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Article

The Feeding Habits and Length–Weight Relationships of the Invasive Black Bullhead Ameiurus melas (Rafinesque, 1820) in the Gruža Reservoir, Central Serbia

by
Milena Radenković
1,*,
Nataša Kojadinović
1,
Aleksandra Milošković
2,
Tijana Veličković
1,3,
Milica Stojković Piperac
4,
Aleksa Cvetković
1 and
Vladica Simić
1
1
Department of Biology and Ecology, Faculty of Science, University of Kragujevac, Radoja Domanovića 12, 34000 Kragujevac, Serbia
2
Department of Science, Institute for Information Technologies Kragujevac, University of Kragujevac, Liceja Kneževine Srbije 1A, 34000 Kragujevac, Serbia
3
Department of Organisms and Ecosystems Research, National Institute of Biology, Večna pot 121, 1000 Ljubljana, Slovenia
4
Department of Biology and Ecology, Faculty of Sciences and Mathematics, University of Niš, Višegradska 33, 18000 Niš, Serbia
*
Author to whom correspondence should be addressed.
Fishes 2026, 11(3), 144; https://doi.org/10.3390/fishes11030144
Submission received: 31 December 2025 / Revised: 23 February 2026 / Accepted: 24 February 2026 / Published: 27 February 2026
(This article belongs to the Special Issue Trophic Ecology of Freshwater and Marine Fish Species)
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Abstract

Invasive freshwater fishes often display high trophic plasticity, facilitating their establishment and persistence in novel environments. This study examined the feeding ecology, growth patterns, and trophic role of the invasive black bullhead Ameiurus melas in the eutrophic Gruža Reservoir (Central Serbia), with emphasis on ontogenetic dietary shifts and potential ecological impact. Diet composition was analyzed in 103 individuals representing three age classes using traditional diet indices, Costello graphical analysis, self-organizing maps (SOMs), and the Indicator Value (IndVal). Chironomidae, Protozoa, and fish eggs were the dominant dietary components across age classes, although their relative importance varied ontogenetically. Younger individuals exhibited a more generalized feeding strategy, whereas older fish showed increased specialization on benthic prey. SOM-IndVal analyses revealed prey taxa associated with specific feeding patterns at the individual level, identifying Diptera as an indicator prey not detected by population-level indices. Length–weight relationships indicated negative allometric growth (b < 3) across all age classes, consistent with a diet dominated by low-energy prey. These feeding patterns may contribute to altered benthic processes, reduced native fish recruitment, and reinforcement of eutrophic conditions. Overall, the results highlight the pronounced trophic flexibility and ecological plasticity of A. melas, supporting its invasive success in degraded freshwater ecosystems.
Keywords:
diet composition; invasive species; freshwater ecosystem; self-organizing maps; IndVal
Key Contribution: This study demonstrates that the invasive black bullhead Ameiurus melas exhibits pronounced trophic plasticity and ontogenetic dietary shifts in a eutrophic reservoir, with individual-level feeding patterns revealed by SOM-IndVal analyses that are not detectable using traditional diet indices. By linking diet composition with negative allometric growth, this study highlights how low-energy prey exploitation may underpin the invasive success of A. melas in degraded freshwater ecosystems.

