# Drift Foraging by Allopatric and Sympatric Atlantic Salmon and Brown Trout Parr Under Rapid Flow Fluctuations

Louis Addo, Lise Meneboo, Mahboobeh Hajiesmaeili, John J Piccolo, Johan Watz

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Louis Addo, Lise Meneboo, Mahboobeh Hajiesmaeili, John J Piccolo, Johan Watz. Drift Foraging by Allopatric and Sympatric Atlantic Salmon and Brown Trout Parr Under Rapid Flow Fluctuations. River Research and Applications, 2025, 41 (7), pp.1546-1557. 10.1002/rra.4464. hal-05461606

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RESEARCH ARTICLE

OPEN ACCESS

# Drift Foraging by Allopatric and Sympatric Atlantic Salmon and Brown Trout Parr Under Rapid Flow Fluctuations

Louis Addo¹ ID | Lise Meneboo¹,² ID | Mahboobeh Hajiesmaeili¹ ID | John J. Piccolo¹ ID | Johan Watz¹ ID

¹River Ecology and Management Research Group RivEM, Department of Environmental and Life Sciences, Karlstad University, Karlstad, Sweden | ²Agroécologie—Institut Agro, INRAE, Univ. Bourgogne, Univ. Bourgogne Franche-Comté, Dijon, France

**Correspondence:** Louis Addo (louis.addo@kau.se)

**Received:** 12 December 2024 | **Revised:** 13 May 2025 | **Accepted:** 20 May 2025

**Funding:** This work was supported by the Svenska Forskningsrådet Formas, Swedish Center for Sustainable Hydropower, and Swedish Energy Agency.

**Keywords:** foraging success | group cohesion | hydropeaking | juvenile salmonid | nearest neighbor distance | Salmo salar | Salmo trutta

## ABSTRACT

Hydropeaking may result in behavioral responses in riverine organisms that experience rapid flow fluctuations. In stream fish, the balance between energy spent on swimming and foraging vs. food intake affects fitness, a balance strongly influenced by flow conditions. This study investigates the effects of rapid flow fluctuations on the foraging success, group cohesion (nearest neighbor distance) and microhabitat segregation of Atlantic salmon (*Salmo salar*) and brown trout (*Salmo trutta*) parr (45–55 mm) in both sympatric and allopatric groups in a controlled laboratory stream environment. Our results indicate that fluctuating flow does not significantly influence the foraging success in either species, regardless of group composition (sympatry or allopatry). We found that group composition, rather than flow fluctuation, significantly influenced group cohesion. Salmon exhibited shorter nearest neighbor distances than trout did in allopatry, but not in sympatry. In terms of microhabitat selection, salmon consistently held position closer to the substrate than trout, regardless of group composition, suggesting selective segregation as the main mechanism driving microhabitat selection. Trout, on the other hand, held position higher above the substrate in allopatry but used a wider range of positions when in sympatry with salmon, highlighting interactive segregation as a possible mechanism driving microhabitat use, which underscores the importance of fish community in shaping patterns of habitat use. Our findings suggest that hydropeaking may not strongly impair the foraging ability and sociability of salmon and trout parr. Additionally, we shed light on the mechanisms influencing microhabitat selection of salmon and trout parr in rivers with a hydropeaking flow regime. One possible application of our results is as inputs for individual-based models currently being developed for understanding the effects of hydropeaking on stream salmonid populations.

## 1 | Introduction

The growing reliance on hydropower for clean and cheap energy is driven by the increasing demand for electrical energy from urbanization, modernization, and a rising global population (Bejarano et al. 2018). Hydropeaking, i.e., an operational mode of hydropower plants involving rapid changes in stream flow, is used for maintaining electrical grid stability and offers the flexibility needed to meet sub-daily variations in energy

demand (Haas et al. 2017; Quaranta et al. 2022). However, the rapid fluctuations in flow due to hydropeaking have been shown to impact physical and biological aspects of fluvial ecosystems, including negative effects on many riverine organisms (Bipa et al. 2024; Moreira et al. 2019).

In the boreal region of northern Europe, where hydropower contributes a significant part of the energy mix, the conservation of salmonid populations is often a critical concern when

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<footnote>© 2025 The Author(s). *River Research and Applications* published by John Wiley & Sons Ltd.</footnote>
hydropower companies need to consider their environmental effects (Burman et al. 2021). Typically, Atlantic salmon (*Salmo salar*) and brown trout (*Salmo trutta*), which often coexist, are the target salmonid species because of their high socio-economic and cultural relevance (Butler et al. 2009; Pokki et al. 2018; Liu et al. 2019; Andersson et al. 2020; Watz et al. 2022). These two species have been previously studied concerning the impacts of flow changes on various life stages, including eggs (Casas-Mulet et al. 2015), fry (Addo et al. 2023), parr (Berland et al. 2004; Robertson et al. 2004), smolts (Alexandre and Palstra 2017) and spawners (Vollset et al. 2016). However, thus far, few studies have considered the behavioral responses, including drift-foraging ability and sociability of juvenile salmonids to rapid flow fluctuations resulting from hydropiking (Armstrong 2010; Enders et al. 2012; Höjesjö et al. 2010; Larranaga et al. 2018; Stradmeyer et al. 2008; Watz et al. 2020).

