# Species-specific drivers explain fish feeding and individual niche variation

Javier Sánchez-hernández, Iñaki Fernández de Larrea, Ioar de Guzmán, José M González, Aitor Larrañaga

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Javier Sánchez-hernández, Iñaki Fernández de Larrea, Ioar de Guzmán, José M González, Aitor Larrañaga. Species-specific drivers explain fish feeding and individual niche variation. Journal of Fish Biology, 2025, 107, pp.1420 - 1432. 10.1111/jfb.70136. hal-05502588

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Submitted on 10 Feb 2026

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# Species-specific drivers explain fish feeding and individual niche variation

Javier Sánchez-Hernández<fnref n="1" />,<fnref n="2" /> ID |

Iñaki Fernández de Larrea<fnref n="3" /> ID |

loar de Guzmán<fnref n="3" />,<fnref n="4" /> ID |

José M. González<fnref n="1" />,<fnref n="2" /> ID |

Aitor Larrañaga<fnref n="3" /> ID |

Correspondence

Javier Sánchez-Hernández, Área de Biodiversidad y Conservación, Departamento de Biología y Geología, Física y Química Inorgánica, Universidad Rey Juan Carlos (URJC), Madrid, Spain.

Email: javier.sanchezh@urjc.es

Funding information

Ministerio de Ciencia, Innovación y Universidades, Grant/Award Numbers: PID2020-115830GB-100 TED2021-129966B-C31, TED2021-129966B-C31; Basque Government, Grant/Award Number: IT1471-22

Abstract

The trophic ecology of animals often results from complex interactions between environmental and biological drivers. Yet, studies that explore trophic ecology across multiple fish species while considering a wide range of environmental variables remain limited. In this study, we analysed diet and niche variation in fish communities across five protected areas in the northern Iberian Peninsula, examining their responses along environmental and biological gradients. Our multiple regression analyses showed that both environmental and biological drivers significantly influence fish feeding behaviour and individual specialisation, but these effects are species-specific. We propose that the primary mechanisms shaping fish feeding strategies, whether benthic foraging or surface-drift feeding, are determined by intrinsic factors such as species-specific feeding behaviours and ontogenetic stage, along with benthic macroinvertebrate density and fish density. Specifically, benthic macroinvertebrate density positively affected individual niche specialisation in brown trout (*Salmo trutta*) and Pyrenean stone loach (*Barbatula quignardi*), whereas it negatively impacted the Pyrenean minnow (*Phoxinus bigerri*). An increase in total fish density, measured as the number of fish per square meter, negatively influenced individual niche specialisation of brown trout. However, brown trout density had a positive effect on individual niche specialisation of Pyrenean stone loach and Pyrenean minnow. The findings have direct implications for conservation and restoration efforts, highlighting the importance of increasing habitat heterogeneity to meet species-specific habitat needs and promoting benthic macroinvertebrate production to support fish feeding requirements.

KEYWORDS

diet, fish, niche variation, riverine-dwelling populations

1 | INTRODUCTION

Trophic ecology of animals is well documented because it provides a comprehensive understanding of ecological interactions and

processes (Hanley & La Pierre, 2015), although there is limited information on the mechanisms driving spatial variations in the feeding behaviour (e.g., Lunghi et al., 2020; Sánchez-Hernández et al., 2021a). The current literature indicates that feeding is a highly flexible

This is an open access article under the terms of the [Creative Commons Attribution License](https://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *Journal of Fish Biology* published by John Wiley & Sons Ltd on behalf of Fisheries Society of the British Isles.

