Invasive Brook trout (Salvelinus fontinalis) presence may reduce dominance-linked growth and shift diel activity in brown trout (Salmo trutta)

Invasive Brook trout (Salvelinus fontinalis) presence may reduce dominance-linked growth and shift diel activity in brown trout (Salmo trutta) | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Invasive Brook trout ( Salvelinus fontinalis ) presence may reduce dominance-linked growth and shift diel activity in brown trout (Salmo trutta ) Benedikte Austad, Libor Závorka, Stefano Mari, Stefan Auer, Johan Höjesjö This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8588818/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract Salmonid fishes optimize their habitat use through temporal and spatial activity patterns to balance foraging success, competitive ability, and predator avoidance. Under stable social hierarchies, dominant individuals typically occupy profitable stream positions and achieve higher growth rates. However, the introduction of non-native salmonid species can disrupt these established social hierarchies and alter optimal habitat use patterns. Moreover, the removal of invasive species is often difficult to fully accomplish and may disproportionately target certain behavioural phenotypes, potentially exacerbating the impact of invasive species. Using seminatural stream flumes, this study compared dominance-growth relationship, habitat use, and diel activity patterns between allopatric brown trout groups and sympatric groups of brown trout and invasive brook trout, under varying flow conditions and brook trout removal scenarios. Our results revealed that brook trout invasion disrupted the fundamental dominance-growth relationship observed in allopatric brown trout, indicating that the costs of maintaining dominance in the presence of brook trout outweigh the benefits. Additionally, brook trout invasion significantly altered brown trout diel activity patterns and spatial habitat distribution, even after partial removal of brook trout. These findings demonstrate that invasive species can disrupt established link between dominance, growth and habitat use in native fish communities. Invasive species diel activity social dominance salmonids brook trout (Salvelinus fontinalis) brown trout (Salmo trutta) Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction To increase foraging and competitive success, and reduce the risk of predation, animals generally optimize habitat use both in time and space (for example: Suselbeek et al., 2014; Kohl et al., 2018; Moiron et al., 2018; Sells & Mitchell, 2020), where decisions reflect an apparent trade-off between the risk of predation/injury and the benefit to be gained from engaging in said activity (Lima & Dill, 1990). Even closely related species, such as different salmonid fishes, demonstrate variation in activity patterns reflecting the species-specific adaptations to the trade-offs between foraging, competitive ability and predator avoidance (Johnson & McKenna, 2015; David et al., 2007). Social hierarchies are common among territorial fish and allow them to reduce the cost of aggressive interactions and facilitate access to resources (Eaton, 2011). Under a stable social hierarchy, subordinate individuals are excluded from the most profitable positions in the stream, but they may still maintain high fitness by avoiding costly interaction while feeding at profitable sites downstream (Nilsson et al., 2004). Alternatively, fish might adjust their activity temporally to avoid costly interactions with familiar dominant individuals (Johnsson, 2010). For example, Alanärä et al. (2001) demonstrated that dominant brown trout ( Salmo trutta ) fed predominantly at the most beneficial times from dusk until early night, when food was abundant and predation risk lower, while subdominant brown trout fed more during the day. Variation in activity, boldness, and aggressiveness within and between the species may be critical for the formation and stability of social hierarchy (Colléter & Brown, 2011, Railsback et al., 2005). For example, Závorka et al. (2016) showed that shy and possibly subdominant brown trout display higher changes in the habitat use between day and night, being more active during the night, while bold and likely more dominant individuals maintained high activity throughout the whole daily cycle. The competition and habitat use of riverine fish is also strongly influenced by the hydrological regime as this defines the availability and distribution of suitable habitats. Increasing rapid changes in water level of streams are expected, both because of the more frequent severe weather events (Benetti et al., 2024) and habitat degradation due to fragmentation of rivers and short-term water regulations. This may force stream fish into suboptimal habitats, locally increase the density of fish and produce unexpected outcomes for their diel activity patterns and competitive interactions. For example, Stradmeyer et al. (2008) found that during rapid dewatering events, juvenile Atlantic salmon in sympatry with brown trout were able to maintain access to pool refuges. This was observed even though brown trout are typically the more dominant and aggressive species and occupied pools during normal flow