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Article

Behavioral Responses of Galaxias platei to Salmo trutta: Experimental Evidence of Competition and Predation Risk

by
Catterina Sobenes
1,2,*,
Evelyn Habit
3,4,
Konrad Górski
5 and
Oscar Link
6
1
Departamento de Ingeniería Civil, Universidad Católica de la Santísima Concepción, Concepción 4090541, Chile
2
Centro de Investigación en Biodiversidad y Ambientes Sustentables (CIBAS), Universidad Católica de la Santísima Concepción, Concepción 4090541, Chile
3
Departamento de Sistemas Acuáticos, Facultad de Ciencias Ambientales, Universidad de Concepción, Concepción 4070386, Chile
4
Centro de Ciencias Ambientales EULA—Chile, Universidad de Concepción, Concepción 4070386, Chile
5
Instituto de Ciencias Marinas y Limnológicas, Facultad de Ciencias, Universidad Austral de Chile, Valdivia 5090000, Chile
6
Departamento de Ingeniería Civil, Universidad de Concepción, Concepción 4070386, Chile
*
Author to whom correspondence should be addressed.
Water 2025, 17(12), 1774; https://doi.org/10.3390/w17121774
Submission received: 7 April 2025 / Revised: 20 May 2025 / Accepted: 22 May 2025 / Published: 13 June 2025
(This article belongs to the Section Biodiversity and Functionality of Aquatic Ecosystems)
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Abstract

The adverse impacts of invasive salmonids on native galaxiids are well documented at the population level in the freshwater ecosystems of the Southern Hemisphere. However, the mechanism underlying these interactions and sub-lethal effects of salmonids on native galaxiids at the individual level remain poorly understood. In this study, a series of controlled experiments was conducted to assess sub-lethal interactions between invasive brown trout (Salmo trutta) and the native Galaxias platei at an individual level. The microhabitat preferences of G. platei were evaluated in response to potential competition with juvenile brown trout and predation risk from piscivorous adults. In addition, the swimming capacity of G. platei was assessed to determine their ability to escape predation. The results show that at increasing densities of juvenile brown trout, G. platei fails to increase refuge use and are more frequently observed in open habitats. Furthermore, G. platei juveniles exhibit significantly lower swimming capacity compared to brown trout. In the presence of predatory trout, G. platei does not display a heightened preference for refuge habitats. These findings suggest that the behavioral response of G. platei could be insufficient to reduce competition and predation risks posed by brown trout and potentially other salmonids.