Juvenile salmonids are considered to be particularly vulnerable to the effects of hydropiking due to their preference for shallow river areas and their limited ability to respond to fast changes in water level and very high currents (Moreira et al. 2019), making them susceptible to stranding (Nagrodski et al. 2012; Saltveit et al. 2001) and downstream displacement (Auer et al. 2017). Additionally, hydropiking-induced flow fluctuations can scour invertebrates from the riverbeds, altering food availability by reducing the density of drifting invertebrates (De Jalon et al. 1994; Bruno et al. 2016; Kjærstad et al. 2018). This, in turn, may influence growth and energetics (Puffer et al. 2017, 2015), reproduction (Vollset et al. 2016), behavior (Vehanen et al. 2000; Scruton et al. 2003) and mortality (Saltveit et al. 2020) with long-term consequences for population dynamics and conservation (Enders et al. 2017). Because the foraging success, and consequently growth and fitness, of juvenile salmonids strongly depends on streamflow and invertebrate drift (Piccolo et al. 2014), rapid flow fluctuations might be expected to alter not only energy intake but also the intra- and inter-specific social interactions among individual fishes that may be competing for foraging positions (Addo et al. 2023; Watz et al. 2020).

Stream salmonids are considered to select microhabitats that maximize their Net Energy Intake rate (NEI) (Hughes and Dill 1990; Jowett et al. 2021), which depends on energy gained from feeding versus energy expended swimming. The successful capture of prey is influenced by physical habitat features such as water depth and velocity (Piccolo et al. 2007, 2008a), prey size (Moss and Beauchamp 2007), prey type (Gustafsson et al. 2010), light intensity (Koski and Johnson 2002), turbidity (Vollset and Bailey 2011) and water temperature (Watz et al. 2014; Watz and Piccolo 2011). Additionally, biological factors such as density of invertebrate drift, fish species and size, and intra- and inter-specific competition and sociality, potentially interacting with the physical habitat, also play a major role in habitat selection and resulting population-level responses (Hearn 1987; Piccolo et al. 2014; Piccolo and Watz 2017). The mechanism underlying habitat segregation may be selective or interactive depending on the range of habitat use (Piccolo et al. 2008a; Berg et al. 2014). For instance, if a fish species uses a wider range of habitats when in allopatry than in sympatry with another species, interactive segregation is said to be the main mechanism influencing habitat segregation. On the other hand, if a species uses the same range of habitats, whether in allopatry or sympatry with another

fish, selective segregation is the mechanism influencing habitat segregation (Berg et al. 2014). Understanding habitat segregation mechanisms and how rapid flow variation, such as those caused by hydropiking, influences microhabitat segregation, foraging success, and growth of these species, therefore, is crucial. This knowledge can inform habitat restoration strategies to mitigate the effects of hydropiking since the possibilities for modifying competitive interaction, whether in selective or interactive segregation, may completely differ (Berg et al. 2014).

Group cohesion is a critical behavior among social fish with strong dominance hierarchies such as salmonids (Ward and Hart 2003), and may impact food consumption, growth, survival, and fitness with potential effects on population dynamics (Fudali and Pietrzak 2024; Hart et al. 2014; Höjesjö et al. 1998; Kelly et al. 2017; Lusardi et al. 2020; Seppä et al. 2001; Watz et al. 2020; Wilson et al. 2014). This influence is evident through mechanisms such as foraging performance (Höjesjö et al. 1998), swimming efficiency (Chen et al. 2016; Wiwchar et al. 2018) and predator avoidance (Griffiths et al. 2003; Ward and Webster 2016). Analyzing the effects of flow changes and competitive environment (sympatry vs. allopatry) on group cohesion and foraging performance may shed light on behavioral responses that potentially influence conservation actions, such as habitat restoration and stocking.