conditions to a larger degree than the salmon did and were thereby predicted to exclude salmon from these habitats. Hence, salmonids adjust their diel patterns and habitat use depending on a whole suite of internal and external factors (Metcalfe et al., 1998; Railsback et al., 2005; Larranaga & Steingrímsson, 2015); flexibility in this trait may be especially beneficial in dynamic environments typical for salmonid nursery streams, where available habitat and density of conspecifics may change rapidly (Metcalfe et al., 1998). Additionally, competition with closely related non-native species has been shown to hamper optimal habitat use and fitness of native fishes (Cucherousset & Olden, 2011; Závorka et al., 2017), which may lead to decrease of population size and local extinction of the native fishes (Houde et al., 2016). Hence, benefits of stable social hierarchy for native fish may be disrupted by presence of non-native species with different behavioural and social traits. For example, Roberge et al. (2008) found that the introduction of rainbow trout ( Oncorhynchus mykiss ) disrupted social hierarchies in Atlantic salmon ( Salmo salar ). Similarly, it has been shown that introduction of brook trout ( Salvelinus fontinalis ) can destabilize the social hierarchy of brown trout (Lovén Wallerius et al., 2022), with impacts on its daily activity patterns (Larranaga et al., 2019) and home range size (Závorka et al., 2017). Consequently, considerable effort is being made to remove non-native fish species from ecosystems (Simberloff, 2009). However, complete eradication of invasive species is frequently not achieved (Britton et al., 2010). Therefore, an important but often overlooked question is how the partial removal of invasive fish species may affect habitat use and fitness of native fishes in the dynamic habitat of headwater streams. This question is even more pressing because removal techniques for invasive fish, such as electrofishing, netting (Biro & Dingemanse, 2009) or spearfishing (Côté et al., 2014), may be biased towards bolder and more active behavioural phenotypes. Therefore, it is reasonable to predict that removal effort of invasive species will change the proportion of certain phenotypes which may influence competitive interactions and fitness consequences in the native fish population. Given the potential for invasive species to alter social hierarchies and the distribution of behavioural phenotypes in native fish populations, the aim of the current study is to test the effect of competition between brook and brown trout in different water regimes and after selective removal of invasive brook trout. Using an artificial flume, we were able to obtain detailed information on habitat use, activity and fitness of native brown trout in various water flows with (sympatric conditions) and without (allopatric conditions) the presence of brook trout. Specifically, we predict that: Dominant brown trout in allopatry will display faster growth rates compared to subdominant conspecifics. Competition with invasive brook trout will hamper the fitness benefits for the dominant brown trout, reflected in their growth. Competition with invasive brook trout will prevent brown trout monopolizing profitable habitats and optimize their diurnal activity. Specifically, we predict sympatric brown trout to stay further downstream in less optimal habitats and to be more active during the day. Partial removal of invasive brook trout from the most profitable habitat will allow the sympatric brown trout to monopolize the most profitable habitats. Methods HyTEC flumes In this study, we investigated the effects of brook trout invasion on brown trout by comparing habitat use, activity pattern and growth between allopatric groups of brown trout with sympatric groups of brown trout and brook trout. The research was conducted at the so called HyTEC facility (Hydromorphological and Temperature Experimental Channels) in Lunz am See (Austria), which consists of two 40 m long artificial stream channels designed to simulate natural stream habitat conditions. The water is supplied by Lake Lunz and discharged back to the natural outflow of the lake (see: https://hydropeaking.boku.ac.at ). Each channel was subdivided longitudinally into four 7 m long enclosures,1.5 m in width, separated by 2 m buffer zones. The cross-sectional profile of each enclosure included a deep and a shallow part. During high water levels, the water depth was ~ 30 cm and ~ 15 cm in the deep and shallow part, respectively, and the average water flow was ~ 7 cm/s. During low water levels, the water depth was ~ 15 cm and ~ 0 cm in the deep and shallow part, respectively, with an average water flow of ~ 16 cm/s. The enclosures were partitioned using fencing with approximately 0.5 cm² mesh size, which allowed continuous water flow while preventing fish movement between sections (Fig. 1 a). Water discharge was maintained at a constant rate of 25 L s⁻¹, with water temperatures ranging from 8°C to 12°C throughout the study period ( Appendix A , Table A1 ). Water levels within the flumes were regulated using adjustable wooden panels positioned at the downstream end of each enclosure, allowing for both low and high-water experimental conditions. To establish