<footnote n="1">¹Área de Biodiversidad y Conservación, Departamento de Biología y Geología, Física y Química Inorgánica, Universidad Rey Juan Carlos (URJC), Madrid, Spain</footnote>

<footnote n="2">²Instituto de Investigación en Cambio Global (IICG), Universidad Rey Juan Carlos, Móstoles, Madrid, Spain</footnote>

<footnote n="3">³Department of Plant Biology and Ecology, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Bilbao, Spain</footnote>

<footnote n="4">⁴Ecologie Comportementale et Biologie des Populations de Poissons (UMR Ecobiop), French National Research Institute for Agriculture, Food and Environment (INRAE), Saint-Pée sur Nivelle, France</footnote>
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functional trait, showing large spatiotemporal variation (Bolnick et al., 2003; Jaeger et al., 2013; Lunghi et al., 2020; Sánchez-Hernández, 2024). Variation in feeding and predator niches has been associated with several drivers such as competitive interactions among sympatric species (Araújo et al., 2011; Costa-Pereira et al., 2018), which can be framed within the niche variation hypothesis suggesting that individuals within populations tend to display niche expansion when they are released from interspecific competition (Van Valen, 1965). Other drivers of predator niche variation include population density as a proxy of intraspecific competition (e.g., Araújo et al., 2011; Svanbäck & Bolnick, 2007; Tinker et al., 2012; Ward et al., 2006), prey availability (e.g., Tinker et al., 2008; Araújo et al., 2011; Sánchez-Hernández et al., 2021a), predation risk (Araújo et al., 2011) and morphological constrains linked to body size (Sánchez-Hernández et al., 2021a). Thus, feeding patterns are often a result of complex interactions between environmental and biological drivers (e.g., Clavero et al., 2003; de Camargo et al., 2014; Sánchez-Hernández, Finstad, et al., 2019).

Bottom-up mechanisms have been commonly considered by researchers as an important mechanism explaining trophic niche and feeding patterns of fish species (Sjödin et al., 2018; Sánchez-Hernández et al., 2021a). Most fish species exhibit flexible feeding behaviours, adjusting their diets to take advantage of pulsed resources, such as the high availability of terrestrial arthropods during summer, when the aquatic invertebrate biomass is usually low, or the emergence of aquatic insects peaking around spring and early summer in temperate systems when the peak of terrestrial invertebrates has not occurred yet (Nakano & Murakami, 2001; Sweeney & Vannote, 1982). The ability to forage at different depths in the water column has been recognised as key to understand differences in feeding (bottom-autochthonous vs. surface-allochthonous feeding) at individual and population levels and coexistence of sympatric species in stream-dwelling fish communities through food resource partitioning (Horká et al., 2017; Nakano et al., 1999; Sánchez-Hernández et al., 2016). For example, when benthic macroinvertebrates are more abundant and accessible than surface prey, riverine brown trout (*Salmo trutta*) is unlikely to specialise in surface-drift foraging (Sánchez-Hernández & Cobo, 2018a). Many researchers have also provided evidence of the importance of morphological constraints on feeding through ontogeny to understand feeding of fish species (Anaya-Rojas et al., 2023; Sánchez-Hernández, Nunn, et al., 2019). In this regard, the most common ontogenetic dietary shifts in apex fish species, such as salmonids, involve an increased consumption of allochthonous resources (terrestrial invertebrates) and fish prey (Sánchez-Hernández, 2024 and references therein). In contrast, riverine populations of omnivorous cyprinid species, such as the northern Iberian chub (*Squalius carolitertii*), show a decrease in detritus consumption over the course of their ontogeny (Sánchez-Hernández & Cobo, 2012).

Additionally, physical habitat characteristics (e.g., flow velocity combined with habitat complexity) can affect feeding conditions, as fish individuals commonly choose feeding patches with current velocities that allow them to maintain their position and feed effectively (Orlov et al., 2006). For example, higher substratum roughness

reduces the conspicuousness and accessibility of benthic invertebrates to brown trout (De Crespin Billy & Usseglio-Polatera, 2002). Understanding the drivers of fish feeding allows ecologists to better predict how environmental changes, such as climate change, pollution and habitat destruction, will impact both their population and food webs and may be useful for fishery management practices. However, research on this topic is typically biased towards common model organisms and socio-economically significant stream-dwelling species, such as salmonids (e.g., Evangelista et al., 2014; Orlov et al., 2006; Sánchez-Hernández & Cobo, 2018b), as well as other fish species of socio-economic importance inhabiting lacustrine and marine environments (e.g., Hayden et al., 2019; Sánchez-Hernández et al., 2021b). To identify general patterns in the drivers of fish feeding, it is essential to expand studies to include a broader range of species, particularly omnivorous and endemic species, beyond just predatory fish.