One significant and growing application of foraging behavior studies has been the development of NEI models for stream salmonids (Hughes and Dill 1990; Piccolo et al. 2014). Most recently, NEI theory has been used to develop dynamic Individual-Based Models (IBMs) that address population-level responses to river management, such as instream flow or hydropiking scenarios (Hajiesmaeili et al. 2023, 2024). Comparing traditional instream physical habitat simulation models (PHABSIM) with IBMs, the latter have shown to be more ecologically realistic and provide the possibility to make testable predictions of fish population responses to river management actions (Railback 2016). For instance, inSTREAM and inSALMO version 7-SD are examples of such specialized IBMs designed for evaluating water management scenarios on the population dynamics and smolt production of stream salmonids in rivers with dynamic flow regimes (Railback et al. 2023). IBM development typically relies on empirical data from experiments. For example, parameters in inSTREAM sub-models that describe intra- and interspecific competitive behavior are based on empirical data (Railback et al. 2023). Nevertheless, the understanding of the effects of flow fluctuation on the foraging performance of sympatric and allopatric salmon and trout and microhabitat segregation is limited.

The IBMs inSTREAM and inSALMO 7-SD assume that flow fluctuation does not affect foraging performance and habitat selection; instead, the flow magnitude at each time step determines the flow response. We examined if this assumption holds; specifically, we tested the effects of flow magnitude and fluctuation on the number of captured drifting prey, the nearest neighbor distance (a measure of group cohesion) and the position used above the substratum of sympatric and allopatric young-of-the-year Atlantic salmon (45-55 mm) and brown trout (47-67 mm) parr (hereafter salmon and trout) in a laboratory stream. We measured the responses of juvenile salmon and trout in low
stable, high stable, and fluctuating flow. We hypothesized that fish would capture more prey in both high and low stable than in the fluctuating flow condition. We also anticipated that trout would achieve better foraging performance than salmon due to their size, stemming from earlier emergence. Furthermore, we predicted that group cohesion would be lower (more spread out) in the fluctuating than in stable flows. Additionally, we hypothesized that trout (which is considered relatively more aggressive than salmon; Heggenes et al. 1999) and salmon (Bremset and Heggenes 2001) would segregate spatially, with salmon predominantly holding positions closer to the substratum than trout and that these effects would be stronger in sympatry than in allopatry. Moreover, we expected that this predicted vertical interactive segregation would be more pronounced in stable than in fluctuating flows.

## 2 | Materials and Methods

We used hatchery-reared salmon and trout parr originating from eggs of wild spawning adults of the Gullspång stocks returning from Lake Vänern to River Kläralven, Sweden. These populations are landlocked and endemic (Piccolo et al. 2012). The study was carried out in Karlstad University's stream aquarium facility, Sweden, during July and August 2022. The fish were housed in 80 L holding tanks containing deionized water buffered with $0.1\,g\,L^{-1}$ NaCl and $0.1\,g\,L^{-1}$ NaHCO$_3$. The fish were kept at 11°C and fed a maintenance ration equal to 3% of their body mass per day until they were used in the trials.

We equipped a flow-through aquarium ($length \times width \times depth = 1.30 \times 0.23 \times 0.40\,m$) with two pumps and a funnel and tube system, designed for delivery of food (Figure 1). Water was pumped from and returned to a 500 L container, enabling recirculating water flow in one direction inside the test arena. Plastic foam screens were positioned at the flow inlet (upstream end) to minimize eddies and prevent fish from escaping the

test environment, and a metal mesh screen (downstream sieve) inclined at a 60° angle at the flow outlet (downstream end) prevented fish from escaping and minimized the risk of impingement against the screen. The bottom substrate had a gradient slope of 2.3% and comprised a mixture of gravel (4–9 mm; 67%), small cobbles (10–30 mm; 17%) and larger cobbles (30–300 mm; 17%).

The two pond pumps used ($P1$: $3.3 \pm 0.02\,L\,s^{-1}$, Hailea Pond Pump P25000, Hailea Group Co. Ltd., Guangdong, China; $P2$: $6.12 \pm 0.10\,L\,s^{-1}$, Länspump GLI 50, E.M.S. Teknik AB, Åhus, Sweden) enabled us to create three distinct flow treatments. We categorized these three flow treatments as (1) low stable flow, using only pump $P1$, (2) high stable flow, using both $P1$ and $P2$, and (3) fluctuating flow, which was characterized by alternating flows between low (with $P1$ only) and high (with both $P1$ and $P2$) flow. In the fluctuating flow treatment, flow variation simulated a hydropeaking flow by manually toggling between low (mean depth and water velocity $\pm SD = 19.5 \pm 0.9\,cm$ and $0.11 \pm 0.01\,m\,s^{-1}$ respectively; File S1, Table S2) and high ($25.7 \pm 1.3\,cm$ and $0.21 \pm 0.01\,m\,s^{-1}$ respectively; File S1, Table S2) flows in two complete cycles over a 50 min period (File S1, Figure S1). Since the width of the aquarium was constant, changes in flow will translate into changes in water depth and velocity. Lights were on from 04:00 am to 10:00 pm, corresponding to the sunrise and sunset times in Karlstad, Sweden, during mid-July. The average ($\pm SD$) environmental parameters recorded in the main test arena under low stable, high stable, and fluctuating flow conditions are presented in File S1, Table S1. The physical conditions in our test arena were designed to remain within the natural preference ranges of young salmonid parr in rivers, with water depths between 0.05 and 0.55 m and flow velocities between 0 and $0.4\,m\,s^{-1}$ (Heggenes et al. 1999; Jonsson and Jonsson 2011; Riley et al. 2006). We ensured that the maximum water velocities did not exceed $0.25\,m\,s^{-1}$ to allow for a successful drift-feeding (Heggenes et al. 1999; Heggenes and Traaen 1988).