1. Introduction

Species invasions are among the leading causes of native freshwater fish declines and extinctions worldwide [1,2], predominantly due to predation, competition and habitat alteration [3,4]. The sub-lethal effects of invasive species often involve trait-mediated interactions, such as niche partitioning, which ultimately manifest at the population level [5]. These effects arise from biotic interactions, such as competition for space and resources, as well as predator–prey dynamics [6].
In South America, the introduction of exotic salmonids has caused significant ecological damage to native fish populations. Studies report changes in native fish abundance, distribution, and trophic roles due to salmonid invasions [7]. Brown trout (Salmo trutta), originally from the Northern Hemisphere, has become widespread in the freshwater systems of the Southern Hemisphere [8,9] and has had a profound impact on native galaxiids [10,11,12]. Predation and competition for food and habitat are the primary negative interactions documented [13,14]. However, the specific mechanisms underlying these impacts at the individual level remain largely unexplored [13,15].
Galaxias platei is the most widespread species of the Galaxiidae family in South America [16,17,18]. In Chile, it occurs from the Toltén River basin in the North (39° Lat. S) to the island of Tierra del Fuego in the South (54° Lat. S) [10,19,20]. This region has been identified as an invasion hotspot for salmonids, which are rapidly colonizing local freshwaters [21]. Galaxias platei is common in the post-glacial rivers and lakes of the region [20,21,22] and frequently coexists with brown trout [9,22]. Galaxias platei is a specialized bottom dweller [23,24] and possesses traits that allow it to successfully utilize deep benthic habitats, including retinas adapted to vision in low-light environments, a cephalic lateral line, gill protection against abrasion, high endurance to low oxygen availability, and physiological adaptation to anaerobic metabolism [24,25]. A preference for deep habitats in Patagonian lakes (>40 m) and low water temperatures (<12 °C) has been identified in this species, with both factors being positively correlated with sexual maturity [26]. Optimal growth for this species has been estimated to occur at temperatures between 10 and 16 °C during the warmer seasons [26]. It exhibits a benthic carnivorous feeding strategy, primarily consuming prey from the littoral zones of lakes. In the presence of invasive trout, G. platei juveniles show slower ontogenetic development and an expansion of their trophic niche toward allochthonous prey [20]. This shift is density-dependent and occurs in reciprocal relation to the trophic position of S. trutta [8]. Juvenile G. platei (<80 mm in length) tend to use littoral habitats during the day and are frequently found in the diets of large conspecifics and brown trout [8,27,28,29]. Larger individuals of G. platei utilize deeper habitats, reaching depths greater than 8 m [11,30], and exhibit primarily nocturnal activity [31]. During the day, they occupy refuge-providing habitats such as submerged vegetation, cobbles and fallen logs [9]. Experimental observations during the day show that juvenile G. platei prefer refuges but may become more active and utilize open habitats as densities increase [32]. Additionally, captive rearing studies have confirmed nocturnal and crepuscular activity patterns, refuge-seeking behavior during daylight, low aggression, and exploratory foraging in benthic areas, supporting previous observations under natural conditions [33]. Consequently, juveniles are at higher risk of interactions with invasive brown trout, particularly since brown trout feed during both day and night [34].
Little is known about the sub-lethal effects of brown trout on G. platei and how they operate. Previous studies have suggested that the pervasive negative effects of salmonids on G. platei, such as morphological changes [24] and trophic niche shifts [35], may explain the declining abundance of G. platei [9]. However, other sub-lethal behavioral effects caused by brown trout, such as shifts in habitat use due to competition for space or predation risk, remain unexplored for G. platei. Studies on other freshwater fish species highlight the potential importance of these sub-lethal effects [36].
Given the ongoing decline of G. platei populations in Patagonia, it is crucial to understand how this species responds behaviorally to the competition and predation risks imposed by brown trout. Previous research has shown that fish species with limited behavioral plasticity are more vulnerable to competition and predation by invasive species [37]. In this context, G. platei may be at a disadvantage compared to salmonids, which exhibit higher metabolic rates, greater feeding efficiency, and superior swimming capacity [38]. Furthermore, the lack of recognition of introduced predators may increase the vulnerability of native fish, reducing the likelihood of effective antipredator responses [39].
This study employs an experimental approach to unravel the sub-lethal interactions between invasive brown trout and the native galaxiid G. platei. The following hypotheses were tested: (i) G. platei increases its use of microhabitats with available refuges in the presence of juvenile brown trout to reduce competition; (ii) G. platei increases its use of refuge microhabitats in the presence of predatory brown trout to mitigate predation risk; and (iii) G. platei exhibits lower critical swimming performance than S. trutta, limiting its capacity to escape from predatory encounters. To test these hypotheses, we conducted experiments assessing the microhabitat preferences of G. platei at different juvenile brown trout densities and in the presence of a predatory brown trout. Additionally, we compared the swimming capacities of G. platei and brown trout to evaluate whether locomotor performance plays a role in predator–prey interactions.

2. Materials and Methods

2.1. Sampling and Fish Maintenance in the Laboratory

A total of 52 G. platei individuals (4.0–8.2 cm total length) were collected from the San Pedro River (39°19′–40°03′ S, 73° 28′–71° 42′ W), and 30 brown trout (S. trutta) individuals (7.0–24.5 cm total length) were collected from the Nonguén River (36°42′–36°56′ S, 72°36′–73°04′ W). Fish were collected using electrofishing (Smith-Root LR24, Vancouver, WA, USA) in June 2012 (austral winter).
All collected fish were housed in 120 L aquariums containing gravel and cobble refuges at the Hydraulics and Environmental Engineering Laboratory, Universidad de Concepción [40]. The fish were fed once a day ad libitum until 24 h before each experiment commenced. All experiments were conducted in the same laboratory where fish were acclimated. To minimize bias associated with human intrusion, aquariums were covered on three sides, and the observations were made through an opening (50 × 50 cm) behind a blind. Water temperature was maintained at 16 ± 0.5 °C throughout the experiments, and the fish were not fed.