In the present study, the primary objective was to identify the main environmental and biological drivers of feeding and individual niche variation in a diverse stream-dwelling fish community in northern Iberian Peninsula, including both omnivorous species such as Pyrenean minnow (*Phoxinus bigerri*) and predator species such as brown trout and Pyrenean stone loach (*Barbatula quignardi*). With this, the current study represents an innovative attempt to identify the relative importance of multiple environmental and ecological factors in shaping dietary patterns, population niche width and individual specialisation across fish taxa based on the following hypotheses:

**Hypothesis 1.** Brown trout heavily relies on terrestrial invertebrates (Sánchez-Hernández & Cobo, 2018a, 2018b), whereas Pyrenean stone loach primarily forages on aquatic invertebrates (Oscoz et al., 2004), and Pyrenean minnow has omnivorous behaviour, consuming both animal and non-animal resources (Oscoz et al., 2001, 2008). These differences in diet composition are likely to be accompanied by variations in dietary specialisation at both individual and population levels, with omnivorous species expected to exhibit broader population-level niche widths.

**Hypothesis 2.** Feeding differences among fish species are driven by species-specific environmental and biological drivers. Overall, low environmental habitat complexity and high availability of aquatic prey drive species towards benthic or aquatic feeding (De Crespin Billy & Usseglio-Polatera, 2002; Orlov et al., 2006; Sánchez-Hernández & Cobo, 2018a). However, because fish species differ in body length, feeding strategies (predatory vs. omnivore) and habitat preferences (e.g., water column use) (e.g., Prenda et al., 1997; Doadrio, 2001), environmental and biological drivers may not affect all species equally. For example, fish length, as a proxy of ontogenetic mechanisms, may have a stronger impact on salmonids because they achieve larger maximum body length allowing them to increase both drift
<figure><img src="image_4.png" /><figcaption>**FIGURE 1** Map of the study area showing sampling locations in the five protected areas. Dotted and grey areas are the catchments of riverine systems and the protected areas, respectively.</figcaption></figure>

foraging ability and surface feeding over ontogeny (Sánchez-Hernández, Nunn, et al., 2019).

**Hypothesis 3.** Differences in individual niche variation may vary among fish species. High fish densities as a proxy of inter- and intraspecific competition would lead weaker competitors to specialise on suboptimal resources and chiefly promote niche variation among individuals within a population, but the effect of intra- and interspecific competition on niche variation can be variable (see Araújo et al., 2011; Sánchez-Hernández et al., 2021a). In this sense, fish density is likely to be particularly relevant for salmonid species, as they typically exhibit density-dependent effects (Grossman & Simon, 2020).

# 2 | METHODS

## 2.1 | Study area

We selected five riverine systems located within five protected areas of the Basque Country and Navarre in northern Iberian Peninsula

(Figure 1). Three of these catchments (Artikutza, Izki and Gorbea) are almost entirely protected; thus they only suffer from mild anthropic perturbations, such as forest exploitation and extensive livestock. Meanwhile, Aizkorri and Aralar catchments are protected only at the highlands; thus anthropic pressure intensifies at lower lands, where villages are also found (Appendix S1).

## 2.2 | Data collection

Environmental variables, prey availability and fish sampling were spatially replicated (5-10 replicates) in each of the five protected areas from 15 June 2022 to 14 July 2022.

## 2.3 | Environmental drivers

Data on riparian forest and habitat complexity were gathered across all protected areas (Appendix S1). Riparian forest quality was described using the Adapted Riparian Quality Index (URA, 2021), and canopy cover percentage was described by means of a Spherical Crown Densiometer (Bain & Stevenson, 1999). Fluvial habitat
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complexity was assessed following the Stream Heterogeneity Index (IHF, Pardo et al., 2002; Valero et al., 2015) and describing the granulometry (Wolman, 1954). The IHF, with values up to 100, evaluates the habitat heterogeneity and physical variables of the stream channel that are influenced by hydrology and substrate composition (see Pardo et al., 2002 for further details). This index has been widely employed by Spanish water agencies and researchers to evaluate river habitat quality (e.g., Fernández et al., 2011; Valero et al., 2015). For describing granulometry, we randomly collected 100 riverbed substrates in each sampling site and assigned them to the size categories established by Wolman (1954). The median values obtained in each site were used as descriptor of the riverbed granulometry.