<figure><img src="image_3.png" /><figcaption>**FIGURE 1** | A schematic diagram of the experimental setup used to test the effects of fluctuating water flows on groups of salmonid parr. The setup was equipped with a thermometer and a video camera to film salmonid behavior. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>
We divided the experimental area into two equal halves: the upstream and downstream arenas (Figure 1). Each arena was further divided into nine equal rectangular cells (measuring 25×9.5cm). Water velocities measured at the midpoint of each cell at high (P1 and P2 on) and low (P1 and P2 off) stable flow conditions are shown in File S1, Table S2.

We randomly selected 132 salmon and 132 trout parr for the experiment, used in 66 trials with three group composition treatments and three flow treatments. After the trial, we euthanized the fish, measured their body mass and total length (mean mass ± SD; salmon mass = 0.71 ± 0.1 g and length = 49.4 ± 2.0 mm; trout mass = 1.14 ± 0.2 g and length = 56.7 ± 3.0 mm) and dissected the stomachs to count the consumed prey items. The larger size of trout compared to salmon is due to their earlier emergence, as trout typically hatch a few weeks before salmon in the river system from which our study fish were acquired (Bjørnås et al. 2023; Hajiesmaeili et al. 2024).

Four individual fish were used in each trial. We used three different group compositions: (1) four trout, (2) four salmon, and (3) two trout and two salmon. Before a trial, the fish were acclimated to the test arena for 30 min (at the low stable flow treatment). During a trial, we administered four chironomids to the test arena five times (after 5, 15, 25, 35 and 45 min; File S1, Figure S1), and we filmed the tanks during the entire trial with video cameras. We maintained consistency in prey availability across treatments by feeding fish constantly with four chironomids at both low and high flows even though peaking flows may lead to catastrophic drift of prey in a natural setting (Bruno et al. 2016). The chironomids drifted with the flow velocity through the water column, allowing fish to intercept them. Uneaten chironomids were transported away from the area unavailable for fish to eat.

Across the three flow treatments and three group compositions, there were no significant variations in mean water pH, oxygen content, conductivity, or light intensity (two-way ANOVAs, *p* > 0.05). A one-way ANOVA conducted to compare the effect of group composition on the coefficient of variation (CV) in length showed no significant differences for the allopatric salmon (mean ± SD: 0.03 ± 0.01) and trout (0.03 ± 0.01) groups (*p* > 0.05), but significantly higher variation for the sympatric groups (0.05 ± 0.01; *F* (2,63) = 24.36, *p* < 0.001, ηₚ² = 0.44). The reason for the differences in variability in length is that young-of-the-year trout are larger than salmon in the Gullspång River (the origin salmonids used in the experiment) because trout eggs hatch about 2 weeks earlier than salmon eggs (Hajiesmaeili et al. 2023).

## 3 | Data Analysis

We considered flow and group composition as independent variables, assessing their effects on (1) number of consumed preys, (2) group cohesion, and (3) distance above the substratum. All analyses were conducted at the group level by using the mean values of the four individuals. We determined the number of consumed prey by counting consumed chironomids after dissecting the stomach of the fish. The foraging

efficiency of the individual that ate the highest number of chironomids in each group (assumed to be the most dominant individual) was assessed. To explore potential dominance hierarchies within groups, we also calculated the coefficient of variation (CV) in prey consumption as a measure of within-group variability. Group cohesion was assessed as the mean nearest neighbor distance between each individual and its group members (Ward and Webster 2016). Differences in the distance above the substratum were assessed as a measure of microhabitat segregation.

