# Potential Trophic Competition Between the European PerchPerca fluviatili and Pikeperch Sander lucioperca

Mathieu Toutain, Phillip J Haubrock, Nadège Belouard, Irmak Kurtul, Sadi

Aksu, Özgür Emiroğlu, Ali Serhan Tarkan, Paride Balzani

► To cite this version:

Mathieu Toutain, Phillip J Haubrock, Nadège Belouard, Irmak Kurtul, Sadi Aksu, et al.. Potential Trophic Competition Between the European PerchPerca fluviatili and Pikeperch Sander lucioperca. International Review of Hydrobiology, 2026, 111 (1), pp.e70033. 10.1002/iroh.70033. hal-05533616

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RESEARCH PAPER

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# Potential Trophic Competition Between the European Perch *Perca fluviatilis* and Pikeperch *Sander lucioperca*

Mathieu Toutain<fnref n="1" />,² | Phillip J. Haubrock²,³ | Nadège Belouard<fnref n="1" /> | Irmak Kurtul³,⁴ | Sadi Aksu⁵ | Özgür Emiroglu⁶ |
Ali Serhan Tarkan⁷ | Paride Balzani²

**Correspondence:** Ali Serhan Tarkan (serhantarkan@gmail.com) | Paride Balzani (balzani@frov.jcu.cz)

**Received:** 28 July 2025 | **Revised:** 24 September 2025 | **Accepted:** 29 January 2026

**Keywords:** fish diet | interspecific competition | Percidae | stable isotope analysis | trophic competition

## ABSTRACT

The European perch *Perca fluviatilis* and pikeperch *Sander lucioperca* can co-occur, potentially competing for food resources. However, few studies have addressed the potential feeding competition between these species. We used carbon and nitrogen stable isotope analysis to assess the potential trophic competition between these species in two Turkish ecosystems, a natural lake where both species are native (Lake Gala), and a reservoir where both have been introduced (Sığırcı). We found a significant positive relationship for *S. lucioperca* between δ¹³C and size in Lake Gala, suggesting higher consumption of benthic/littoral prey in bigger individuals, but not in the Sığırcı reservoir nor for *P. fluviatilis*. A positive relationship between δ¹⁵N and size was also found in the Sığırcı reservoir for both species, indicating that trophic position increased with age, while in Lake Gala *P. fluviatilis* showed a negative relationship, possibly due to consumption of abundant prey from lower trophic levels. In Lake Gala, the isotopic niches overlapped and were larger in *S. lucioperca*, while in the Sığırcı reservoir they were segregated and larger in *P. fluviatilis*. Our results show that the trophic niches of *P. fluviatilis* and *S. lucioperca* can overlap, leading to potential interspecific competition.

## 1 | Introduction

Mesopredatory piscivorous fishes play an important function in freshwater ecosystems as they predate on smaller species' populations through a process known as *top-down control* (Hairston et al. 1960; Rosenfeld 2000; Baum and Worm 2009). Variation in the abundance of these predators can therefore have cascading consequences for lower trophic levels (Carpenter et al. 1985; Rooney et al. 2006). At the same time,

they serve as prey for top predators, exerting a *bottom-up* control on species from higher trophic levels (Baum and Worm 2009; Nishijima et al. 2014). As such, mesopredatory fishes act as intermediate players in the energy and biomass transfer across trophic levels (Heithaus et al. 2008; Sivy et al. 2018; Rahman et al. 2023).

In the Northern Hemisphere's freshwater ecosystems, many species of Percidae (a family encompassing over 230 species;

<footnote n="1">¹Université de Rennes, CNRS, ECOBIO [(Ecosystèmes, Biodiversité, Évolution)], Rennes, France | ²Faculty of Fisheries and Protection of Waters, South Bohemian Research Center of Aquaculture and Biodiversity of Hydrocenoses, University of South Bohemia in České Budějovice, Vodňany, Czech Republic | ³Faculty of Health, Environment and Medical Sciences, Bournemouth University, Dorset, Poole, UK | ⁴Marine and Inland Waters Sciences and Technology Department, Faculty of Fisheries, Ege University, İzmir, Türkiye | ⁵Vocational School of Health Services, Eskişehir Osmangazi University, Eskişehir, Türkiye | ⁶Department of Biology, Faculty of Science, Eskişehir Osmangazi University, Eskişehir, Türkiye | ⁷Department of Ecology and Vertebrate Zoology, Faculty of Biology and Environmental Protection, University of Lodz, Lodz, Poland</footnote>