## 2.4 | Biological drivers

Availability of potential aquatic prey was estimated from five randomly located 0.09 m² Surber samples of benthic invertebrates collected at each site (e.g., Sánchez-Hernández & Cobo, 2018b; Sánchez-Hernández et al., 2021a). All invertebrates retained in a 1-mm sieve were identified to family following Tachet et al. (2010). Benthic macroinvertebrates density (ind./m²) and taxa richness (the number of families) were calculated. Moreover, macroinvertebrate densities were combined with the affinity of each family to the locomotion traits from Tachet et al. (2010) to calculate swimmer, crawler and sessile macroinvertebrate densities.

Fishes were collected using pulsed D.C. backpack electrofishing equipment (Hans Grassl GmbH, IG200), identified to species level, counted and measured (fork length) to the nearest 1 mm. The density (fish per square meter) of each fish species was roughly calculated using a single-pass electrofishing to avoid resource-intensive sampling. Four fish species were captured, including brown trout, Pyrenean stone loach, Pyrenean minnow and European eel (Anguilla anguilla). Individuals were returned to the river, except for a subset covering a subsample representative of a length-frequency distribution of each population, which were immediately killed using cerebral concussion followed by an overdose of anaesthetic (protocols were approved by the Animal Care Committee of the University of the Basque Country-M20/2022/136). Fish individuals were stored at -20°C for later laboratory analysis of their feeding habits. Although the European eel was captured at several sampling sites, individuals were only euthanised in one river system (Aralar), and it was dropped from subsequent analyses. Pyrenean stone loach was only captured in three out of five protected areas. In total, 345 individuals were used for feeding analyses. Because fish populations are often regulated by density-dependent mechanisms (Grossman & Simon, 2020; Henderson & Magurran, 2014), fish density – both within and among species – can serve as an indicator of intra- and interspecific competition, respectively (e.g., Hasegawa, 2016; Sánchez-Hernández et al., 2021a). To capture this, we considered total fish density within communities and further partitioned it into species-specific densities for brown trout, Pyrenean stone loach and Pyrenean minnow as proxies for intra- and interspecific competition.

## 2.5 | Feeding analyses

The 345 guts were opened to first visually determine the total fullness, ranging from empty (0%) to full (100%) according to the relative fullness method (Amundsen & Sánchez-Hernández, 2019), and to assign then relative contribution of each food resource to the total stomach fullness. Food resources were classified as aquatic prey, surface prey (i.e., terrestrial arthropods and emerged aquatic insects), non-animal (detritus and vegetal rest) and fish remains (scales). Thus, prey categories were also used to inform about foraging modes (benthic and surface-drift foragers), assuming that benthic foraging included individuals feeding mainly on aquatic prey, whereas individuals performing surface-drift foraging feed chiefly surface prey (e.g., Sánchez-Hernández et al., 2016; Sánchez-Hernández & Cobo, 2018a, 2018b). Animal prey items in each gut were identified to the same taxonomic level (mostly family, Appendix S2) as prey availability.

Niche width at the population level was calculated using Levins' index. The proportional similarity index (PSi) was calculated to study the individual dietary specialisation (Bolnick et al., 2002). This index compares each individual's diet with the diet at the population level, with values ranging between 0 and 1. For individuals specialising on a single or few prey items, the PSi values tend to be low, whereas for individuals that consume resources in a similar proportion as the entire population, the PSi values approach 1 (Bolnick et al., 2002).

## 2.6 | Statistics

All data analyses were conducted in R version 4.3.1 (R Core Team, 2023). Dietary niche variation at the population (Levins') and individual (PSi) levels was calculated using the *RinSp* package (Zaccarelli et al., 2013). Graphical visualisations were conducted using the *ggplot2* package version 3.5.1 (Wickham, 2016) and *rstatix* package version 0.7.2 (Kassambara, 2023) to perform Tukey's post hoc tests related to fish feeding and individual niche variation (hypothesis 1).