1. Introduction

Freshwater ecosystems harbour disproportionately high biodiversity per unit area compared to marine and terrestrial systems, yet they are among the most heavily impacted by biological invasions [1]. When non-native species become invasive, they pose serious threats to biodiversity, ecosystem functioning, and ecosystem services, often inducing profound alterations in native faunal assemblages [2]. Through mechanisms such as competition, habitat degradation, or pathogen transmission, invasive species can drive declines or local extinction of native taxa and disrupt trophic dynamics at the community level [3,4,5].
Invasive species usually have a larger trophic niche than native species, which indicates higher trophic plasticity and adaptability to newly invaded ecosystems, making them strong competitors [6]. Globally, the scale and impacts of biological invasions are increasing [7]. Biological invasions of freshwater ecosystems have a variety of known and potential effects on community structure and ecological function [1]. A comprehensive understanding of the ecological and behavioural characteristics of invasive species is a critical prerequisite for developing or refining effective population control strategies [5]. Non-native fish fauna in Serbia are dominated by species of North American, Asian, and Ponto-Caspian origin, with a total of 23 non-native species currently recorded [8]. Among them, several species, including black bullhead Ameiurus melas (Rafinesque, 1820), brown bullhead Ameiurus nebulosus (Lesueur, 1819), pumpkinseed Lepomis gibbosus (Linnaeus, 1758), and Prussian carp Carassius gibelio (Bloch, 1782), exhibit particularly wide distributions, occupying more than 50% of the national territory [9]. Despite legal restrictions on the introduction of non-native species—such as the ban on stocking in many Balkan countries (enacted in Serbia in 2000)—dispersal through major hydrological pathways, particularly the Danube River as a key southern invasive corridor, remains an ongoing vector of spread [10].
Ameiurus melas is a North American ictalurid catfish that has become one of the most widespread non-native freshwater fish species in Europe [11]. In the Gruža Reservoir, A. melas has been regularly recorded in recent years and is considered an established non-native species, occurring across multiple age classes and indicating successful population persistence [12,13]. It typically inhabits lentic or slow-flowing, turbid, and nutrient-rich lakes and reservoirs with muddy substrates and abundant macrophyte cover [11,14]. The species exhibits high ecological tolerance to low oxygen levels and temperature fluctuations, rapid growth, tolerance to pollution, early maturation, and parental care—traits that contribute to its invasive success [10,15,16]. As an opportunistic benthophagous feeder, A. melas consumes a wide variety of prey, including insect larvae, crustaceans, molluscs, detritus, and small fish [17,18], enabling flexible trophic adaptation to local food resources. Once established, it often becomes dominant in fish assemblages, potentially affecting the structure and trophic balance of invaded communities [4].
Feeding ecology is a central component of fish biology because diet reflects both the ecological role of a species and its interactions within the food web. Trophic position, prey preferences, and feeding flexibility provide insight into the mechanisms through which a species acquires energy and influences ecosystem structure [19,20]. Invasive fishes, in particular, may alter food web dynamics by shifting energy pathways or increasing competition with native species through dietary overlap. Therefore, analysis of diet composition is essential not only for understanding species-specific feeding strategies but also for evaluating their ecological impact in newly colonized environments [2,21].
Length–weight relationships (LWRs) are fundamental tools in fish biology, providing insights into growth patterns, somatic condition, and energy allocation [22]. The parameters of the LWR (slope b and intercept a) are widely applied to infer species-specific ecological traits and to detect temporal or spatial variations in growth associated with environmental or trophic factors [23]. Complementing these metrics, Fulton’s condition factor (K) [24] is among the most frequently used indices for evaluating the overall health and physiological status of fish in relation to their habitat conditions [25]. When interpreted with diet composition, these indicators offer a comprehensive framework for understanding how feeding strategies shape growth efficiency—a perspective particularly valuable in studies of invasive species adapting to new ecosystems.
Accordingly, this study investigates whether non-native fish species pose a competitive threat to native fauna, with particular emphasis on interspecific trophic interactions. To address this, we analyzed the feeding patterns of non-native fish in the eutrophic Gruža Reservoir as a proxy for potential dietary overlap and resource competition. Previous studies in the Gruža Reservoir have documented the feeding ecology of native fish species [26,27,28,29], providing a useful basis for understanding trophic relationships within this system. Building on this background, the present study focuses on the invasive A. melas, aiming to elucidate its ecological impact relative to native fauna.
The specific objectives of this study were to (i) analyze the diet composition of A. melas in the Gruža Reservoir and compare it with previously published data on native species; (ii) use length–weight relationship and Fulton’s condition factor as supportive, complementary metrics in the interpretation of feeding ecology; and (iii) discuss the implications of the species’ feeding ecology for potential trophic competition and ecosystem functioning in an eutrophic reservoir.

2. Materials and Methods

2.1. Study Area and Fish Sampling

The Gruža Reservoir is an artificial lake located in Central Serbia (43°55′18.84″ N, 20°41′20.04″ E, Figure 1), created in 1984 by damming the Gruža River to supply drinking water to the city of Kragujevac. With a surface area of 9.34 km2, an average depth of 6.5 m, and a maximum depth of 35 m, it represents the largest water body in Central Serbia and is predominantly lowland in nature [30]. Although primarily intended for water supply, the reservoir is also used for recreational activities and is subjected to notable anthropogenic influence from the surrounding agricultural landscape and the presence of unregulated settlements along the shoreline. Gruža is classified as a eutrophic reservoir [31], and its fish community is dominated by pikeperch Sander lucioperca (Linnaeus, 1758), C. gibelio, roach Rutilus rutilus (Linnaeus, 1758), and common bream Abramis brama (Linnaeus, 1758) [26,27]. The reservoir also supports a diverse assemblage of non-native species, among which silver carp Hypophthalmichthys molitrix (Valenciennes, 1844), bighead carp Hypophthalmichthys nobilis (Richardson, 1845), L. gibbosus, and A. melas are particularly prominent [32].
Fieldwork was carried out in July and August 2025. Specimens were obtained in cooperation with recreational anglers operating at multiple locations along the reservoir shoreline, in accordance with national fisheries regulations. As an invasive species, A. melas is exempt from harvest limits and closed seasons under the Law on the Protection and Sustainable Use of Fish Stocks of the Republic of Serbia [33], allowing for unrestricted collection for research purposes. Although angling is a commonly applied method for sampling invasive benthophagous fishes, we acknowledge that it may preferentially capture individuals actively foraging in nearshore habitats.
In the laboratory, each specimen (103 individuals in total) was measured to the nearest mm for total length (TL) and to the nearest g for body weight (W). Age was estimated using length–frequency analysis following the Bhattacharya method [34], as applied by Jaćimović [35]. After measurements were taken, the specimens were dissected, and their intestines were removed and preserved in 96% ethanol for further analysis under a stereomicroscope. Prey items were identified to the lowest possible taxonomic level, counted under the stereomicroscope, and subsequently stored in 70% ethanol for preservation.