<footnote>Ali Serhan Tarkan and Paride Balzani co-senior authors.</footnote>

<footnote>This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>

<footnote>© 2026 The Author(s). *International Review of Hydrobiology* published by Wiley-VCH GmbH.</footnote>
Froese and Pauly 2024) are recognized as mesopredators (Craig 2008; Tarkan et al. 2023). Two of these species, the European perch *Perca fluviatilis* (Linnaeus, 1758) and the pikeperch *Sander lucioperca* (Linnaeus, 1758), are widely distributed in Eurasia, with sympatric ranges (Freyhof and Kottelat 2007). These two species are commonly introduced to support recreational and commercial fisheries, increasing their co-occurrence (Kopp et al. 2009; Banha et al. 2015; Specziár and Turcsányi 2017). *Perca fluviatilis* is a widely distributed fish in Eurasia due to its ability to adapt to different aquatic environments (Urho 1996; Ceccuzzi et al. 2011). Perch undergoes an ontogenetic diet change (Olsson et al. 2000; Estlander et al. 2010; Jacobson et al. 2019), with young individuals consuming zooplankton and small benthic animals such as insects and their larvae (Berzina and Strel'nikova 2001; Lorenzoni et al. 2007). As adults, *P. fluviatilis* can reach up to 50 cm and 2 kg (Pimakhin et al. 2015; Jacobson et al. 2019) and are mainly opportunistic schooling predators, mainly consuming invertebrates and small fish, becoming piscivorous when reaching sizes of > 120 mm standard length (SL; Freyhof and Kottelat 2007). Cannibalistic behavior has also been observed (Wang and Eckmann 1994; Yazıçıoğlu et al. 2016).

*Sander lucioperca* is native to Central and East Europe and Western Asia and often plays a key ecological role as a predator, structuring aquatic food webs, while also holding significant socio-economic value (Schlumberger and Proteau 1996; Hansson et al. 1997; Alieva et al. 2023). It can be found in most freshwater ecosystems (Lehtonen et al. 1996) and can grow to over a meter in total length (Willemsen 1977; Riede 2004). Similar to *P. fluviatilis*, the diet of *S. lucioperca* varies throughout its life. During the first year, this species mainly consumes small prey (e.g., zooplankton; Buijse and Houthuijzen 1992; Ginter et al. 2011). Subsequently, its diet consists mostly of smaller fish species including the European perch (Balik et al. 2006; Dörner et al. 2007; Bousseba et al. 2024), but can also include smaller prey (such as insects, frogs, and crustaceans), and cannibalism of juveniles is also reported (Hansson et al. 1997; Yilmaz and Ablak 2003). Its diet, however, strongly depends on prey availability (Persson and Brönmark 2002; Dörner et al. 2007; Alieva et al. 2023).

Despite ample research on both species' ecology, comparative evidence on their potential feeding competition across different ecological contexts remains limited (but see Popova and Sytina 1977; Schulze et al. 2012; Didenko and Gurbyk 2016; Westrelin et al. 2023). In regions where both species coexist, they likely exhibit a form of spatial partitioning, with *P. fluviatilis* favoring shallower, vegetated waters and *S. lucioperca* preferring deeper areas (Persson and Eklov 1995; Westrelin et al. 2018; Blabolil et al. 2023). Moreover, *P. fluviatilis* is a diurnal predator, while *S. lucioperca* feeds mostly during dawn and dusk (Jacobsen et al. 2002; Poulet et al. 2005; Horký et al. 2008). Nonetheless, as the life stages of both species rely on similar prey, they—depending on their respective sizes—potentially compete for food and other resources.