We employed multiple regression modelling using generalised linear models (GLMs) with quasi-binomial distribution and the default logit link function to examine the effects and identify the most influential environmental and biological drivers shaping feeding patterns (hypothesis 2) – specifically, the proportions of aquatic and surface prey – and individual niche variation (hypothesis 3) across the three fish species. In total, we fitted nine models (three response variables × three species), using trophic niche components – proportion of aquatic prey, proportion of surface prey and PSi – as response variables. Explanatory variables included environmental factors (canopy cover, substrate granulometry and IHF) and biological factors (benthic macroinvertebrate richness, total macroinvertebrate density, crawler, swimmer and sessile macroinvertebrate densities, total fish density, species-specific fish densities and fish length).

Prior to modelling, variance inflation factors (VIF) were calculated to assess multicollinearity, that is, correlations among predictor
<figure><img src="image_6.png" /><figcaption>**FIGURE 2** Estimated means for the proportion of different prey categories in the gut contents for brown trout (*Salmo trutta*), Pyrenean minnow (*Phoxinus bigerri*) and Pyrenean stone loach (*Barbatula quignardi*). Error bars represent 95% confidence intervals. Only statistically significant pair-wise comparisons are reported (*p* < 0.05; **p** < 0.01; ***p** < 0.001).</figcaption></figure>

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had clearer effects on larger and more mobile fish species such as brown trout and Pyrenean minnow.

## 4.1 | Feeding and niche variation

Our first hypothesis predicting that fish species exhibit distinct feeding strategies leading to differences in niche variation was supported by the results. Consistent with previous studies, we found that brown trout predominantly consumed surface prey (Sánchez-Hernández & Cobo, 2018a, 2018b), Pyrenean stone loach primarily fed on aquatic macroinvertebrates (Oscoz et al., 2004), and Pyrenean minnow

exhibited an omnivorous diet, incorporating both animal and non-animal resources (Oscoz et al., 2001, 2008). It should be kept in mind that the Pyrenean stone loach is benthivore and primarily occupies microhabitats such as sheltered substrate areas for feeding and avoiding predators (Prenda et al., 1997), suggesting that its feeding behaviours and ecological interactions are highly localised and specific to certain types of habitats. Thus, these results are in accordance with the proposal made by Worischka et al. (2015) that diet in benthivore fish species is better linked to benthic macroinvertebrate metrics (macroinvertebrate size, abundance and locomotion mode) than to habitat complexity (microhabitat and current velocity preference). It is possible that this kind of species (small, benthivore and little mobile),
<figure><img src="image_8.png" /><figcaption>FIGURE 3 Estimated means for individual diet specialisation quantified as the proportional similarity index (PSi) of the diet utilisation for brown trout (*Salmo trutta*), Pyrenean minnow (*Phoxinus bigerri*) and Pyrenean stone loach (*Barbatula quignardi*). Error bars represent 95% confidence intervals. Low PSi values indicate a high level of individual specialisation. Only statistically significant pair-wise comparisons are reported (*p* < 0.05; **p** < 0.01; ***p** < 0.001; ****p** < 0.0001).</figcaption></figure>

such as loaches (e.g., *Cobitis* spp. and *Barbatula* spp.) and other small fish (e.g., *Cottus* spp. and *Gobio* spp.), requires approaches at micro- and meso-habitat scales to disentangle the role of environmental and biological drivers in its feeding (Copp & Vilizzi, 2004; MacKenzie & Greenberg, 1998; Wegscheider et al., 2020), scale resolutions that have not been considered in the design of the current study.

## 4.2 | Species-specific drivers explaining feeding

Our second hypothesis was also supported, as species-specific drivers accounted for the observed dietary differences among the three fish species. Our models reveal the positive effect of benthic macroinvertebrate density on feeding on aquatic prey by both brown trout and Pyrenean minnow. This outcome is supported by previous studies showing the link between prey availability and fish diet (Kreiling et al., 2021; Sánchez-Hernández & Cobo, 2018a): when aquatic macroinvertebrates are more abundant and accessible than surface prey, brown trout individuals do not specialise on surface prey (surface-drift foraging) and feed mainly on aquatic prey. In addition, we observed that the consumption of aquatic prey by Pyrenean minnow was negatively associated with density of sessile prey, demonstrating

that sessile macroinvertebrates seemed to be avoided by this omnivorous fish species. We posit that its feeding depends on the ability to detect either sessile macroinvertebrates or more preferred food resources when available.