2.2. Digestive Tract Content Analysis

To determine the importance of individual prey categories, diet composition was quantified using the Prominence Value (PV), calculated according to Hickley et al. [36] and Lorenzoni et al. [37]:
PV = %N√(%FO)
%PV = 100PV × ΣPV−1
where %FO is the frequency of occurrence, representing the proportion of digestive tracts containing a given food item relative to the total number of non-empty digestive tracts, and %N is the numerical percentage of each prey category relative to the total number of prey items [38].
To assess the feeding strategy of the species, we applied the graphical method proposed by Costello [39] and later modified by Amundsen et al. [40]. This approach plots prey-specific abundance (Pi) against frequency of occurrence (%FO) to distinguish between specialized and generalist feeding patterns. Prey-specific abundance was calculated as follows:
Pi = 100ΣSi × ΣSti−1
where Si represents the number of prey type i and Sti the total number of prey items in the digestive tracts containing prey i. In the resulting plot, prey types positioned toward the upper part of the graph indicate specialization, whereas those near the lower part reflect a generalist feeding strategy. To complement the Costello-Amundsen graphical method, dietary specialization was also assessed using Levins’ standardized niche breadth index, computed according to the following formulas [41,42]:
B = 1/Σpij2
BA = (B − 1)/(n − 1)
where B is Levins’ niche breadth index, BA is the standardized Levins’ index, pij is the proportion of prey category j used by group i, and n is the total number of prey categories. The standardized index BA ranges from 0 to 1, where values close to 0 indicate a narrow trophic niche (specialist feeding), whereas values approaching 1 indicate a broad trophic niche (generalist feeding) [42].

2.3. Length–Weight Relationship and Condition Factor

The length–weight relationship (LWR) of A. melas is described by the following equation:
W = aTLb
where W is the body weight (g), TL is the total lenght (mm), a is the intercept, and b is the allometric growth coefficient [43,44,45]. The parameters a and b were estimated by linear regression of log-transformed data:
logW = loga + blogTL.
Deviations of the coefficient b from the isometric value (b = 3) were tested following Froese [22], where b > 3 indicates positive allometric growth and b < 3 negative allometric growth. The correlation between the variables was evaluated through the coefficient of determination (r2) [22].
Fish condition was assessed using Fulton’s condition factor (K) [22,24], calculated as follows:
K = W/TL3 × 100
which provides an index of the overall well-being and energy reserves of individuals [43]. Higher K values typically indicate better nutritional status and favourable feeding conditions.

2.4. Statistical Analysis

Data derived from digestive tract analyses may be affected by noise, as fragmented or highly digested prey items are often difficult to identify, and the recorded composition does not necessarily reflect the actual ingested proportions [46]. Kohonen’s self-organizing map (SOM) [47], which is robust to noisy data [48,49], was therefore applied to identify patterns in digestive tract contents. This method is well suited for clustering and visualizing large, high-dimensional datasets and for exploring both linear and non-linear relationships among variables [50,51].
The SOM consisted of an input and an output layer composed of neurons [47,52]. The input dataset comprised a matrix of 98 columns, each representing a single digestive tract, and 18 rows corresponding to prey taxa. Digestive tracts containing exclusively detritus were excluded, as they did not provide sufficient information for prey-specific dietary pattern analysis. Relative abundance data were log-transformed (log (x + 1)), normalized, and scaled prior to analysis. During the learning process, the input data were iteratively introduced into the network, resulting in a two-dimensional grid of output neurons forming a codebook matrix, in which differences between neuron models increased with their mutual distance. Clusters of neurons on the trained SOM were identified using the k-means algorithm [53]. Because inappropriate map resolution may obscure or exaggerate data structure [54], the optimal map resolution was determined using the approaches proposed by Vesanto et al. [55] and Park et al. [56], while minimizing the occurrence of empty output neurons [50], based on these criteria, a 6 × 7 grid was selected. The SOM Toolbox provided grey-scale visualizations of associations between prey categories and SOM regions, although these visualizations were not used for statistical inference [57]. All analyses were performed using the SOM Toolbox implemented in Matlab ver. 6.1.0.450 (http://www.cis.hut.fi/projects/somtoolbox, accessed on 20 February 2026).
Because SOM is primarily a visualization tool and does not provide statistical inference, the Indicator Value (IndVal) [58] was applied to identify prey categories significantly associated with individual SOM clusters. IndVal for prey category i in the cluster j was calculated as the product of its relative abundance (Aij) and relative frequency of occurrence (Fij), according to the following equations:
Aij = mean massij/mean massi
Fij = N digestive tractsij/N digestive tractsj
IndValij = Aij × Fij × 100
Statistical significance was assessed using a Monte Carlo permutation test (100 permutations) implemented in PC-ORD statistical software (ver. 6.06) [59]. Prey categories with IndVal > 25 and with both relative abundance and frequency exceeding 50% were considered characteristic of a given cluster.