All response variables exhibited normal distribution (Kolmogorov-Smirnov tests, *p* > 0.05), and the variances among treatments were similar (Levene's tests, *p* > 0.05). To investigate the effects of flow treatment and group composition on foraging performance (including both the number of prey items eaten and its variability), the number of chironomids consumed by the most dominant individual, sociability (group cohesion), and vertical microhabitat segregation (distance from substratum), we used two-way ANOVAs. For all analyses, we used SPSS 28 (IBM, Armonk, USA).

## 4 | Results

### 4.1 | Foraging Success

Groups ate on average (mean ± SE) 9.3 ± 0.6 out of the 20 chironomids encountered in a trial. Allopatric trout groups ate 10.9 ± 1.2 chironomids, allopatric salmon groups 8.8 ± 0.9, and sympatric groups 8.5 ± 0.8 chironomids. In the low stable flow treatment, most groups ate between 6 and 10 chironomids, whereas groups in the high flow treatment ate between 8 and 10 and in the fluctuating flow between 8 and 12. There was no significant effect of group composition, flow treatment, or their interaction on the number of consumed chironomids (group composition: *F* (2,57) = 1.892, *p* = 0.16, ηₚ² = 0.062; flow treatment: *F* (2,57) = 0.423, *p* = 0.657, ηₚ² = 0.015; group composition × flow: *F* (4,57) = 0.553, *p* = 0.698, ηₚ² = 0.037; Figure 2a). Within the sympatric groups, the trout groups ate on average 6.5 ± 1.9 chironomids in fluctuating flow, 5.6 ± 1.2 in high stable flow, and 2.4 ± 0.8 in low stable flow, whereas the salmon groups ate on average 2.5 ± 1.2 chironomids in fluctuating flow, 4.3 ± 1.2 in high stable flow, and 4.1 ± 1.0 in low stable flow (Figure 3). Flow treatment, species, or their interaction term did not significantly influence the number of chironomids eaten in sympatry (*p* > 0.05; File S1, Table S3). With regard to the foraging efficiency of the most dominant individual within a group, there were no significant effects of group composition, flow treatment, or their interaction (group composition: *F* (2,57) = 1.1, *p* = 0.34, ηₚ² = 0.037; flow treatment: *F* (2,57) = 2.942, *p* = 0.061, ηₚ² = 0.094; group composition × flow: *F* (4,57) = 1.342, *p* = 0.265, ηₚ² = 0.086; File S1, Figure S2).

There were no differences in the coefficient of variability in prey consumed between group compositions (*F* (2,57) = 0.123, *p* = 0.885, ηₚ² = 0.004), between flow treatments (*F* (2,57) = 0.867, *p* = 0.426, ηₚ² = 0.030) or the interaction between group composition and flow treatments (*F* (4,57) = 2.208, *p* = 0.079, ηₚ² = 0.134; Figure 2b).
<figure><img src="image_4.png" /><figcaption>**FIGURE 2** | Boxplot showing (a) number chironomids consumed and (b) coefficient of variation in chironomids consumed by groups of four salmonid parr. The groups consisted of allopatric salmon (AS) (n=22), allopatric trout (BT) (n=22) and mixed groups of both species (MIX) (n=22) across the different flow conditions: low and high stable, as well as fluctuating flow treatments.</figcaption></figure>

## 4.2 | Nearest Neighbor Distance

The allopatric salmon groups had the shortest NND to any fish (mean ± SE) (17.3 ± 0.7 cm), followed by the sympatric groups at 18.4 ± 1.1 cm and the allopatric trout at 22.3 ± 0.9 cm. The results showed that group composition had a significant effect on nearest NND ($F(2,57)=7.331, p=0.001, \eta_p^2=0.205$; Figure 4), and post hoc tests (Bonferroni corrected) revealed a significant difference in the NND between the allopatric salmon and allopatric trout groups ($p=0.002$) and between allopatric trout and the sympatric group ($p=0.022$) but not between the allopatric salmon group and the sympatric group ($p=1.0$). Flow treatment had no effect on the NND ($F(2,57)=0.211, p=0.81, \eta_p^2=0.01$), nor did the interaction term ($F(4,57)=0.343, p=0.848, \eta_p^2=0.024$; Figure 4). Within the sympatric groups, the nearest neighbor distance was not influenced by flow treatment, fish species, or their interaction, regardless of distances from

salmon to salmon or salmon to trout or vice versa ($p>0.05$; File S1, Table S3).