We observed a significant effect of total fish density only on the feeding of brown trout: as total fish density increased, aquatic feeding increased and terrestrial feeding decreased. Theory predicts that fish densities (as a proxy of inter- and intraspecific competition) have an impact on feeding and niche variation in stream-dwelling salmonids (Boujard et al., 2002; Karjalainen, 1992; Sánchez-Hernández & Cobo, 2018b). For example, Sánchez-Hernández and Cobo (2018b) observed that increasing brown trout density was associated with a reduced proportion of surface prey in their diet. Notably, although most salmonid species have shown evidence of density dependence in growth, mortality, fecundity and recruitment (Grossman & Simon, 2020), our findings indicate that such effects may not uniformly extend to feeding dynamics in this context. That is, we found no significant effect of brown trout density – used as a proxy for intraspecific competition – on feeding behaviour; but total fish density had a negative influence on surface-drift foraging. This suggests that interspecific competition might play a more prominent role than intraspecific interactions in shaping brown trout feeding strategies,
<table><caption>TABLE 1 Summary table of the best model simulations for the proportional similarity index (PSi) and proportions of aquatic and surface prey for each fish species according to the model selection method (automated backward stepwise).</caption><thead><tr><th colspan="4">Environmental drivers</th><th colspan="4">Biological drivers</th></tr><tr><th>Variable</th><th>Intercept</th><th>Granulometry</th><th>Canopy cover</th><th>IHF</th><th>Benthic macroinvertebrate richness</th><th>Benthic macroinvertebrate density</th><th>Benthic macroinvertebrate density (swimming)</th><th>Benthic macroinvertebrate density (sessile)</th><th>Total fish density</th><th>Brown trout density</th><th>Fish length</th></tr></thead><tbody><tr><td rowspan="4">ST</td><td>Aquatic</td><td>1.399*</td><td>—</td><td>—</td><td>—</td><td>—</td><td>0.002**</td><td>—</td><td>—</td><td>-1.723*</td><td>—</td><td>-0.018**</td></tr><tr><td>Surface</td><td>-1.012</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>-0.011*</td><td>—</td><td>0.411</td><td>—</td><td>0.014**</td></tr><tr><td>PSi</td><td>-0.205</td><td>-0.009***</td><td>-0.009</td><td>0.012</td><td>-0.059*</td><td>0.002***</td><td>-0.004</td><td>0.001</td><td>-1.077**</td><td>—</td><td>—</td></tr><tr><td>PSL</td><td>4.803</td><td>—</td><td>—</td><td>—</td><td>-0.033</td><td>—</td><td>—</td><td>—</td><td>-1.365</td><td>—</td><td>—</td></tr><tr><td rowspan="4">PM</td><td>Aquatic</td><td>24.900</td><td>—</td><td>—</td><td>-1.206</td><td>—</td><td>0.031</td><td>—</td><td>—</td><td>—</td><td>-11.180</td><td>—</td></tr><tr><td>Surface</td><td>1.890</td><td>—</td><td>-0.022</td><td>—</td><td>—</td><td>-0.004***</td><td>0.0184**</td><td>0.005*</td><td>—</td><td>13.232***</td><td>—</td></tr><tr><td>PSi</td><td>1.343**</td><td>-0.009**</td><td>—</td><td>—</td><td>—</td><td>0.001*</td><td>—</td><td>-0.005*</td><td>—</td><td>—</td><td>—</td></tr><tr><td>PSL</td><td>-1.602</td><td>-0.011</td><td>0.039</td><td>0.0271</td><td>—</td><td>-0.001</td><td>0.009</td><td>0.001</td><td>-0.154</td><td>1.361</td><td>-18.860**</td><td>0.002</td></tr><tr><td></td><td>PSi</td><td>-1.094***</td><td>—</td><td>—</td><td>—</td><td>—</td><td>-0.001***</td><td>—</td><td>—</td><td>—</td><td>—</td><td>3.289**</td><td>—</td></tr></tbody></table>

Note: The parametric coefficients with significance values are given for each variable.