Stable isotope analysis (SIA) of carbon (δ¹³C) and nitrogen (δ¹⁵N) is a methodology that provides time-mediated information on the trophic structure of species and communities (Layman et al. 2007), granting an understanding of how species coexist and on their role in food webs (Layman et al. 2012; Post 2002). Carbon isotopes track the main energy sources, while the

nitrogen isotopes indicate the trophic position of consumers (Boecklen et al. 2011). Thus, by using isotopic signatures, trophic levels can be estimated and ecological niches quantified (Post 2002; Newsome et al. 2007; Boecklen et al. 2011). Since the isotopic niche is a good proxy of the trophic niche (Bearhop et al. 2004; Nielsen et al. 2018; Matich et al. 2021), SIA can be used to assess the potential feeding competition between co-occurring species, discriminating cases of niche partitioning (Polačik et al. 2014) from those in which the niches overlap (Balzani et al. 2016, 2020). A niche overlap indicates that species could compete for shared resources, should they be limitant (e.g., Haubrock et al. 2020; Aksu et al. 2025).

To assess the potential effects of competing interactions between *P. fluviatilis* and *S. lucioperca*, we (1) used SIA to characterize the trophic niche of both species in two Turkish water bodies, a natural lake where both species are native, and a reservoir where both have been introduced in the end of the 1990s. In particular, we (2) assessed changes in trophic position and feeding habitat with fish size and (3) compared the isotopic niche width between species in each site. Finally, we (4) assessed the potential trophic competition between these two fishes in the two sites, as competition is often context-dependent, especially where interactions involve non-native populations that share no co-evolutionary history (e.g., Haubrock et al. 2019; Bissattini et al. 2021). Based on the available literature, we hypothesized (1) an increase in trophic position with size for both species, (2) a wider niche in *P. fluviatilis* than *S. lucioperca*, and (3) trophic competition to occur.

## 2 | Materials and Methods

### 2.1 | Study Sites

This study was conducted in Lake Gala (40.780834 N; 26.200358 E) and the Sığırcı reservoir (40.833216 N; 26.323628 E) in Türkiye (Figure 1). Lake Gala is a small natural lake of 5.6 km² with a maximum depth of 2 m, whereas the Sığırcı reservoir was formed from Lake Gala by a dam, and covers an area of 6.5 km² with a maximum depth of 6 m. The two water bodies are approximately 11.5 km straight-line distance apart. Due to the increase in agriculture over several decades, both water bodies have undergone ecological succession and have transitioned into meso-eutrophic environments (Tarkan et al. 2023).

In Lake Gala, the fish community consisted of a total of 11 species, namely *Abramis brama*, *Alburnus alburnus*, *Carassius gibelio*, *Cyprinus carpio*, *Esox lucius*, *Gambusia holbrooki*, *P. fluviatilis*, *Pseudorasbora parva*, *Rutilus rutilus*, *S. lucioperca*, and *Silurus glanis* (Toutain et al. 2024). The Sığırcı reservoir, established in 1999, hosts a fish community composed of seven species: *C. gibelio*, *C. carpio*, *G. holbrooki*, *Lepomis gibbosus*, *P. fluviatilis*, *R. rutilus*, and *S. lucioperca* (Toutain et al. 2024). All these species are believed to have been introduced within a few years of the reservoir's establishment.

### 2.2 | Sampling

Since both ecosystems are shallow and environmentally homogeneous, sampling was carried out near (~3 m) the shore for both ecosystems for 3 days during autumn 2021 at a depth of
<figure><img src="image_2.png" /><figcaption>FIGURE 1 | Map of the study areas in Türkiye.</figcaption></figure>

~2 m using multi-mesh gillnets, consisting of 12 panels with mesh sizes ranging from 5 to 55 mm. Each panel was 2.5 m in length and 1.5 m in width, and the nets were exposed for a duration of 12 h, from 18:00 to 06:00. Following their capture, fishes were transported to the laboratory on an ice water slurry. The collected fishes were measured for their total length and weight (to the nearest mm and g, respectively) and stored at –20°C until further processing.

## 2.3 | Stable Isotope Analysis

All specimens were thawed and a sample of dorsal muscle tissue was excised from each individual for bulk carbon and nitrogen SIA. The sampled muscles were then dried in an oven at 60°C for 24 h before being ground to a fine powder using an agate pestle and mortar. Samples of 1 (± 0.1) mg of homogenized tissue were analyzed with a continuous flow interface (ConFlo IV, Thermo Fisher Scientific, Germany) and an isotopic ratio mass spectrometer (Delta V Advantage, Thermo Fisher Scientific, Germany) at the Biological and Chemical Research Centre in Warsaw, Poland. The isotope compositions were expressed in the δ notation (‰), calculated based on δ¹³C or δ¹⁵N = ((Rsample/Rstandard) – 1) × 1000, where R represents the ¹³C:¹²C or ¹⁵N:¹⁴N ratios, and the standards were Vienna Pee Dee Belemnite for carbon and atmospheric N₂ for nitrogen. To ensure the reliability of the isotopic analysis,

samples were analyzed in duplicates. The average standard errors were 0.03‰ for δ¹³C and 0.11‰ for δ¹⁵N.