## 4.3 | Height of Fish Above Substrate (Microhabitat Segregation)

The salmon in allopatric groups positioned themselves (mean ± SE) at 1.6 ± 0.2 cm above substrate, whereas the trout in allopatric groups and fish in sympatry positioned themselves at a mean heights of 5.8 ± 0.5 cm and 3.4 ± 0.3 cm, respectively (Figure 5). Results from a two-way ANOVA revealed significant effects of group composition ($F(2,57)=36.021, p<0.001, \eta_p^2=0.558$) and flow treatment ($F(2,57)=4.085, p=0.022, \eta_p^2=0.125$), but not for their interaction term ($F(2,57)=0.41, p=0.801, \eta_p^2=0.028$). A post hoc test with Bonferroni correction revealed a significant difference in the average height of
<figure><img src="image_5.png" /><figcaption>**FIGURE 3** | Boxplot showing the number of chironomids consumed by Atlantic salmon (n = 22) groups and brown trout (n = 22) group in the mixed group of same species across the three flow conditions: Low stable flow, high stable flow and fluctuating flow. The figure shows how the different flow regime influence foraging behavior of both species when grouped together.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**FIGURE 4** | Boxplot showing mean distance between individual fish within groups of four salmonid parr. The groups were allopatric salmon (AS, n = 22), allopatric trout (BT, n = 22) and mixed species groups (MIX, n = 22) under the three different flow conditions: low stable flow, high stable flow, and fluctuating flow treatments. This figure highlights the differences in spatial behavior among the groups in response to varying flow regimes.</figcaption></figure>

fish among all three group compositions (*p* < 0.01). With regard to the flow treatment, significant differences were recorded between fluctuating flow and high stable flow (*p* = 0.044) and between high and low stable flows (*p* = 0.008), while the difference between fluctuating flow and low stable flow was not significant (*p* = 0.501).

There were significant differences in the coefficient of variation in the height above substrate between the group compositions (F (2,57) = 7.7, *p* = 0.001, ηₚ² = 0.213), but not between flow treatments (F (2,57) = 0.392, *p* = 0.677, ηₚ² = 0.014) or their interaction term (F (4,57) = 0.689, *p* = 0.602, ηₚ² = 0.046; Figure 6). A post hoc test with Bonferroni correction revealed a significant difference between allopatric salmon and

sympatric groups (*p* = 0.002) and allopatric trout and sympatric groups (*p* = 0.006) but not between allopatric salmon and trout group (*p* = 0.887).

Within the mixed group, trout occupied positions higher above the substrate than salmon across all three flow treatments (Figure 7). Specifically, trout positioned themselves at (mean ± SE) 4.13 ± 0.8 cm compared to 1.54 ± 0.3 cm for salmon in low stable flow, 6.59 ± 1.5 cm versus 2.1 ± 0.6 cm in high stable flow, and 4.13 ± 0.9 cm versus 2.1 ± 0.8 cm in fluctuating flow. There was a significant effect of fish species on height above substrate within the sympatric group (F (1,38) = 17.15, *p* < 0.001, ηₚ² = 0.31). However, there was no effect of flow treatment (F (2,38) = 1.5, *p* = 0.236, ηₚ² = 0.073) or
<figure><img src="image_7.png" /></figure>

**FIGURE 5** | Boxplot showing the average height of fish above substrate across the different flow treatments for allopatric salmon group (AS, *n* = 22), allopatric brown trout group (BT, *n* = 22) and the mixed species groups (MIX, *n* = 22). The figure illustrates how each group adjusts its vertical positioning in response to low stable flow, high stable flow and fluctuating flow conditions. Each group consisted of four salmonid parr.

<figure><img src="image_8.png" /></figure>

**FIGURE 6** | Boxplot showing the coefficient of variation in average height of fish above substrate for groups of four salmonid parr. Groups were composed of allopatric salmon (AS, *n* = 22), allopatric trout (BT, *n* = 22) and the mixed species groups (MIX, *n* = 22) across the three different flow conditions: low and high stable and fluctuating flow treatments. The figure highlights the variability in vertical positioning behavior across the different flow regimes and group compositions.

the interaction term between fish species and flow treatment (F (2,38) = 1.046 *p* = 0.361, *η*<sup>2</sup><sub>p</sub> = 0.052). A post hoc test with Bonferroni correction revealed that on average, trout positioned themselves significantly higher than salmon across all flow treatments (*p* < 0.01).