Abbreviations: IHF, Stream Heterogeneity Index; PM, Pyrenean minnow (Phoxinus bigerni); PSi, the proportional similarity index; PSL, Pyrenean stone loach (Barbatula quignardi).

*p < 0.05, **p < 0.01, ***p < 0.001.
<figure><img src="image_9.png" /><figcaption>**FIGURE 4** Variance explained by each driver for brown trout (*Salmo trutta*), Pyrenean minnow (*Phoxinus bigerri*) and Pyrenean stone loach (*Barbatula quignardi*) with respect to aquatic prey, surface prey and the proportional similarity index (PSi). Crawling = density of crawling macroinvertebrates; Density = macroinvertebrate density; Fish = fish density; Granulometry = substratum roughness; IHF = Stream Heterogeneity Index; Length = fork length; Richness = benthic macroinvertebrate richness; Sessile = density of sessile macroinvertebrates; Swimming = density of swimming macroinvertebrates; TreeCover = canopy cover; Trout = brown trout density.</figcaption></figure>

particularly in their preference between benthic and surface-drift prey as indicated by the findings of the present study.

Only brown trout showed an influence of fish length on the feeding. Salmonids commonly exhibit size-structured dominance hierarchies, where larger individuals dominate smaller ones, leading to differences in feeding behaviour between dominant and subordinate individuals (e.g., Kohda et al., 2008; Nakano, 1995; Nakano et al., 1999). The observed pattern, that is, the increasing reliance of allochthonous resources (terrestrial invertebrates) with ontogeny, matches with previous studies focusing on brown trout (Sánchez-Hernández & Cobo, 2016) and on stream-dwelling salmonids in general (reviewed by Sánchez-Hernández, 2024). In contrast, the lack of ontogenetic effect on the feeding of Pyrenean stone loach and Pyrenean minnow may be attributable to morphological constraints linked to mouth gape limitation, which prevents the consumption of alternative and bigger prey types, and the limitations when capturing large individuals. A fish's ability to handle prey generally scales with mouth

gape size, which in turn typically scales with its body size (Sánchez-Hernández, Nunn, et al., 2019). Specifically, Pyrenean stone loach, a small benthivore species, likely does not change diet composition through ontogeny but changes occur depending on prey size. Although we can attribute the lack of any ontogeny shift to the fact that we did not observe the largest individuals recorded for this species (120 mm of maximum body length; Leunda et al., 2017), Oscoz et al. (2006) also observed a similar diet composition of this species irrespective of body size.

## 4.3 | Species-specific drivers explaining individual niche variation

Our third hypothesis predicting that drivers of individual specialisation are species specific was also supported; as, for example, our models found that total fish density had a negative influence on PSi in brown
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**FIGURE 5** Generalised linear models (GLMs) with quasi-binomial distribution explaining the association between feeding (aquatic and surface prey) of brown trout and its significant drivers. Note panels only included the statistically significant predictors based on Table 1.</figcaption></figure>

trout, but brown trout density had a positive influence on PSi of Pyrenean stone loach and Pyrenean minnow. This supports the view that resource competition promotes niche variation among individuals within a population (e.g., Araújo et al., 2011; Svanbäck & Bolnick, 2007; Costa-Pereira et al., 2019; Sánchez-Hernández et al., 2021a). More specifically, species-specific differences found in the current study may be linked to the differences in the inherent feeding habits of each fish species, with less-important effects of fish density

on niche variation in omnivorous fish species (as it is the case for the Pyrenean minnow) compared to those species relaying exclusively on animal resources.

We also found that PSi of brown trout increased with macroinvertebrate density but decreased with macroinvertebrate richness. This finding partially supports previous research on Scandinavian brown trout populations (Sánchez-Hernández et al., 2021a), which reported a negative influence of benthic macroinvertebrate richness
on PSi, although no statistically significant relationship was observed between benthic macroinvertebrate density and PSi. Behavioural diversification in feeding is generally driven by prey diversity under scenarios with no extensive food resource limitation, whereas prey density may become more important when resource limitations are severe (Sánchez-Hernández et al., 2021a). It is possible that reported macroinvertebrate densities in the current study (224–2032 ind./m², including only macroinvertebrates higher than 1 mm) are low compared to other more productive in similar rivers of northern Iberian Peninsula (Barba et al., 2010), which can lead to individual niche diversification (Araújo et al., 2011; Sánchez-Hernández et al., 2021a).