## 2.4 | Statistical Analyses

To assess changes in the trophic position or feeding habitat with body size, linear models between δ¹³C or δ¹⁵N as response variable and total length were performed for each species and site. Then, to describe the species isotopic niches within each site, we calculated Layman's metrics (Layman et al. 2007) and the corrected standard ellipse area (SEAc, Jackson and Parnell 2023) considering 40% of the δ¹³C and δ¹⁵N data points using the R package SIBER (Stable Isotope Bayesian Ellipses in R; Jackson and Parnell 2023). The δ¹⁵N and δ¹³C ranges (NR and CR) refer to the interindividual variability in the trophic position and in the carbon source, respectively. The mean distance to the centroid (CD) and the total area encompassing all individual measures (TA) are indices of the species' niche width. The mean nearest neighbor distance (MNND) and its standard deviation (SDNND) relate to the distance among individuals and indicate the degree of trophic redundancy (i.e., the intraspecific, interindividual competition). SEAc is a measure of niche width that is less affected by extreme values than TA. Additionally, the 95% Bayesian standard ellipse area (SEAb) was calculated using the R package SIBER (Jackson and Parnell 2023). To assess the interspecific differences in niche
size within each site, we calculated the proportion of values in the SEAb posterior distribution for one species that were lower than those for the other species.

To assess the differences in the isotopic niche, we followed the complementary approach of Balzani et al. (2024). First, we identified if the occupied trophic niches were significantly different between species within each site using two permutational multi-variate analyses of variance (PERMANOVAs, with Euclidean distance and 9999 permutations) on the δ¹⁵N and δ¹³C, followed by permutational analysis of variance (PERANOVAs) as post hoc tests, using the *adonis2* function implemented in the R package *vegan* (Oksanen et al. 2024). Second, the ratio between the overlap area and the sum of both niche areas for the 95% (maximum likelihood and SEAb) and 40% (SEAc) standard ellipse areas (SEA) were calculated for each site using the R package *SIBER* (Jackson and Parnell 2023). Third, the directional probability of an individual of a species to occur within the niche of the other species (considered as the 95% and 40% SEA) was estimated for each site by applying a Monte Carlo estimation (chain-length: 10,000 steps) using the R package *nicheROVER* (Swanson et al. 2015).

All the statistical analyses were performed in the R environment (v. 4.2.2; R Core Team 2022).

## 3 | Results

Across the two sites, a total of 39 specimens were sampled: Nine individuals of *P. fluviatilis* and 10 of *S. lucioperca* in Lake Gala, and 10 individuals of both *P. fluviatilis* and *S. lucioperca* in the Sığırcı reservoir (Table S1).

We found that in Lake Gala, δ¹³C values significantly increased with size in *S. lucioperca* (*F*₁,₈ = 7.55, *p* < 0.05, adj. *R*² = 0.42) but not in *P. fluviatilis* (*F*₁,₇ = 2.09, *p* = 0.19, adj. *R*² = 0.12; Figure 2a), while in the Sığırcı reservoir, the relationship was not significant for both species (*P. fluviatilis*: *F*₁,₈ = 3.26, *p* = 0.11, adj. *R*² = 0.20; *S. lucioperca*: *F*₁,₈ = 0.09, *p* = 0.77, adj. *R*² = -0.11; Figure 2b). We also found a significant decrease in δ¹⁵N with size in Lake Gala for *P. fluviatilis* (*F*₁,₇ = 8.51, *p* < 0.05, adj. *R*² = 0.48) but not for *S. lucioperca* (*F*₁,₈ = 0.02, *p* = 0.90, adj. *R*² = -0.13; Figure 2c). A significant but opposite (i.e., increasing) relationship between δ¹⁵N and size was found in the Sığırcı reservoir for both *S. lucioperca* (*F*₁,₈ = 23.56, *p* < 0.01, adj. *R*² = 0.71) and *P. fluviatilis* (*F*₁,₈ = 44.2, *p* < 0.001, adj. *R*² = 0.84; Figure 2d).