2.2. Microhabitat Preference Experiments

A 900 L glass observation tank (1.5 × 1.0 × 0.6 m) divided into four microhabitats (substrates) of equal size (0.5 × 0.75 m each) was used. Environmental variables were maintained under controlled conditions with water temperature at 14 °C, dissolved oxygen at 8 ppm, and pH at 7. A 12:12 h light–dark photoperiod was implemented using fluorescent lighting. Two microhabitats contained refuge: a vegetated patch (Egeria densa, 1 plant/0.01 m2; ‘Vegetation’) and a cobble patch (70–160 mm in size, ‘Cobble’). The remaining two microhabitats lacked refuge: a sand patch (grain size 2–38.5 mm, ‘Sand’) and a slab-stone patch (‘Slab stone’; Figure 1). These microhabitats were selected according to the habitats utilized by G. platei in its natural environment [9].
Diurnal microhabitat uses and preference by G. platei was studied in four different experiments with varying fish densities (Table 1). All experimental fish were juveniles (~6 cm length), with no significant size differences among trials. Microhabitat use was registered by direct observations every 10 min from 09:00 AM to 1:00 PM (morning) and from 3:00 PM to 6:00 PM (afternoon), over a total period of 48 h, supported with videos for the observer’s review and consultation, according to [32]. After each trial, both, S. trutta and G. platei were removed from the experimental tank.
To account for potential intraspecific interactions, the microhabitat preferences of juvenile G. platei (mean length 6.8 ± 1.1 cm; mean weight 1.3 ± 2.1 g, N = 27) were analyzed at different densities (4, 8, 12, 16 and 27 individuals, E1; Table 1). To minimize potential side effects from the repeated use of individuals, each fish had a rest period between trials, and all test sequences were randomized. Behavioral trials were short and designed to capture immediate responses to environmental cues, rather than long-term conditioned behaviors.

2.3. Competitive and Predator–Prey Interactions Experiments

An intermediate density of 12 G. platei (density: 0.016 ind./l, corresponding to densities observed in natural systems) [9] was used in subsequent experiments (Table 1). The selection of G. platei and S.trutta individuals for the experimental trials was based on naturally occurring size classes. Juvenile and adult brown trout were chosen to simulate realistic competitive and predatory interactions, respectively. To assess potential competitive interactions, 12 G. platei individuals (mean length 6.5 ± 0.6 cm; mean weight 1.3 ± 0.1 g) were exposed to juvenile S. trutta (mean length 8.3 ± 0.5 cm; mean weight 5.3 ± 0.8 g) at varying densities of juvenile S. trutta (6, 12, 18, 24 ind.; E2). During these experiments, only G. platei exhibited two distinct behaviors within the cobble patch: either swimming above cobbles or resting within them. Consequently, the “Cobble” microhabitat was divided into the “Above-Cobble” (fish swimming above) and “Within-Cobble” (fish resting within the cobble substrate) microhabitats. This distinction was incorporated into a subsequent experiment (E3) to evaluate habitat use preferences across five microhabitats between juveniles of G. platei (mean length 6.1 ± 0.17 cm; mean weight 1.3 ± 0.14 g) and S. trutta (mean length 8.1 ± 0.7 cm; mean weight 5.1 ± 0.8 g).
Finally, the microhabitat preference of G. platei in the presence of a potential predator (S. trutta > 20 cm; [41]) was tested (E4). Since S. trutta adults were larger than G. platei, the experimental design included structured microhabitats and refuge zones to prevent direct aggressive encounters and allow for behavioral responses to be evaluated independently of size-based dominance. This experiment was conducted using a new batch of 12 G. platei individuals (mean length 6.7 ± 0.7 cm; mean weight 1.8 ± 0.5 g) and three larger S. trutta individuals (mean length 22.07 ± 3.0 cm; mean weight 97.12 ± 34.98 g). The microhabitat use of G. platei was recorded in three trials. Only two individuals of G. platei were preyed upon, and they were immediately replaced with new individuals of similar size to maintain experimental density.
Jacob’s selectivity index (D) [42] was used to assess microhabitat preference for both G. platei and S. trutta:
D = (r − p)/(r + p − 2rp)
where r is the ratio of habitat used in categoryi to the total habitat use, and p is the ratio of the available habitat in categoryi to the total available habitat. This index ranges from −1 (complete avoidance of habitat category) and +1 (complete selection of habitat category), with 0 indicating neutral selection.