natural prey conditions, each enclosure was initially stocked with benthic invertebrates collected from a nearby stream ( Appendix A , Table A2 ). Approximately 60 000 macroinvertebrates were introduced according to the method described in Mari et al. (2025), establishing a density of 5000–6000 individuals per m². This density was maintained throughout the experimental period by reinoculating the enclosures (three times in total). Each enclosure was divided into three distinct habitat sections. The upstream section, considered the "best" habitat, had a substrate composed of gravel and pebbles and a shelter consisting of a half-buried flowerpot with a piece of wood mounted to represent a more complex structure preferred by salmonids (Bjornn & Reiser, 1991; Whiteway et al., 2010). The "intermediate" habitat had a gravel substrate and two shelters without additional wood enrichment. The downstream "poorest" habitat consisted of a sandy substrate with a single shelter. Additionally, the upstream buffer zone was inoculated with invertebrates and regularly moved to promote the drift of food items into the enclosure. This design created a continuous gradient within the enclosure, from the most to the least favourable habitat (Fig. 1 b). Each enclosure was equipped with two stationary RFID antennas (Multiple Antenna PIT tag reader, Oregon RFID, USA) and an IR-sensitive camera (RLC-810A, Reolink, China) mounted on an overhead metal frame, providing complete video coverage of the enclosure area. This allowed for tracking of fish to be used for determination of dominant fish and activity rates. The cameras were programmed to record 30-minute intervals throughout both day and night, specifically during periods when the experimental area was undisturbed by human activity. To allow for nighttime observations, infrared lights were installed on the metal frames. Experimental design Each enclosure contained six size-matched fish: either six brown trout (allopatric treatment) or three brown trout and three brook trout (sympatric treatment). The brown trout were wild-caught or hatchery-reared from wild parental broodstock, with the latter having been exposed to long-term laboratory captivity before the experiment (Závorka et al. 2025, Mari et al. 2025). This experiment was not specifically designed to compare hatchery-reared and wild fish; however, hatchery fish were included in accordance with the 3R’s principle of animal research (Russell and Burch, 1959). This approach exemplifies the principle of reduction by avoiding the use of additional wild fish to increase statistical power, thereby minimizing the number of research animals used. We argue that pooling these two groups is appropriate as the patterns we observed did not differ between the hatchery and wild fish. All brook trout were wild caught. For transfer to and from the enclosures the fish were collected using electrofishing (EFKO 1500, Germany). Each experimental replicate utilized new brown trout individuals; however, brook trout were reused multiple rounds of the experiment to further reduce the use of wild fish. The brook trout were redistributed among the enclosures before each experimental round to minimize their familiarity with conspecifics and to reduce any advantage they might gain over brown trout due to the prior resident effect. Prior to each experimental run, fish were held in an acclimation zone at the downstream end of the channels for approximately one week. Individual fish were measured, weighed, and then placed in one of the eight enclosures. Each experimental runs lasted 10 days, during which daily monitoring was conducted using a portable RFID antenna (HPR Plus reader with BP Plus Portable Antenna, Biomark, USA) to record each fish's precise location within the enclosure (measured in meters from downstream to upstream, including shelter usage). This scanning procedure was performed three times a day always in the morning, with the consecutive runs divided by ~ 15 minutes intervals between them. The scanning was always conducted from down- to upstream direction. Water levels were systematically manipulated throughout the experimental run following this schedule: Days 1–4: High water level Days 5–6: Lowered water level Days 7–10: Return to high water level On day 7, one fish was removed from each enclosure. The selection was based on behavioural data collected from both stationary and portable antennas (activity rate and time spent upstream) that were continually assessed throughout the experimental runs, targeting the individual assumed to be the boldest; in the sympatric enclosures the boldest brook trout was selected. During the experiment a subset of the fish was subjected to an emergence test in the laboratory, where latency to leave a shelter was used as the measure of boldness. Although the results were only available after the experiments were finished, we used them to evaluate the legitimacy of our removal selection specifically for brook trout ( Appendix A , Fig. 1 a). On day 10 all fish were collected, weighed and length measured to determine individual growth. Brown trout were euthanized with an overdose of anesthetics (0.5 ml·L⁻¹ 2‑phenoxyethanol) to take samples of brain, muscle and liver for a different study (Závorka