The isotopic niche metrics showed some differences in the two sites (Table 1; Figure 3). In Lake Gala, *S. lucioperca* showed a broader isotopic niche than *P. fluviatilis*, as indicated by the higher TA, CD, and SEAc, while also exploiting a more diverse range of resources, as shown by the higher CR. On the other hand, *P. fluviatilis* was feeding over more trophic levels, as indicated by the higher NR values, along with a more differentiated individual diet, as suggested by the slightly higher values of NND and SDNND. In the Sığırcı reservoir, *P. fluviatilis* occupied a broader isotopic niche than *S. lucioperca*, with SEAc and all Layman's metrics being higher in the former species than in the latter, except for NND. However, SEAb values (Table 1; Figure 3c,d) were similar in both Lake Gala (P(SEAb<sub>P. fluviatilis</sub> < SEAb<sub>S. lucioperca</sub>) = 0.88) and the Sığırcı reservoir (P(SEAb<sub>S. lucioperca</sub> < SEAb<sub>P. fluviatilis</sub>) = 0.71), indicating that differences in the niche area were not so marked.

The multivariate analyses showed that the isotopic niches of *P. fluviatilis* and *S. lucioperca* were not significantly different in Lake Gala (pseudo*F*₁,₁₇ = 2.53, *p* = 0.11), whereas in the Sığırcı reservoir they were significantly different (pseudo*F*₁,₁₈ = 4.38, *p* < 0.05). In particular, this segregation was due to differences in δ¹³C (higher in *P. fluviatilis*; pseudo*F*₁,₁₈ = 8.84, *p* < 0.01) but not in δ¹⁵N (pseudo*F*₁,₁₈ = 3.36, *p* = 0.08). However, some niche overlap was observed in both sites. Indeed, the niche overlap was 38% in Gala and 45% in the Sığırcı reservoir when considering the 95% maximum likelihood ellipses, and 35% in Gala and 41% in the Sığırcı reservoir when considering the SEAb. On the other hand, the SEAc overlap was considerably lower and similar in the two sites (5% in Lake Gala and 3% in the Sığırcı reservoir). The probability for an individual of *P. fluviatilis* to fall in the 95% isotopic niche of *S. lucioperca* was 79% in Gala (Figure S1a) whereas the probability of the reverse situation was 47.6% (Figure S1e). In Sığırcı, the probability of an individual of *P. fluviatilis* falling into the *S. lucioperca* 95% isotopic niche was 54.5% (Figure S1b), while the reverse was around 68.1% (Figure S1f). The probability for an individual of *P. fluviatilis* to fall in the 40% isotopic niche of *S. lucioperca* was 26.1% in Lake Gala (Figure S1c) whereas the opposite probability was 14.8% (Figure S1g). In the Sığırcı reservoir, the probability of an individual of *P. fluviatilis* falling into *S. lucioperca* 40% isotopic niche was 13.9% (Figure S1d), while the reverse was 15.6% (Figure S1h).

## 4 | Discussion

Feeding competition is an important ecological interaction between species (e.g., Whitfield et al. 2024). In this study, we aimed to evaluate the potential trophic competition between *P. fluviatilis* (European perch) and *S. lucioperca* (pikeperch) by characterizing their trophic niches using stable isotope analysis (SIA) in two distinct freshwater ecosystems in Türkiye: a lake where both species are native (Lake Gala) and a reservoir where both species have been introduced (Sığırcı reservoir). We observed differences in trophic niche overlap between the two species across the sites.