5 | Discussion

The experiment presented here was designed to test the ef-
fects of flow and group composition on the behavior (foraging,

sociability and position) of Atlantic salmon and brown trout
parr. Our findings reveal no distinct effects of flow treatment
or group composition on prey consumed by the fish or the num-
ber of prey consumed by the most dominant individual within
a group. Given that the amount of prey consumed is essential
for growth and survival (Elliott 1976), our results indicate that
hydropeaking during the growth period for parr may not sig-
nificantly impact the foraging performance of young sympatric
trout and salmon parr, at least within the range of flows and at
the temperatures used in this experiment. This finding is par-
ticularly interesting considering the natural size advantage of
<figure><img src="image_9.png" /><figcaption>**FIGURE 7** | Boxplot showing the average height of fish above substrate for the Atlantic salmon (n = 22) and brown trout (n = 22) within the mixed species group for the different flow treatments: Low stable flow, high stable flow and fluctuating flow. The figure shows how each species together on average adjust their height above the substrate when coexisting under varying water flow regime.</figcaption></figure>

trout over salmon in the Gullspång River, where trout typically hatch about 2 weeks earlier than salmon (Bjørnås et al. 2023; Hajiesmaeili et al. 2024). This earlier hatching gives trout a developmental head start, often resulting in larger body size during early life stages. Based on this size advantage, we initially expected trout to exhibit superior foraging performance compared to salmon. However, our results did not support this assumption, suggesting that other factors beyond size, such as species-specific foraging strategies or behavioral plasticity, may play a role in determining foraging success under varying flow conditions.

Flodmark et al. (2006) subjected juvenile (parr) brown trout (*Salmo trutta L.*) (158–170 mm) to stable and varying flow in artificial outdoor stream channels. Surprisingly, despite observing a generally poor growth rate, there was no effect of flow treatment on growth. Although our experiment did not measure growth rate, the lacking effect of flow treatment on prey consumed could indirectly indicate that there was no effect of flow treatment on growth, similar to the conclusions made by Flodmark et al. (2006). In a laboratory study, Flodmark et al. (2004) analyzed the effect of fluctuating water levels typically found downstream of a hydroppeaking hydropower plant on juvenile brown trout (107–117 mm). They found higher feeding and growth rates with high stable water levels than with low stable or fluctuating water levels, indicating a potential negative effect of low or fluctuating flows of hydroppeaking on the growth of juvenile brown trout. The discrepancies between our study and that by Flodmark et al. (2004) could be related to differences in fish size (47–67 mm vs. 107–117 mm), food (chironomid vs. dry granulated feed with added ¹³³Cs) and possibly the method of estimating food consumption (counting under microscope vs. ¹³³Cs concentration; Forseth et al. 2001).

Using an outdoor stream channel, Puffer et al. (2015) investigated the effects of hydroppeaking on the growth of allopatric juvenile Atlantic salmon (73–90 mm) during summer and found that fluctuating flows led to reduced growth, as evidenced by

lower final body mass. In a follow-up study, Puffer et al. (2017) introduced juvenile brown trout to examine the additional effects of interspecific competition on Atlantic salmon (60–80 mm). Their findings revealed that hydroppeaking-induced changes in water level leading to stranding were not the only factors affecting growth. Thus, competition alone, even in the absence of stranding, also had negative effects corroborating our findings. Notably, in Puffer et al. (2017), interspecific competition itself significantly contributed to reduced salmon growth. In our study, there was no effect of group composition on food consumption, suggesting that the effect of intra- and interspecific competitive effects on growth was likely similar. Differences in temperature (11 vs. 13°C), flow velocity (0.11–0.21 vs. 0.002–0.81 ms⁻¹), as well as fish size (45–64 vs. 60–80 mm) may potentially have affected the competition (Watz et al. 2019). Furthermore, in a study conducted by Addo et al. (2023) on sympatric salmon (28.6–30.6 mm) and trout (30.8–34 mm) fry, no effect of flow changes on salmon growth was observed, but a significant negative effect on trout growth was observed. This difference between the species could possibly be attributed to differences in body physiology enabling salmon fry to handle rapid flow changes better than trout fry. In the current study, we found no effects of flow; we found no effects of flow magnitude or fluctuations (either as a main effect or as an interaction with group composition) on prey consumption. Given that food consumption is likely more important than swimming cost for the net energy intake rate (Piccolo and Watz 2017), our results indicate a low impact of flow changes on the growth of salmon and trout parr. Similarly, Lauters et al. (1996) found no effect of flow fluctuations on the prey capture success of brown trout (120–200 mm) in a hydroppeaking river. In other experiments conducted in hydroppeaking rivers, flow fluctuations have been shown to increase the amount of prey found in fish gut, resulting in enhanced growth in species such as rainbow trout (*Oncorhynchus mykiss*), brown trout (115–400 mm) (Miller and Judson 2014; Rocaspana et al. 2016) and brook trout (*Salvelinus fontinalis*) (Kelly et al. 2017). The results of these studies, those of Addo et al. (2023) and ours, indicate that there is likely no
general negative effect of hydropкеaking on food consumption
and growth, and when such effects do exist, they are possibly
fish size, life stage, and site-specific.