3. Results

3.1. Diet Composition

A total of 103 individuals of A. melas, ranging from 11.5 to 20.6 cm in total length, were examined to assess diet composition; mean total length (± SD) was 13.56 ± 0.81 cm for age class 1+, 15.82 ± 0.55 cm for age class 2+, and 17.73 ± 0.99 cm for age class 3+. Nineteen prey taxa were identified, showing pronounced variation in frequency of occurrence and numerical contribution among the three age classes. The relative abundance (%N), frequency of occurrence (%FO), and prominence value (%PV) of each prey category across age classes are presented in Table 1. Although 19 prey taxa were recorded overall, not all were present in each age class. Detritus was excluded from quantitative analysis because it consisted of highly degraded material that could not be reliably assigned to any specific prey category and was therefore recorded only as presence.
Of the recorded prey taxa, 11 occurred in all three age classes, whereas Rotifera and Hydrachnidia were found exclusively in the youngest age class (1+). Protozoa, Chironomidae, and fish eggs represented the dominant prey groups across all age classes, although their relative importance varied. In age class 1+, fish eggs were the most important prey item, followed by Protozoa and Chironomidae. In age class 2+, Protozoa predominated, while fish eggs and Chironomidae contributed to a lesser extent. In age class 3+, Chironomidae constituted the primary prey category, with lower contributions of fish eggs and Protozoa.
The Costello plot revealed distinct differences in feeding strategy among age classes (Figure 2). In age class 1+, A. melas exhibited a predominantly generalist feeding pattern. Chironomidae, Insecta, and Bryozoa occupied the lower region of the graph, indicating generalized consumption. Fish eggs, however, were positioned in the upper part of the plot and were classified as a specialist prey. Protozoa occupied an intermediate position (%FO = 54.54, Pi = 39.22), suggesting neither strict specialization nor generalization. Diptera showed high prey-specific abundance but low frequency of occurrence, indicating occasional specialization, while all remaining prey categories were rare.
In age class 2+, Chironomidae, Diptera, and Insecta were consumed as generalist prey, where Bryozoa displayed a specialist pattern. Fish eggs and Protozoa occupied intermediate position. Leptodora kindtii (Focke, 1844) and Coleoptera were characterized by high prey-specific abundance but low frequency of occurrence, indicating intensive consumption by a limited number of individuals.
In age class 3+, both fish eggs and Chironomidae formed distinct specialist prey categories. Bryozoa, Protozoa, Diptera, and Copepoda were consumed as generalist prey, occupying the central and lower regions of the graph and indicating consistent use across individuals of the oldest age class.
The patterns revealed by Costello plots were further supported by the standardized Levins’ niche breadeth index (BA). The highest BA was recorded in the 1+ age class (BA = 0.44), indicating the most generalized feeding strategy, whereas lower BA values in the 2+ (BA = 0.38) and 3+ (BA = 0.36) age classes suggest a gradual narrowing of the trophic niche with increasing age. This concordance between graphical and index-based approaches confirms an ontogenetic shift from a more generalized to a more specialized feeding strategy in A. melas.

3.2. Results of Length–Weight Relationships and Condition Factor

Length–weight relationships (LWRs) and Fulton’s condition factor (K) for all age classes and for the total sample are summarized in Table 2. All estimated allometric coefficients (b) were below 3, indicating negative allometric growth across age classes. When considering the total sample, the estimated b value was 2.660, further confirming an overall negative allometric growth pattern in the studied population. The b values ranged from 2.660 in age class 1+ to 2.868 in age class 3+, with the highest value observed in the oldest individuals. The LWR for the total sample showed a strong relationship between length and weight (r2 = 0.875), exceeding the values obtained for individual age classes. The strength of the LWR, expressed as r2, was highest in age class 1+ (0.758), followed by age class 3+ (0.689), whereas the weakest relationship was observed in age class 2+ (0.393).
Fulton’s condition factor showed relatively stable values among age classes, ranging from 1.185 to 1.238. The highest mean K was recorded in age class 2+, suggesting slightly better overall condition compared to the other age groups.

3.3. Results of Statistical Analysis

Three neuron clusters (I, II, and III) were identified in the SOM output network (Figure 3). Cluster I contained the highest number of neurons and samples, dominated by individuals from age class 3+, whereas age class 1+ was least represented. Cluster II included an intermediate number of neurons and samples and was dominated by individuals from age class 2+. Cluster III comprised the fewest neurons and samples, with equal representation of age classes 2+ and 3+.
Significant IndVal values were identified for four of the eighteen recorded prey categories, excluding detritus (Figure 4; Table 3). Two prey categories were significantly associated with clusters I and III, while one prey category characterized cluster II. Diptera and Chironomidae were significant indicator prey for cluster I. Chironomidae were also significant for cluster III. Protozoa were identified as the characteristic prey for cluster II and were present in all digestive tracts assigned to this cluster. Fish eggs were a significant indicator for cluster III and, unlike Chironomidae, occurred in the digestive tracts of all individuals within this cluster.