Both young *P. fluviatilis* and *S. lucioperca* are known to feed on littoral prey (Cicala et al. 2020), with a shift towards more pelagic food sources with increasing size (Persson 1987; Bousseba et al. 2024). However, the positive relationship between δ¹³C and size found in Lake Gala for *S. lucioperca* suggests a higher consumption of benthic or littoral prey in larger individuals and a higher reliance on pelagic prey in smaller ones. On the other hand, such a significant relationship was not found in the Sığırcı reservoir, possibly because of its simplified habitat structure (Toutain et al. 2024). We also did not observe such an ontogenetic shift in feeding habitat in *P. fluviatilis*, potentially due to the narrow size range of the animals, which prevented the detection of any trend. Both species usually increase their trophic position with age and size (Linzmaier et al. 2018; Trudeau et al. 2024; Bousseba et al. 2024), and our results from the Sığırcı reservoir support this general trend. However, in Lake Gala *P. fluviatilis* showed a decreasing trophic position with size. Although contrary to our expectations, larger individuals could have relied on some more abundant prey from lower trophic levels since this species is very opportunistic (Dörner et al. 2003; Akin et al. 2011).
<figure><img src="image_3.png" /><figcaption>**FIGURE 2** | Regression lines of the carbon (a, b) and nitrogen (c, d) in function of the total length (mm) of *Perca fluviatilis* (yellow) and *Sander lucioperca* (blue) in the different sampling sites (a and c: Lake Gala, b and d: Sığırcı reservoir).</figcaption></figure>

<table><caption>**TABLE 1** | Layman's metrics, corrected standard ellipse area (SEAc) and mode of the Bayesian standard ellipse area (SEAb) of *Perca fluviatilis* and *Sander lucioperca* in the two sampling sites.</caption><thead><tr><th>Site</th><th>Species</th><th>NR</th><th>CR</th><th>TA</th><th>CD</th><th>NND</th><th>SDNND</th><th>SEAc</th><th>SEAb (mode)</th></tr></thead><tbody><tr><td rowspan="2">Lake Gala</td><td><i>P. fluviatilis</i></td><td>2.938</td><td>3.323</td><td>4.683</td><td>1.078</td><td>0.765</td><td>0.635</td><td>3.001</td><td>2.444</td></tr><tr><td><i>S. lucioperca</i></td><td>1.769</td><td>6.266</td><td>7.474</td><td>2.227</td><td>0.648</td><td>0.588</td><td>5.178</td><td>4.330</td></tr><tr><td rowspan="2">Sığırcı reservoir</td><td><i>P. fluviatilis</i></td><td>2.213</td><td>1.289</td><td>1.649</td><td>0.950</td><td>0.278</td><td>0.205</td><td>1.024</td><td>0.884</td></tr><tr><td><i>S. lucioperca</i></td><td>1.801</td><td>1.230</td><td>1.396</td><td>0.671</td><td>0.322</td><td>0.180</td><td>0.846</td><td>0.677</td></tr></tbody></table>

Moreover, this ontogenetic trend in *P. fluviatilis* has been shown to be less pronounced where the proportion of shallow water is low (Trudeau et al. 2024), as it is the case in Lake Gala.

Niche metrics revealed that in Lake Gala, *S. lucioperca* had a wider niche than *P. fluviatilis*, indicating a more generalist diet (as also indicated by the results of the mixing models, see below) and a greater exploitation of carbon sources. Although this species is mainly piscivorous (e.g., Schulze et al. 2012),

other studies found similar results (e.g., Dörner et al. 2003; Cicala et al. 2020). In the same site, *P. fluviatilis* relied on prey from multiple trophic levels, while having a greater inter-individual feeding differentiation, meaning a lower potential for intraspecific competition (Layman et al. 2012). On the contrary, in the Sığırcı reservoir, *P. fluviatilis* had a larger and more redundant (i.e., with individuals closer to each other) niche than *S. lucioperca*, indicating a more generalist niche and a
<figure><img src="image_4.png" /><figcaption>**FIGURE 3** | Bplot of the carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic niches of *Perca fluviatilis* (yellow) and *Sander lucioperca* (blue) in the different sampling sites (a: Lake Gala, b: Siğirci reservoir). Ellipses represent 95% (transparent) and 40% (opaque) of the data. Boxplot of SEAb of *P. fluviatilis* (yellow) and *S. lucioperca* (blue) in the different sampling sites (c, d).</figcaption></figure>

higher potential intraspecific competition for *P. fluviatilis*. This opposite pattern in the two sampled ecosystems is particularly interesting, as this result is accompanied by a more aggressive behavior observed in *P. fluviatilis* populations introduced in reservoirs (Tarkan et al. 2023; pers. comm. Necati Ayvaz). On the other hand, we cannot exclude the possibility that the larger niche of *S. lucioperca* in Lake Gala is the result of the larger specimens caught there.