2.4. Swimming Performance Experiments

To assess the swimming performance of G. platei, 25 individuals (mean length 6.6 ± 0.41 cm; mean weight 1.6 ± 0.12 g) were used (Table 1). Swimming performance trials were conducted using a 2.75-L Blazka-type swimming respirometer [43] placed in a 140 L bath tank at a water temperature between 15.0 and 16.5 °C. Critical swimming speed (Ucrit), which represents the maximum sustainable swimming speed of a fish, was measured by increasing flow rate in increments of 0.5 body length (BL) every 10 min. Ucrit was calculated as follows [44]:
Ucrit = U + (t/ti × Ui)
where U = penultimate speed, Ui = velocity increment (0.5 BL), t = time swum in the final velocity increment and ti = set time interval for each velocity increment (10 min). For S. trutta, critical swimming speed at a mean water temperature of 15 °C was obtained from published sources ([45,46]; Table 2). All experimental procedures conformed to the national guidelines for animal usage in research [47] and were approved by the bioethics committee of the Universidad de Concepción.

2.5. Statistical Analyses

To assess the effects of S. trutta density on G. platei microhabitat use, Jacob’s indices were calculated separately for each microhabitat and trout density. Microhabitat preferences among treatments were compared using permutational multivariate analysis of variance (PERMANOVA; [48,49]). Separate models were used for each habitat type to assess significant differences in preferences across fish density and potential predator presence. Models were run based on an Euclidean distance matrix. Mean differences in critical swimming speeds between G. platei and S. trutta were compared applying the Mann–Whitney test.

3. Results

3.1. Galaxias platei Microhabitat Preferences

Significant differences in microhabitat use, expressed by Jacobs’s index, were found for G. platei at different densities of conspecifics (E1; Figure 2, left panel; Table 3). At lower densities (4, 8 and 12 individuals), microhabitats with cobbles and vegetation were preferred (Figure 2, left panel). Conversely, at higher densities (16 and 27 individuals), G. platei tended to avoid the vegetated patch and exhibited a stronger preference for cobble microhabitat. Sand and slab stone were consistently avoided across all densities (Figure 2, left panel).
At a fixed G. platei density (E2), juvenile S. trutta strongly preferred the cobble patch at all densities of conspecifics, while sand was consistently avoided (Figure 2, right panel; Table 3). Vegetated and slab stone patches were avoided at lower densities but were utilized at higher densities (Figure 2, right panel; Table 3). As S. trutta densities increased (E2), G. platei’s preference for the cobble patch decreased, while their use of vegetated and slab stone patches increased (Figure 2, left panel; Table 3).

3.2. Experiment with Five Microhabitats (E3)

In the experiment involving five microhabitats (E3), S. trutta predominantly occupied areas above the cobble at all densities, whereas the within-cobble, vegetated, and sand patches were consistently avoided (Figure 3, right panel, Table 4). The slab-stone patch was avoided by S. trutta only at intermediate densities but utilized at lower and higher densities.
Galaxias platei exhibited a preference for within-cobble and vegetated microhabitats, while avoiding above-cobble, sand and slab-stone microhabitats in the absence of S. trutta (Figure 3, left panel; Table 4). However, as S. trutta densities increased, the preference of G. platei for the within-cobble microhabitat decreased (Figure 3, left panel). At higher S. trutta densities, G. platei also avoided the above-cobble microhabitat (Figure 3, left panel). At the highest experimental S. trutta density (18 fishes), G. platei exhibited reduced avoidance of sand and used the slab-stone microhabitat (Figure 3, left panel).

3.3. Microhabitat Preferences in Presence of a Predator (E4)

Microhabitat preferences of G. platei varied significantly in the presence of a potential predator (E4; Figure 4, Table 5). The presence of the predator significantly reduced the preference of G. platei for within-cobble and vegetated microhabitats, while simultaneously increasing their use of slab stone habitat. Salmo trutta, used as potential predators in this experiment, exhibited high mobility across all habitats except within- cobble microhabitats, which were inaccessible to them due to the inter-cobble space size in comparison to the size of brown trout (Figure 4; Table 5).

3.4. Swimming Performance

The mean critical swimming speed of G. platei was 8.34 cm·s−1 (range: 2.6 to 12.8 cm·s−1), recorded at a mean temperature of 16.2 °C (Figure 5). The swimming capacity of S. trutta was significantly higher than that of G. platei of the same length (Mann–Whitney U test: U = 0, N1 = 7, N2 = 25, p < 0.001; Figure 5, Table 2).