4. Discussion

4.1. Feeding Ecology and Ontogenetic Dietary Shifts

Invasive freshwater fishes often rely on flexible feeding strategies that facilitate their establishment and spread in newly colonized environments [11,60]. As a highly opportunistic species, A. melas is known to exhibit considerable trophic plasticity, allowing it to exploit a broad range of prey resources and adjust its diet according to local prey availability [6]. Invasive species are well known to restructure freshwater food webs [1,61], with feeding flexibility modifying trophic relationships, influencing community structure, and increasing interactions with native species [1].
The results of this study reflect these characteristics and provide new insights into how A. melas utilizes available food resources across different age classes in the Gruža Reservoir. Although the general food categories were similar across age classes, each age group exhibited distinct predominant prey items. Ontogenetic dietary shifts, a common phenomenon in freshwater fishes, typically arise from size-related changes in habitat use, prey accessibility, and trophic requirements [62,63]. In A. melas, the observed patterns indicate that feeding habits change not only between juvenile and adult stages but also continue to develop among older individuals as prey preferences are progressively adjusted. This was reflected in a transition from Protozoa and small invertebrates in younger fish toward larger and more energy-rich benthic prey in older age classes, consistent with previous findings. Leunda et al. [64] similarly observed ontogenetic dietary shifts, with Chironomidae remaining the primary prey across age classes, while secondary prey groups (e.g., microcrustaceans) were gradually replaced by larger invertebrates or fish as individual grew. Czeglédi et al. [65] further confirmed these trends and highlighted an increasing proportion of fish in the diet with body growth. In the present study, this ontogenetic progression was reflected in the dominance of fish eggs, Protozoa, and Chironomidae in the youngest age class (1+), a more balanced and transitional diet in age class 2+, and a marked specialization toward Chironomidae in age class 3+. Such increasing specialization on benthic prey likely enhances foraging efficiency in older individuals and may amplify their ecological impact on benthic communities within the Gruža Reservoir.
A comparable trend was noted by Leunda et al. [64], who observed that A. melas predominantly feeds on Chironomidae in running waters, while in lakes and reservoirs the diet shifts toward zooplankton crustaceans as the principal prey category. These findings support the view that prey dominance in A. melas is strongly shaped by habitat characteristics and local prey availability. Our results also revealed the presence of zooplankton in the 2+ and 3+ age classes, although their overall contribution was relatively low. Notably, Copepoda (%FO = 23.25) were more prominent in the oldest age class (3+), while Cladocera (%FO = 10.52) were primarily consumed by the 2+ age class.
Our findings regarding the occurrence of fish remains in the diet of A. melas are largely consistent with those of Leunda et al. [64], who reported piscivory in this species, generally at low proportions. This modest contribution may partly reflect methodological constraints, as soft fish tissues degrade rapidly during digestion and often leave few identifiable structures, resulting in their underrepresentation in gut-content data [66]. In addition, the prevalence of benthic and invertebrate prey in Gruža Reservoir may limit the actual availability of fish prey, further contributing to their low detected proportion. In the present study, fish prey was detected in all three examined age classes, but its overall contribution remained limited and was highest in the oldest age class (3+). In contrast, several studies have documented a stronger piscivorous tendency in A. melas, with fish constituting a dominant dietary component [18,67]. Similarly, Czeglédi et al. [65] identified fish as the dominant prey, followed by Chironomidae and Bivalvia. Although these prey categories were also recorded in our study, fish and Bivalvia occurred at lower proportions compared to Chironomidae. In contrast to the aforementioned findings, Ruiz-Navaro et al. [68] identified plant material as the dominant dietary component of A. melas, with a high frequency of occurrence, followed by Chironomidae. Fish prey was also recorded in their study, but with a comparatively lower overall contribution.
Because prey items in digestive tracts undergo varying degrees of digestion, dietary data often involve a trade-off between taxonomic resolution and information loss [69,70], which may introduce methodological bias [46]. In this context, self-organizing maps represent a valuable analytical tool for studies of fish feeding ecology, as they effectively handle complex, non-linear relationships among variables and accommodate both normally distributed and skewed data [46,57]. Despite their widespread application in biocenological research, the combined use of self-organizing maps and the IndVal approach remains relatively uncommon in studies of fish diet composition. Traditional diet indices such as frequency of occurrence or prominence value have been widely used for decades and provide a useful, population-level overview of feeding composition. However, these metrics inherently summarize dietary information across groups of individuals and may therefore mask prey items that are important only for specific subsets of the population. In contrast, self-organizing maps (SOMs), which are particularly well suited for complex, non-linear ecological data, allow dietary patterns to be explored at the level of individual digestive tracts and provide a more detailed overview of trophic structuring than linear ordination approaches [50,71,72,73].
In the present study, both traditional indices and the IndVal analysis consistently highlighted Protozoa, Chironomidae, and fish eggs as dominant dietary components of A. melas, confirming their overall importance across age classes. However, Diptera emerged as a significant prey category exclusively through the IndVal analysis. This indicates that, although not abundant or frequent enough across all individuals to be emphasized by traditional population-level indices, Diptera were strongly associated with a specific cluster of digestive tracts identified by the SOM. Such prey items may therefore be ecologically relevant at the level of individual feeding strategies, despite contributing little to overall dietary averages. By combining SOM clustering with the IndVal index, this approach increases the resolution and interpretability of dietary data by revealing prey-predator relationships that may remain obscured in population-level analyses [46,74]. This approach therefore bridges population-level summaries with individual-level feeding strategies, offering a more nuanced interpretation of trophic organization in invasive fish populations.
In addition to natural prey, A. melas consumed maize and wheat kernels—material introduced into the reservoir through angler baiting practices targeting common carp Cyprinus carpio Linnaeus, 1758 (these items were classified under detritus in the quantitative analysis because many kernels were partially digested and could not be reliably counted). Such anthropogenic food subsidies may artificially enhance the feeding opportunities of this invasive species, potentially strengthening its competitive advantage and adding to the cumulative pressures on native fish populations. Detritus represented a major component of the diet of A. melas in all examined age classes, with the 2+ group showing its consistent presence in every digestive tract analyzed. Although detritus is often abundant in freshwater habitats, it is generally considered a poor-quality food source with low nutritional value [75]. A similar pattern was reported by Leunda et al. [64], who found that A. melas frequently ingests detritus, likely as a consequence of its benthic foraging behaviour and the incidental intake of sediment while searching for invertebrate prey. The high occurrence of detritus in the diet observed in the present study therefore likely reflects the species’ feeding mode rather than a targeted preference [76].