Our results demonstrated that the group cohesion of juvenile
salmon and trout (measured as the mean distance to the near-
est neighbor) was significantly influenced by group composition
rather than by flow fluctuation or magnitude. Notably, salmon
generally exhibited significantly shorter nearest neighbor dis-
tances than trout, irrespective of whether they were part of allo-
patric or sympatric groups. This difference between the species
may be attributed to the more aggressive behavior of trout rela-
tive to salmon (Höjesjö et al. 2003). Our findings imply that hy-
dropeaking alone may not exert a discernible effect on the group
cohesion of sympatric or allopatric salmon and trout parr. In
contrast, Watz et al. (2020) reported that hydropкеaking disrupts
the group cohesion of juvenile European grayling (Thymallus
thymallus) (152–190 mm) and that the effect of flow on sociabil-
ity may be mediated by the physical environment to some extent.

In our experiment, salmon maintained a position significantly
closer to the substrate than trout under sympatric conditions,
consistent with prior observations of fish from the same popula-
tions used in our study (Addo et al. 2023). However, it was sur-
prising that salmon maintained this position also in the absence
of trout, as we expected that the presence of trout was limiting
salmon habitat use (Heggenes et al. 1999). Allopatric trout, on
the other hand, positioned themselves significantly higher above
the substrate than sympatric trout. These findings align with
previous studies by Heggenes (1996); Heggenes et al. (1999)),
which noted that brown trout often occupy positions high above
stony substrates, whereas Atlantic salmon tend to position
themselves close to the substrate. Similar results have been re-
ported by others (Allen 1969; Bremset and Berg 1999; Bremset
and Heggenes 2001; Heggenes et al. 2002), and these studies at-
tributed the tendency of salmon to occupy positions closer to the
substrate than trout to interactive segregation from spatial com-
petition. Berg et al. (2014) reported that salmon parr occupied
higher positions above the substrate in allopatry, and that their
habitat use did not change whether they were in sympatry with
trout or in allopatry. Our experiment showed similar results in
the sense that salmon maintained the same habitat (close the
substrate) with or without trout, giving an indication of selective
habitat segregation. Furthermore, our study revealed that allo-
patric trout generally positioned themselves higher above the
substrate when in allopatry but used a wider range of vertical
positions when in sympatry with salmon, which indicates that
interactive segregation may influence the vertical positioning of
trout (Berg et al. 2014). In a real river, low-flow conditions re-
duce water levels and may decrease the availability of suitable
habitats, leading to increased fish densities in deeper pools (e.g.,
Stradmeyer et al. 2008). This crowding can potentially intensify
interspecific interactions. Although our study did not explic-
itly measure behavioral responses such as aggression, previous
findings (e.g., Höjesjö et al. 2010) suggest that trout are likely to
dominate the upper parts of pools, while salmon tend to occupy
areas closer to the substrate.

Our laboratory study offers insights into the effects of flow fluc-
tuation and group composition on the foraging behavior, socia-
bility, and vertical positioning of Atlantic salmon and brown

trout parr. We studied the parr life stage, and the generalizabil-
ity of our findings to other life stages may be limited. Moreover,
the study did not consider the spatial complexity of fish habitats
and their interactions with variables such as water temperature
and predation risk (Bret et al. 2016; Hajiesmaeili et al. 2023,
2024; Watz et al. 2023). Addressing such interactions in future
research will provide a more comprehensive understanding
of the behavioral responses to flow fluctuations of sympatric
salmonids.

6 | Conclusions

We investigated the effects of flow fluctuations and group com-
position (sympatry and allopatry) on the behavior of Atlantic
salmon and brown trout parr, and we did not find strong effects
of flow treatment or group composition on food intake. Thus, the
results from our study do not support the hypothesis that the for-
aging success of sympatric trout and salmon parr is negatively
impacted by hydropкеaking during summer, at least within the
range of flows and temperatures tested. Additionally, our study
highlighted the significance of group composition in influencing
group cohesion and how that affects microhabitat segregation of
salmonids. Individual-based models of salmonids in regulated
rivers do not typically consider effects of flow fluctuation and
group composition on foraging, sociability, or microhabitat se-
lection, and our study provides evidence that this assumption
likely does not reduce their performance in accurately modeling
energy intake rates of salmonid parr.

Acknowledgments

The study was funded by the Swedish research council Formas (grant number 2019-00543), the Swedish Energy Agency (P2022-00111 and P2023-01155) and the Swedish Center for Sustainable Hydropower (VKU 31016). The experiment was carried out with an ethical permission from the Animal Ethical Board of Sweden (5.8.18-03390/2019 and 5.2.18-2717/17). J. Norrgård, Gammalkroppa lax AB provided salmon and trout parr.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

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## Supporting Information

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