio, Leuciscus cephalus, and Aspius aspius species, and positively selected Leucaspius delineatus, Scardinius erythrophthalmus, and Rhodeus sericeus. Also, they reported that wels catfish mostly preferred to eat small-sized foods. Zaikov et al. [62], in their research conducted under controlled conditions, found that S. glanis individuals positively selected P. parva species in their feeding activities, while they negatively selected carp fish. On the other hand, there are also studies where wels catfish do not show any food preferences [40].

In conclusion, this study provides a comprehensive view of how the feeding habits of wels catfish (S. glanis) vary according to seasonal changes, age groups, and individual sizes. These findings allow us to understand how this species adapts to its environment and biological conditions and modifies its feeding strategies. The feeding habits of wels catfish offer critical insights for understanding the species’ role in the ecosystem. Its opportunistic and diverse feeding behavior enables this species to adapt to different ecological conditions, which in turn affects population dynamics. From a management perspective, these findings provide important information for habitat management and fishing regulations for the wels catfish. For instance, considering the increased feeding activities during the breeding season and Winter months, it might be advisable to reduce fishing pressure during these periods. Additionally, the feeding preferences of different age and size groups are crucial factors to consider for population health and sustainable fishing practices. These details help us better understand the behaviour and feeding habits of wels catfish in their natural habitats. Furthermore, the applicability of this information in fisheries management and conservation strategies can contribute to the balance and sustainability of the species within the ecosystem.