Diet overlap has been demonstrated between several co-occurring Percidae species (Hlohowskyj and White 1983; Jamet 1994; Rezsu and Specziár 2006; Lorenzoni et al. 2007), as well as cases of trophic niche partitioning (Dukowska et al. 2012; Dukowska and Grzybkowska 2014; Hopper and Tobler 2016; Kraus et al. 2021). In our study, the isotopic niches of the two species overlapped in Lake Gala, indicating a high potential trophic competition if resources would become limiting (Balzani et al. 2016). Such potential competition would also be not symmetric, since individuals of *P. fluviatilis* showed a higher probability of being within the niche of

*S. lucioperca* than the opposite. This trophic niche overlap was accompanied by a similar diet in the two species. One explanation for this overlap could be a limited prey availability (Jamet 1994). However, in Lake Gala only *G. holbrooki* was more abundant than the other suitable prey (cf. tab. S2 in Toutain et al. 2024), and was not preferentially consumed by neither local species. An alternative explanation that at the same time could mitigate competition is spatial partitioning between *P. fluviatilis* and *S. lucioperca*, as observed in some temperate reservoirs (Westrelin et al. 2021). A shift towards the littoral habitat by *P. fluviatilis* was also observed after the experimental introduction of *S. lucioperca* in a lake, to reduce interspecific competition (Schulze et al. 2006). Similarly, differences in the activity patterns between the two species (Jacobsen et al. 2002; Poulet et al. 2005; Horký et al. 2008) could mitigate competition by reducing direct interactions (i.e., interference competition), while exploitative competition could still occur, as niche overlap rather indicates competition for shared resources. Additionally, it is important to remember that in Lake Gala
S. lucioperca was larger than P. fluviatilis, thus making spatial partitioning a probable mechanism to avoid competition, as well as predation by large pikeperch on small perch.

On the contrary, and against our expectations, in the Sığırcı reservoir, the niches of the two species were relatively separate, due to differences in δ¹³C values, while both species occupied a similar trophic position. A low diet overlap between P. fluviatilis and S. lucioperca was also observed in a Ukrainian reservoir (Didenko and Gurbyk 2016). In the Sığırcı reservoir, the relative abundance of the suitable preys was very similar (cf. tab, S2 in Toutain et al. 2024), and the diet of the two fishes differed due to a marked preference for small R. rutilus by P. fluviatilis. However, this segregation was not complete, so that some potential overlap (with a higher probability for individuals of S. lucioperca of falling in the niche of P. fluviatilis) was still present. In any case, in both sites the overlap of core niches was limited compared with the overall niche overlap, indicating that it is some of the most extreme individuals from each species that compete for resources. Extreme individual values indicate opportunistic feeding or individual specialization (Araújo et al. 2011). Therefore, in these two sites, competition for occasional prey may be more likely than competition for prey that contribute the most to both P. fluviatilis and S. lucioperca's diets. Such a trophic differentiation mechanism was observed between co-occurring asp (Leuciscus aspius) and pikeperch populations in two Central European reservoirs (Vašek et al. 2018) to identify ranges with low-moderate interspecific niche overlap (0%-65%). Despite ontogenetic dietary shifts in both predators toward piscivory, asp and pikeperch exhibited marked differences in prey use (e.g., cyprinids vs. percids), showing how resource partitioning may allow coexistence even in the presence of potential competition. This is consistent with our conclusion that niche partitioning, including spatial and dietary segregation, is context-dependent, and can be dynamic among ecosystems. While individual specialization reduces intraspecific competition (Bolnick et al. 2003; Vander Zanden et al. 2010), individuals specialized in shared resources may be more subject to interspecific competition than individuals feeding on unshared resources, potentially causing intrapopulation differences in fitness (e.g., Evangelista et al. 2014). It is also important to highlight the presence of other predatory species in Lake Gala, namely S. glanis and E. lucius, which could also compete with S. lucioperca or predate on juveniles of P. fluviatilis.