4.2. Growth Patterns and Length–Weight Relationships

Available literature indicates that A. melas exhibits considerable geographic and environmental variability in LWRs, ranging from negative to strongly positive allometric growth. For example, Pedicillo et al. [77] reported a distinctly positive allometry, with b values significantly greater than 3, whereas negative allometric growth has been documented in non-native populations, particularly under altered or suboptimal environmental conditions [78]. At a broader scale, Copp et al. [79] reported a mean slope of b = 3.03 across European non-native populations, suggesting near-isometric growth on average while also emphasizing substantial local deviations, a pattern also observed in Serbian waters where seasonal and size-specific differences influence growth trajectories [35]. In the present study, LWRs were applied as complementary descriptors of population growth patterns, rather than as primary analytical tools, and were interpreted cautiously in the context of sample size and age-class subdivision. Accordingly, the LWR calculated for the full sample provides a more robust description of population-level growth patterns, whereas age-specific estimates should be interpreted with greater caution. Within this framework, negative allometric growth observed in the Gruža Reservoir (b < 3) likely reflects local environmental conditions, prey availability, population density, and ontogenetic structure, all of which are known to shape growth patterns in invasive fish populations [80].
The negative allometric growth observed in A. melas (b < 3) is consistent with the feeding patterns dominated in this study. Across all age classes, the diet was dominated by low-energy prey such as Protozoa, Chironomidae, and, to a lesser extent, zooplankton, while high-caloric items such as fish [65] were consumed only sporadically. Given the predominance of low-energy prey, it is possible that the overall energetic intake is insufficient to support proportional weight gain, which might help explain the negative allometric growth observed in this population. However, this pattern may also reflect ontogenetic shifts in energetic demands, whereby smaller individuals rely on easily accessible prey, while larger individuals increase consumption of energetically richer prey types without fully transitioning to piscivory. In addition, the moderate presence of zooplankton in older age classes suggests potential trophic competition with native zooplanktivores, which may further constrain access to high-value prey resources. The consumption of maize and wheat introduced by anglers does not improve the nutritional status of A. melas, as these items are of low digestibility [81,82] and therefore may provide only limited energetic benefit for this species. Together, these factors likely underlie both the negative allometry and the moderate condition coefficients recorded in the Gruža Reservoir population.

4.3. Ecological Implications for Native Communities and Reservoir Functioning

When compared with the native fish community of the Gruža Reservoir, A. melas demonstrates a markedly broader and more flexible feeding niche. Native zooplanktivorous species such as common bleak Alburnus alburnus (Linnaeus, 1758) and juvenile R. rutilus rely heavily on Cladocera, Daphnia sp., and Copepoda, forming a relatively narrow feeding spectrum dominated by zooplankton [27,28,29]. In contrast, A. melas incorporates these prey groups only opportunistically and supplements them with benthic invertebrates, Protozoa, fish eggs, and occasionally fish remains. This indicates only partial trophic overlap with native zooplanktovores. Furthermore, because piscivorous species in Gruža Reservoir (e.g., S. lucioperca, European perch Perca fluviatilis Linnaeus, 1758, and Wels catfish Silurus glanis Linnaeus, 1758) feed almost exclusively on fish [26], the limited piscivory observed in A. melas suggests low overlap with top predators as well. Altogether, these comparisons emphasize that A. melas occupies a trophically flexible and distinct niche, capable of exploiting resources that native species do not use extensively, which may enhance its invasive success. In Serbia, A. melas is considered one of the most impactful invasive fish species, exerting negative effects on native ichthyofauna through direct predation and competition for shared food resources [83]. This conclusion is further supported by the standardized Levins’ index values obtained in this study (BA = 0.44 for age class 1+, 0.38 for 2+, and 0.36 for 3+), all of which fall within the range typically associated with generalist feeders, confirming that A. melas exhibits a broad trophic niche with moderate specialization in older age classes.
Regional risk assessment studies further corroborate the invasive potential of A. melas in the Balkan region. Using the Fish Invasiveness Screening Kit (FISK), Simonović et al. [84] classified A. melas in Serbia as a moderately high-risk species, emphasizing that several long-established non-native fishes in Balkan inland waters exhibit consistent medium-to-high invasiveness scores. Subsequent applications of FISK and Aquatic Species Invasiveness Screening Kit (AS-ISK) in Croatia and Slovenia confirmed A. melas as a high-risk species [85,86], highlighting its ecological plasticity, wide distribution, and capacity for successful establishment. These regional assessments align with the trophic flexibility and ecological tolerance documented in the present study, supporting the view that A. melas represents a persistent and potentially impactful component of non-native fish assemblages in eutrophic Balkan reservoirs.
Given that the Gruža Reservoir experiences pronounced anthropogenic impacts and persistent eutrophic conditions [31], the feeding behaviour of invasive species may further alter its already unstable trophic dynamics. The feeding ecology of A. melas also carries important implications for the reservoir’s trophic state. By heavily exploiting Chironomidae, the organisms reduce benthic fauna responsible for organic matter processing and nutrient recycling [87,88]. Although zooplankton constituted a smaller proportion of the diet, predation on Cladocera and Copepoda may still diminish grazing pressure on phytoplankton, potentially reinforcing eutrophic tendencies [89]. In addition, the consumption of fish eggs, likely originating from native species, may reduce recruitment success and thereby represent an additional pressure on native fish populations. Altogether, these feeding patterns suggest that A. melas is likely to exacerbate, rather than mitigate, existing eutrophic conditions in the Gruža reservoir.
The growth pattern observed in A. melas further illustrates traits that support its invasive success. The negative allometric growth observed in Gruža Reservoir suggests a reliance on low-energy prey, yet the species maintains stable condition values, indicating its ability to function effectively even under modest nutritional input. This combination of physiological tolerance and trophic opportunism reflects a high degree of ecological plasticity, enabling A. melas to persist and spread even in resource-limited or degraded environments [15,80]. Together, these traits underline the capacity of A. melas to persist and expand under suboptimal trophic conditions, reinforcing its invasive potential in eutrophic reservoirs.