4. Discussion

Understanding the negative impacts of salmonids on galaxiids is crucial for the conservation and management of native fish fauna in the Southern Hemisphere [1,13,15]. In this study, an experimental approach was used to assess the behavioral responses of Galaxias platei to competition and predation risks posed by invasive brown trout. The use of manipulative experiments minimized confounding effects [50] and allowed for a clear understanding of the impact that brown trout can exert on G. platei, a species endemic to Patagonia and highly affected by salmonid invasion [8,9,22,24,51]. The behavioral patterns observed in juveniles of G. platei in the presence of brown trout provide insight into its vulnerability to competition and predation by non-native salmonids. Indeed, G. platei experiences a shift in its trophic ecology in the presence of invasive trout, delaying its piscivorous behavior and reducing its trophic level and ontogenetic development [35]. Consistently, ref. [52] found that non-diadromous galaxiids showed reduced individual growth rates when exposed to trout competition following flood disturbances, emphasizing the persistent negative impact of trout even in the context of habitat renewal.
While the present study offers valuable insights into the behavioral responses of G. platei to the presence of the invasive S. trutta, several methodological limitations must be acknowledged. Experimental research is a fundamental tool in ecological science [53], yet it is often constrained by logistical and practical challenges that can influence the outcomes and their interpretation. Previous studies have shown that spatial confinement can affect physiological and behavioral traits in fish, with increased tank volume being associated with enhanced growth and feed intake [54], as well as altered swimming behavior, such as increased activity in larger enclosures compared to natural conditions [55]. In this study, we maintained environmental parameters representative of the native habitats of the species involved and incorporated environmental enrichment following established guidelines [56,57]. Nevertheless, it is possible that tank size and other artificial conditions may have influenced the behavioral patterns observed in G. platei. This possibility should be addressed in future studies.
Moreover, the absence of replication across certain density treatments presents an additional limitation, potentially reducing the generalizability of our findings. This issue, commonly encountered in behavioral experiments with live fish, raises concerns about pseudoreplication [58]. Although such experimental designs are frequently used to explore interspecific interactions under controlled conditions, we acknowledge that increased replication would improve the robustness and statistical power of this study. In this context, the use of non-parametric statistical analyses allowed us to draw conservative inferences without relying on assumptions of data normality [59], thus partially mitigating the constraints imposed by the experimental design.
Although some individuals were used more than once, we implemented randomized test sequences and adequate rest periods to mitigate potential carryover effects. Furthermore, the observed behavioral responses were consistent across treatments, supporting the validity of our findings despite potential individual habituation. Similar approaches have been successfully used in other experimental studies focusing on short-term behavioral responses in fish [60].

4.1. Do Galaxias platei Avoid Potential Competition of Juvenile Brown Trout?

In the absence of brown trout, juvenile G. platei exhibited a clear preference for refuge microhabitats, such as vegetated areas and cobble patches. However, contrary to what was hypothesized, increasing trout density did not lead to a significantly greater preference for refuge habitats in G. platei. Instead, as the density of brown trout increased, G. platei juveniles were observed to occupy more open habitats, potentially increasing the likelihood of interactions between the two species. The absence of avoidance behaviors observed in our experiments suggest that G. platei lack effective mechanisms to reduce competitive interactions with brown trout, which could further exacerbate interspecific competition.
Behavioral plasticity plays a crucial role in competition between native and invasive fish species. However, species with limited adaptability to novel competitors are at greater risk of displacement [37]. Furthermore, G. platei juveniles may be at a disadvantage due to their relatively slow metabolic rates compared to salmonids, which generally exhibit higher feeding rates and growth efficiency [38].
One limitation of the experimental design is the difference in body size between S. trutta and G. platei. However, this size asymmetry is ecologically realistic and expected to occur in natural predator–prey and competitive contexts. Previous studies have shown that size-structured interactions are common in freshwater systems and that body size plays a fundamental role in shaping behavioral and spatial dynamics [61]. In our experiments, refuge availability and habitat heterogeneity allowed for the assessment of behavioral responses without direct physical dominance.