A previous study investigating the effects of the introduction of S. lucioperca in a German lake found limited dietary niche overlap with perch, together with a compressed dietary niche in this latter species (Schulze et al. 2012). The authors explained these findings with increased interspecific competition, which led to a niche shift in P. fluviatilis. A niche shift in P. fluviatilis was also suggested by Fobert et al. (2011) following the introduction of a competitive species such as L. gibbosus. Another previous study using SIA from three sampling years found isotopic niche partitioning between perch and pikeperch young-of-the-year in 2 years and niche overlap in one (Westrelin et al. 2023). Indeed, the trophic niche characteristics result from multiple complex circumstances, such as intra- and interspecific competition, prey availability and local abiotic variables, thus being subject to temporal (and spatial) variability (Haubrock et al. 2021; Cicala et al. 2024; Balzani et al. 2025). Moreover, Westrelin et al. (2023) found that young-of-the-year P. fluviatilis showed a consistent lower trophic position compared to S. lucioperca. A more detailed understanding of the coexistence

mechanisms of P. fluviatilis and S. lucioperca is possible by paralleling our results with those of Didenko and Gurbyk (2016) and Westrelin et al. (2023).

Several limitations of this study require attention. First, the small number of samples of the two species and the narrow size range of the individuals caught imply that our results should be taken with caution, especially those about the niche metrics (see Syväranta et al. 2013). Moreover, the absence of samples of aquatic invertebrates prevented us from running robust stable isotope mixing models (Parnell et al. 2013) to reconstruct the diet of the two species. Such data, together with stomach content analysis on a robust sample number (also not performed), could have provided a more comprehensive picture of the trophic interactions, particularly for P. fluviatilis, known to consume invertebrates more readily than S. lucioperca (Schulze et al. 2012). Finally, our study includes only one site per ecosystem type, so that the effect of this could be described but not properly tested. For example, the two ecosystems differed in a number of other variables (e.g., higher number of fish species from high trophic levels in Lake Gala; see Table S1 in Toutain et al. 2024), which could represent confounding effects.

## 5 | Conclusions

Our results partially align with our expectations and provide insights into how different ecosystems and species life history may drive resource competition in co-occurring mesopredatory fish populations. In particular, the invasive potential of P. fluviatilis has been recognized as a threat to native biodiversity due to its aggressive feeding behavior and competitive advantage (Beeck 2003). Proactive measures are essential to mitigate the risks associated with perch introductions, particularly given the recent surge in popularity of perch as an angling target, which has led to widespread introductions in reservoirs (Tarkan et al. 2023). The active involvement and support of local and national angling clubs indicate that perch may continue to spread rapidly in the coming years, carrying significant ecological implications. Given that niche overlap and resource competition can lead to significant ecosystem changes, managing fish populations in non-native environments requires careful consideration of potential ecological disruptions. In scenarios where niche overlap is high, the introduction of mesopredators like S. lucioperca may intensify competition.

## Author Contributions

Conceptualization: Paride Balzani. Methodology: Irmak Kurtul, Sadi Aksu, and Özgür Emiroglu. Formal analysis and investigation: Mathieu Toutain. Writing – original draft preparation: Mathieu Toutain. Writing – review and editing: Phillip J. Haubrock, Nadège Belouard, and Ali Serhan Tarkan. Funding acquisition: Sadi Aksu. Resources: Sadi Aksu; Supervision: Paride Balzani, and Ali Serhan Tarkan.

## Acknowledgments

This work has been supported by Eskişehir Osmangazi University Scientific Research Projects Coordination Unit under grant number FCD-2021-2146. Part of this work was carried out with the support of VVI CENAKVA Research Infrastructure (ID 90238, MEYS CR, 2023-2026). Open access publishing facilitated by Jihoceska Univerzita v Ceskych Budejovicich, as part of the Wiley - CzechELib agreement.
Conflicts of Interests

The authors declare no conflicts of interest.

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**Supporting Information**

Additional supporting information can be found online in the Supporting Information section.

**Figure S1:** Distribution of the directional probability of an individual of a species (a-d: *Perca fluviatilis*; e-h: *Sander lucioperca*) to occur within the 95% or 40% niche of the other species. Blue distributions correspond to the *S. lucioperca* niche and yellow distributions correspond to the *P. fluviatilis* niche. Tiles a, c, e and g correspond to the Gala lake and tiles b, d, f and h correspond to the Sığırcı reservoir. Dashed lines correspond to the 95% or 40% confidence interval and full lines correspond to the mean. **Table S1:** Morphometric measures (total length and weight) of Perca fluvatilis and Sander lucioperca in the two study sites.