5. Conclusions

Understanding the dietary traits, feeding behaviour, and prey preferences of invasive species is essential for assessing their impact on native fish communities and for developing effective conservation and management strategies in the Gruža Reservoir. Evaluating the status and ecological impact of non-native species within already invaded ecosystems is a necessary prerequisite for preventing or mitigating their further spread. The findings of this study demonstrate pronounced trophic plasticity and clear age-dependent shifts in the feeding patterns of A. melas, highlighting its ability to exploit a wide spectrum of food resources, including benthic macroinvertebrates, fish eggs, and, occasionally, zooplankton. Such dietary flexibility likely enhances the competitive advantage of this species and contributes to cumulative pressures on native fish communities. Although the present study represents a seasonal snapshot based on summer sampling, it offers an ecologically meaningful insight into the feeding strategies of A. melas during a period of high biological activity. We also acknowledge that the use of angling-based sampling may introduce certain biases, such as the preferential capture of actively foraging individuals in nearshore habitats, and that future studies incorporating multiple seasons, standardized sampling gear, and broader spatial coverage would further strengthen ecological inference. Continued monitoring of feeding ecology, abundance, and spatial distribution is therefore essential for informing adaptive management measures and mitigating further ecological impacts within the reservoir.

Author Contributions

Conceptualization, M.R.; methodology, M.R., N.K., A.M.; software, M.S.P.; validation, V.S.; formal analysis, M.R., T.V.; investigation, M.R., T.V., A.C.; writing—original draft preparation, M.R.; writing—review and editing, M.R., N.K., A.M., T.V., A.C., V.S.; visualization, M.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research is funded by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (Grant: No. 451-03-136/2025-03/200122) and the Slovenian Research Agency supporting T.V. under research programme No. P1-0255.