4.2. How Does Galaxias platei Respond to Predation Risk Posed by Brown Trout?

The most severe impacts of brown trout on galaxiids appear to be associated with larger, piscivorous trout and direct predation [62,63]. Interestingly, only two individuals of G. platei experienced direct predation mortality during the 144 h of exposure to potential predation in this study. In natural systems, predation rates are expected to fluctuate depending on prey abundance, and brown trout are well-documented as highly effective predators [64].
Galaxias platei is vulnerable to predation by brown trout. However, our findings indicate that in the presence of predatory trout, G. platei juveniles did not increase their preference for refuge habitats. It is reasonable to expect G. platei juveniles to exhibit anti-predatory behaviors at night, given their evolutionary history of cannibalistic predation pressure from larger conspecifics, which are primarily nocturnal. Our experiments, conducted during the day, aimed to evaluate responses to introduced predatory brown trout, which are known to feed during both day and night [65,66]. The lack of behavioral response in juvenile G. platei to the predation risk posed by brown trout could be attributed to an inability to detect the scent of this introduced predator, as previously demonstrated in Galaxias maculatus when exposed to non-native rainbow trout Onchorhynchus mykiss [67]. Invasive carnivore fish species are especially problematic when native prey lack appropriate anti-predator adaptations [5], such as G. platei in the presence of brown trout. Indeed, brown trout exhibit high dietary plasticity, consuming native galaxiids even at small body sizes, making them effective predators in diverse environments [68].
Avoidance strategies, such as reducing activity to minimize predator encounters, have been documented in various fish species [69,70]. This strategy may be particularly relevant for G. platei, given its relatively slow pace of life compared to the more active brown trout [71]. Indeed, our results confirm that in comparison with brown trout, G. platei exhibits lower swimming capacity, as measured by the critical swimming speed. Previous studies have established that critical swimming speed is directly linked to fish maximum swimming speed, which is crucial in prey–predator interactions [45,72,73,74].
Additionally, predator recognition in native fish species can be influenced by evolutionary history. Some native fish exhibit learned recognition of invasive predators through exposure in natural habitats, while others fail to develop effective responses, making them more vulnerable to predation [39]. If G. platei lacks innate or learned predator recognition for brown trout, it may struggle to implement effective antipredator behaviors, further increasing its susceptibility.
The behavioral responses observed in G. platei appear insufficient to mitigate the risk of competition and predation posed by brown trout and possibly other salmonids. Our results indicate that G. platei has a limited swimming capacity and does not actively seek refuge in response to competitive or predatory threats from brown trout in an experimental setting. The observed spatial displacement of G. platei in the presence of S. trutta can be interpreted as a behavioral response to competitive pressure, where differences in swimming ability may underlie asymmetries in habitat use and dominance [75]. Consequently, the sustainable coexistence of G. platei and invasive salmonids seems improbable, as mutually exclusive distributions have already been documented in Patagonia [9] and previously reported for other galaxiids in Australia and New Zealand [3]. Given these findings, management and conservation efforts should prioritize the extensive protection of the few remaining salmonid-free lakes [9]. These lakes could serve as essential refuges for galaxiids and play a critical role in the successful conservation of G. platei.

Author Contributions

Conceptualization and methodology C.S., E.H. and O.L. Investigation C.S., E.H., K.G. and O.L.; data curation and visualization C.S., E.H. and K.G.; resources C.S., E.H. and O.L.; writing—original draft C.S., E.H. and K.G. Funding acquisition C.S., E.H. and O.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by CONICYT (doctoral fellowships D21080436 and AT24110081 to C.S.) and the Ingeniería 2030 (ING222010004) and InES de Género (INGE220011) projects. E.H. acknowledges Fondecyt (project 1110441). K.G. is supported by Fondecyt (project 1230617).

Institutional Review Board Statement

All experiments complied with Chilean law (Law 20.380) and were approved by the Ethics, Bioethics, and Biosafety Committee of the Universidad de Concepción. Fish were housed in tanks with natural plants and gravel, mimicking their natural habitat. This study followed the ASAB/ABS guidelines, and no signs of stress were observed in the fish. At the end of the experiment, all individuals were relocated to maintenance tanks. Fish collection and experimental procedures complied with the law of the country (Chile) under Fishing Research authorization (Res. Ex. N° 55/2010) and were registered in aquaculture (R.N.A N° 21750/2011). The experimental procedures were approved by the Universidad de Concepción.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