In brown trout, rougher substrate was linked to a higher individual specialisation. This can be linked to higher substratum roughness reducing the conspicuousness and accessibility of aquatic invertebrates to brown trout (De Crespin Billy & Usseglio-Polatera, 2002), making individuals more specialised in foraging and feeding. In contrast, higher substratum roughness is linked to higher consumption of aquatic prey in Pyrenean minnow, likely as a consequence of its higher reliance on prey attached to the substratum such as Ancylidae (see Appendix S2).

## 5 | CONCLUSIONS

Our study provides a novel way to model the links between environmental and biological drivers and fish feeding in riverine systems, showing species-specific effects to explain differences in diet and individual niche variation in fish. Among all the environmental factors, granulometry was the main driver explaining variations in feeding and individual niche differences in all three species. Among biological factors, benthic macroinvertebrate density emerged as the most frequent driver of fish feeding. Thus, fish conservation and management plans should not only focus on direct actions as stocking but also on restoration works to increase habitat quality to stimulate production of benthic macroinvertebrates. These actions will lead long-term ecological benefits that directly support fish populations by balancing prey availability and resource competition. The ways in which different fish species adapt their feeding strategies to various environmental and ecological drivers underscore the importance of expanding research beyond predatory species, particularly on omnivorous and endemic species. In this regard, the limited published literature on the trophic ecology of species other than brown trout (i.e., Pyrenean minnow and Pyrenean stone loach) has constrained our ability to discuss these species in equal depth. This highlights the need for greater attention to less-studied species, which, despite their lower socio-economic importance, play crucial ecological roles within fish assemblages. Generating restoration actions that target multiple fish species represents a challenge for the next generation of fish ecologists and managers.

### AUTHOR CONTRIBUTIONS

Javier Sánchez-Hernández performed the data analysis and wrote the first draft of the manuscript. All authors commented on previous versions, read and approved the final manuscript.

### ACKNOWLEDGEMENTS

Sincere thanks are due to Arturo Elosegi and Jon Garrastatxu for their assistance in the field sampling, and to María Plaza, Oier Frias and Carlos Jiménez for their skilful laboratory assistance. We also appreciate the constructive feedback given by Jafrin Farha Hussain and another anonymous reviewer.

### FUNDING INFORMATION

This work was supported by the Spanish Ministry of Science, Innovation and Universities through the project PID2020-115830GB-100 (RiMSEC) and the Basque Government (Consolidated Research Group IT1471-22). The authors also acknowledge the Spanish Ministry of Science, Innovation and Universities (I de Guzmán, PID2020-115830GB-100 and TED2021-129966B-C31), as well as financial support from pre-doctoral (I Fernandez de Larrea, PIF21/172) and post-doctoral fellowships (I de Guzman, POS-TUPV24/47) from the University of the Basque Country.

### CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

### DATA AVAILABILITY STATEMENT

Data will be made available on reasonable request.

### ORCID

Javier Sánchez-Hernández https://orcid.org/0000-0001-9684-4774

Iñaki Fernández de Larrea https://orcid.org/0000-0002-5910-6905

Ioar de Guzmán https://orcid.org/0000-0001-8894-8477

José M. González https://orcid.org/0000-0001-7289-8321

Aitor Larrañaga https://orcid.org/0000-0002-0185-9154

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## SUPPORTING INFORMATION

Additional supporting information can be found online in the Supporting Information section at the end of this article.

**How to cite this article:** Sánchez-Hernández, J., Fernández de Larrea, I., de Guzmán, I., González, J. M., & Larrañaga, A. (2025). Species-specific drivers explain fish feeding and individual niche variation. *Journal of Fish Biology, 107*(4), 1420-1432. https://doi.org/10.1111/jfb.70136