Institutional Review Board Statement

The study did not require approval from an institutional ethics committee, as all sampling was conducted exclusively on an invasive fish species, Ameiurus melas. According to the Law on the Protection and Sustainable Use of Fish Stocks of the Republic of Serbia (“Official Gazette of RS”, Nos. 128/2014 and 95/2018), this species is exempt from harvest limits and closed seasons. Consequently, its collection for research purposes is permitted without restrictions. All specimens were obtained in cooperation with recreational anglers and in full compliance with national fisheries regulations. No protected or native species were involved, and no experimental procedures requiring ethical approval were applied.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank the members and volunteers of the Ekomar Association (Kragujevac, Serbia) for their assistance during field sampling.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of Gruža Reservoir.
Figure 1. Location of Gruža Reservoir.
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Figure 2. Costello graph. Prey-specific abundance versus frequency of occurrence in diet of different age classes (1+ to 3+) of Ameiurus melas from Gruža Reservoir. Rare prey items are encircled.
Figure 2. Costello graph. Prey-specific abundance versus frequency of occurrence in diet of different age classes (1+ to 3+) of Ameiurus melas from Gruža Reservoir. Rare prey items are encircled.
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Figure 3. Distribution of 98 digestive tracts of Ameiurus melas from the Gruža Reservoir across 42 (6 × 7) SOM output neurons grouped into clusters I, II, and III. Each digestive tract is labelled by age class (1+, 2+, or 3+) and individual identification number.
Figure 3. Distribution of 98 digestive tracts of Ameiurus melas from the Gruža Reservoir across 42 (6 × 7) SOM output neurons grouped into clusters I, II, and III. Each digestive tract is labelled by age class (1+, 2+, or 3+) and individual identification number.
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Figure 4. Distribution patterns of 18 food categories in the diet of Ameiurus melas from the Gruža Reservoir. Shading intensity is scaled independently for each food category and reflects corresponding IndVal values, with darker shades indicating higher indicator values and progressively lighter shades representing lower IndVal values.
Figure 4. Distribution patterns of 18 food categories in the diet of Ameiurus melas from the Gruža Reservoir. Shading intensity is scaled independently for each food category and reflects corresponding IndVal values, with darker shades indicating higher indicator values and progressively lighter shades representing lower IndVal values.
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Table 1. Diet composition of Ameiurus melas from the Gruža Reservoir based on frequency of occurrence (%FO), relative abundance (%N), and prominence value (%PV).
Table 1. Diet composition of Ameiurus melas from the Gruža Reservoir based on frequency of occurrence (%FO), relative abundance (%N), and prominence value (%PV).
1+2+3+
%FO%N%PV%FO%N%PV%FO%N%PV
Protozoa54.5430.6533.9957.8926.8431.1637.2115.1113.86
Rotifera9.090.250.11
Bryozoa27.271.891.4821.0518.6213.0318.615.863.8
Hydrachnidia9.090.250.11
Cladocera 10.520.590.292.330.110.02
Daphnia sp. 2.630.030.0074.650.330.11
Leptodora kindtii4.540.080.032.630.030.007
Copepoda9.090.160.0710.520.530.2623.256.434.66
Amphipoda9.090.490.222.630.030.007
Insecta45.452.142.1623.680.520.386.970.790.31
Diptera4.543.120.9931.573.192.7332.558.687.45
Chironomidae86.3612.1616.9789.4714.8621.4588.3738.2254.06
Coleoptera 2.635.411.342.331.130.25
Nematoda9.090.160.0710.520.210.114.650.230.07
Bivalvia 5.260.070.024.650.90.29
Fishes9.090.160.72.630.030.00711.630.560.29
Fish roe36.3648.1543.5944.7328.3828.9620.9321.4214.74
Algae4.540.330.115.260.630.222.330.230.05
Detritus90.91 100 97.67
Table 2. Descriptive statistics and estimated parameters of the length–weight relationships and Fulton’s condition factor for Ameiurus melas; n—number of individuals, a—intercept of length–weight relationship, b—allometric growth coefficient (slope), SE(b)—standard error of b, r2—coefficient of determination, K—Fulton’s condition factor, SE(K)—standard error of K.
Table 2. Descriptive statistics and estimated parameters of the length–weight relationships and Fulton’s condition factor for Ameiurus melas; n—number of individuals, a—intercept of length–weight relationship, b—allometric growth coefficient (slope), SE(b)—standard error of b, r2—coefficient of determination, K—Fulton’s condition factor, SE(K)—standard error of K.
Age GroupnabSE(b)r2KSE(K)
1+220.0052.6600.4280.7581.1850.071
2+380.0022.7910.7490.3931.2380.185
3+430.0062.8680.3420.6891.1990.116
Total sample1030.0092.6600.1170.8751.2090.016
Table 3. Relative frequency (%FO), relative abundance (%N), and indicator values (IndVal) of food categories of Ameiurus melas from the Gruža Reservoir. Significant IndVal values (p ≤ 0.05) are shown in bold for each cluster (I, II, III); exact significance levels are presented in Figure 4.
Table 3. Relative frequency (%FO), relative abundance (%N), and indicator values (IndVal) of food categories of Ameiurus melas from the Gruža Reservoir. Significant IndVal values (p ≤ 0.05) are shown in bold for each cluster (I, II, III); exact significance levels are presented in Figure 4.
Fish Diet GroupIIIIII
%FO%NIndVal%FO%NIndVal%FO%NIndVal
Protozoa241010094945253
Rotifera0000001010010
Bryozoa24912115102982
Hydrachnidia00046225382
Cladocera438184435181
Daphnia sp.21508857000
Leptodora kindtii22910005713
Copepoda16102278824520
Amphipoda00082425764
Insecta1214238532129339
Diptera35863023511491
Chironomidae944340922523903329
Coleoptera41004000000
Nematoda10394413010485
Bivalvia67950005211
Fishes1041400014598
Fish eggs4004232141006868
Algae24044010939
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Radenković, M.; Kojadinović, N.; Milošković, A.; Veličković, T.; Piperac, M.S.; Cvetković, A.; Simić, V. The Feeding Habits and Length–Weight Relationships of the Invasive Black Bullhead Ameiurus melas (Rafinesque, 1820) in the Gruža Reservoir, Central Serbia. Fishes 2026, 11, 144. https://doi.org/10.3390/fishes11030144

AMA Style

Radenković M, Kojadinović N, Milošković A, Veličković T, Piperac MS, Cvetković A, Simić V. The Feeding Habits and Length–Weight Relationships of the Invasive Black Bullhead Ameiurus melas (Rafinesque, 1820) in the Gruža Reservoir, Central Serbia. Fishes. 2026; 11(3):144. https://doi.org/10.3390/fishes11030144

Chicago/Turabian Style

Radenković, Milena, Nataša Kojadinović, Aleksandra Milošković, Tijana Veličković, Milica Stojković Piperac, Aleksa Cvetković, and Vladica Simić. 2026. "The Feeding Habits and Length–Weight Relationships of the Invasive Black Bullhead Ameiurus melas (Rafinesque, 1820) in the Gruža Reservoir, Central Serbia" Fishes 11, no. 3: 144. https://doi.org/10.3390/fishes11030144

APA Style

Radenković, M., Kojadinović, N., Milošković, A., Veličković, T., Piperac, M. S., Cvetković, A., & Simić, V. (2026). The Feeding Habits and Length–Weight Relationships of the Invasive Black Bullhead Ameiurus melas (Rafinesque, 1820) in the Gruža Reservoir, Central Serbia. Fishes, 11(3), 144. https://doi.org/10.3390/fishes11030144

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