We thank R. Jara and A. González for their assistance with the experiments.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic diagram of the experimental aquaria for assessing microhabitat use in Galaxias platei with different substrates (cobble, vegetation, sand and slab stone).
Figure 1. Schematic diagram of the experimental aquaria for assessing microhabitat use in Galaxias platei with different substrates (cobble, vegetation, sand and slab stone).
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Figure 2. Mean ± SD Jacob’s index (D) for Galaxias platei (a) and Salmo trutta (b) at different S. trutta densities across four microhabitats. Significant differences (PERMANOVA) between S. trutta densities within each habitat type (cobble, vegetation, sand and slab stone) are denoted by different letters.
Figure 2. Mean ± SD Jacob’s index (D) for Galaxias platei (a) and Salmo trutta (b) at different S. trutta densities across four microhabitats. Significant differences (PERMANOVA) between S. trutta densities within each habitat type (cobble, vegetation, sand and slab stone) are denoted by different letters.
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Figure 3. Mean ± SD Jacob’s index (D) for Galaxias platei (a) and Salmo trutta (b) at different S. trutta densities across five microhabitats. Significant differences (PERMANOVA) between S. trutta densities within each habitat (within-cobble (W. Cobble), above-cobble (A. Cobble), vegetation, sand and slab stone) are denoted by different letters.
Figure 3. Mean ± SD Jacob’s index (D) for Galaxias platei (a) and Salmo trutta (b) at different S. trutta densities across five microhabitats. Significant differences (PERMANOVA) between S. trutta densities within each habitat (within-cobble (W. Cobble), above-cobble (A. Cobble), vegetation, sand and slab stone) are denoted by different letters.
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Figure 4. Frequency (Mean ± SE) of behavioral categories of Galaxias platei in the absence and presence of Salmo trutta potential predators. W. Cobble, within-cobble; A. Cobble, above-cobble.
Figure 4. Frequency (Mean ± SE) of behavioral categories of Galaxias platei in the absence and presence of Salmo trutta potential predators. W. Cobble, within-cobble; A. Cobble, above-cobble.
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Figure 5. Critical swimming speed (U) and total length (L) for Galaxias platei and Salmo trutta.
Figure 5. Critical swimming speed (U) and total length (L) for Galaxias platei and Salmo trutta.
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Table 1. Experimental trials on microhabitat preferences based on the abundance of Galaxias platei and Salmo trutta.
Table 1. Experimental trials on microhabitat preferences based on the abundance of Galaxias platei and Salmo trutta.
ExperimentGalaxias plateiSalmo truttaMicrohabitats
NLength (cm)Weight (g)nLength (cm)Weight (g)
E147.0 ± 0.81.3 ± 0.30 Vegetation, Cobble, Slab Stone and Sand.
86.7 ± 0.81.3 ± 0.20
127.2 ± 0.71.4 ± 0.10
166.8 ± 0.71.3 ± 0.20
276.8 ± 1.11.3 ± 0.10
E2126.5 ± 0.61.3 ± 0.168.1 ± 0.624.9 ± 0.96Vegetation, Cobble, Slab Stone and Sand.
12127.9 ± 0.564.7 ± 0.94
12188.4 ± 1.155.6 ± 0.53
12248.3 ± 0.505.3 ± 0.80
E3126.1 ± 0.171.3 ± 0.140 Vegetation, Above-Cobbles, Within-Cobbles, Slab Stone and Sand.
126
1212
1218
E4126.7 ± 0.71.8 ± 0.50--Vegetation, Above-Cobbles, Within-Cobbles, Slab Stone and Sand.
12119.1062.20
12122.0097.10
12125.10132.10
Table 2. Salmo trutta critical swimming speeds (source: [45,46]).
Table 2. Salmo trutta critical swimming speeds (source: [45,46]).
Lenght (cm)Swimming Velocity (cm·s−1)
545.0–49.6
1059.8–73.6
1593.0–97.6
25141.0–145.6
Table 3. PERMANOVA results on fish habitat preferences (Jacobs index, four habitat types) at different densities; df: degrees of freedom; SS: sums of squares; pseudo-F: distance-based pseudo F-statistic; p: probability values obtained using 9999 unrestricted permutations of raw data.
Table 3. PERMANOVA results on fish habitat preferences (Jacobs index, four habitat types) at different densities; df: degrees of freedom; SS: sums of squares; pseudo-F: distance-based pseudo F-statistic; p: probability values obtained using 9999 unrestricted permutations of raw data.
Galaxias plateiSalmo trutta
SubstrateSourcedfSSPseudo-FpdfSSPseudo-Fp
CobblesDensity869.72174.491<0.001317.5889.491<0.001
Residual69681.428 34822.788
VegetationDensity848.11546.687<0.00139.424129.553<0.001
Residual69689.661 34836.991
Slab stoneDensity8118.62125.56<0.001320.51100.75<0.001
Residual69682.191 34823.614
SandDensity827.11422.457<0.00131.24993.6703<0.05
Residual696105.04 34839.503
Table 4. PERMANOVA results on fish habitat preferences (Jacobs index, five habitat types) at different densities; df: degrees of freedom; SS: sums of squares; pseudo-F: distance-based pseudo F-statistic; p: probability values obtained using 9999 unrestricted permutations of raw data.
Table 4. PERMANOVA results on fish habitat preferences (Jacobs index, five habitat types) at different densities; df: degrees of freedom; SS: sums of squares; pseudo-F: distance-based pseudo F-statistic; p: probability values obtained using 9999 unrestricted permutations of raw data.
Galaxias platei Salmo trutta
SubstrateSourcedfSSPseudo-FpdfSSPseudo-Fp
Above cobblesDensity of S. trutta323.42664.976<0.00129.497261.043<0.001
Residual34841.822 26120.304
Within cobblesDensity of S. trutta38.884524.263<0.00127.147219.16<0.001
Residual34842.476 26148.679
VegetationDensity of S. trutta311.44362.324<0.00122.85910.421<0.001
Residual34821.299 26135.804
Slab stoneDensity of S. trutta36.622319.45<0.001213.7479.971<0.001
Residual34839.495 26122.421
SandDensity of S. trutta37.265719.721<0.00120.251750.91453NS
Residual34842.737 26135.923
Table 5. PERMANOVA results on fish habitat preferences in the presence of a potential S. trutta predator (Jacobs index, five habitat types) at different densities; df: degrees of freedom; SS: sums of squares; pseudo-F: distance-based pseudo F-statistic; p: probability values obtained using 9999 unrestricted permutations of raw data.
Table 5. PERMANOVA results on fish habitat preferences in the presence of a potential S. trutta predator (Jacobs index, five habitat types) at different densities; df: degrees of freedom; SS: sums of squares; pseudo-F: distance-based pseudo F-statistic; p: probability values obtained using 9999 unrestricted permutations of raw data.
Galaxias platei Salmo trutta
SubstrateSourcedfSSPseudo-FpdfSSPseudo-Fp
Above cobblesPresence of predator (S. trutta)313.46130.552<0.00121.965337.286<0.001
Residual34851.109 2616.8784
Within cobblesPresence of predator (S. trutta)31.49655.3644<0.001____
Residual34832.36 __
VegetationPresence of predator (S. trutta)317.27253.53<0.00128.7008433.82<0.001
Residual34837.428 2612.6173
Slab stonePresence of predator (S. trutta)321.16677.375<0.00127.1299198.57<0.001
Residual34831.732 2614.6858
SandPresence of predator (S. trutta)31.20043.6105<0.0520.5023223.957<0.001
Residual34838.567 2612.7363
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Sobenes, C.; Habit, E.; Górski, K.; Link, O. Behavioral Responses of Galaxias platei to Salmo trutta: Experimental Evidence of Competition and Predation Risk. Water 2025, 17, 1774. https://doi.org/10.3390/w17121774

AMA Style

Sobenes C, Habit E, Górski K, Link O. Behavioral Responses of Galaxias platei to Salmo trutta: Experimental Evidence of Competition and Predation Risk. Water. 2025; 17(12):1774. https://doi.org/10.3390/w17121774

Chicago/Turabian Style

Sobenes, Catterina, Evelyn Habit, Konrad Górski, and Oscar Link. 2025. "Behavioral Responses of Galaxias platei to Salmo trutta: Experimental Evidence of Competition and Predation Risk" Water 17, no. 12: 1774. https://doi.org/10.3390/w17121774

APA Style

Sobenes, C., Habit, E., Górski, K., & Link, O. (2025). Behavioral Responses of Galaxias platei to Salmo trutta: Experimental Evidence of Competition and Predation Risk. Water, 17(12), 1774. https://doi.org/10.3390/w17121774

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