et al., 2025). Analysis The single, most dominant individual in each enclosure was identified by analyzing videos and cross-referencing the observations with data from antenna passings. To ensure accuracy, the fish were given a settling period before dominance was assessed. In most cases, the social hierarchy appeared to be established by day 8, to determine the most dominant individual displaying antagonistic behaviour (i.e. chasing, biting and guarding of territory) towards other fish. It is reasonable to believe the group structure would have settled on a hierarchy within this period (Ejike & Schreck, 1980). As we were identifying only the most dominant a fish this was a sufficient method and the additional measurable metrics usually required in observational studies (Metcalfe, 1989) were not necessary. The fish were thus categorized as either dominant (1) or non-dominant (0). We compared brown trout in allopatry and in sympatry with brook trout to determine whether an eventual growth benefit would be gained from being most dominant in both treatments. The specific mass growth rate of mass was used as the growth proxy. Position in the stream were measured in meters from the poorest habitat furthest downstream, resulting in individual detections receiving a score ranging from 0 (poorest) to 7 (best habitat). The average position score per experimental day, across all the experimental runs, could then be compared between individuals and species both in sympatric and allopatric groups. Similarly, shelter use was quantified from the data from active telemetry with emphasis on the later days of the experimental runs. Fish utilizing a shelter were noted daily when scanning the enclosures. Fish activity was quantified as detected movements between both antennas (if a fish is detected at antenna A and then at antenna B). This metric was used, as opposed to a pure count of antenna detections, to avoid mischaracterizing a stationary fish sitting within detection vicinity of one antenna as active. Statistics Statistical analyses were performed using R version 4.3.3 (R Core Team, 2023) and the package ggplot2 (Wickam, 2016) was used for visualizations. To assess the dominance-growth relationship, we performed linear regression analyses separately for allopatric and sympatric treatment groups. Model fit and normality assumptions were assessed by checking the distribution of residuals using histograms. The response variable used was mass growth rate, with dominance status and size rank as fixed effects. For the analysis of position within the enclosure, Kruskal-Wallis tests were performed, with pairwise Wilcoxon rank-sum tests. Bonferroni correction for multiple comparisons were used to identify which specific groups differed. Fish position (meters from downstream) served as the response variable and were compared between treatment groups and species. Shelter use were analyzed using a generalized linear mixed model (GLMM) with binomial distribution to examine the probability of detected fish being found in shelter locations. The model included species and experimental day as fixed effects, with day 2 as reference level. Model fitting was performed using maximum likelihood estimation. The analysis examined both main effects and interaction terms. The model structure allowed for examination of baseline shelter use probabilities, species and treatment differences in overall shelter use patterns, and temporal changes in shelter use across experimental days. Fish activity was analyzed using position changes per hour as a proxy for activity. The analysis included both daily activity comparisons and diel activity patterns. Prior to analysis, outliers were identified and removed using the interquartile range method applied separately within each species × treatment to ensure robust inference ( Appendix A , Fig. A2 ). Daily activity differences between groups were tested using Wilcoxon rank-sum tests with Bonferroni correction for multiple comparisons. Diel activity was analysed by categorizing observations into four periods (sunrise at 0400–1000, day at 1000–1600, sunset at 1600–2200, night at 2200 − 0400) and comparing activity between groups within each period using Wilcoxon rank-sum tests with Bonferroni correction. Results Dominance and growth For allopatric brown trout, dominance was a significant positive predictor of specific growth rate (t = 2.268, p = 0.025) (Fig. 2 ), whereas size did not have a significant effect (t = -0.126, p = 0.900). In stark contrast, for sympatric brown trout dominance was not a significant predictor of growth, the model showing effectively no relationship at all (t = 0.000, p = 1.000) (Fig. 2 ). Effect of size was not significant either (t = -0.865, p = 0.390). In sympatric groups, brown trout was the most dominant fish in 66.7% of the experimental trails. Overall, specific growth rates did not differ between treatment groups. For specific growth rate in mass, sympatry fish showed slightly higher values than allopatry fish (mean ± SD: -0.035 ± 0.475 vs -0.075 ± 0.522, respectively; t = 0.557, p = 0.578). Similarly, for specific growth rate in length, allopatric fish had marginally higher values than sympatry fish (0.025 ± 0.130 vs 0.008 ± 0.142, respectively; t = -1.010, p = 0.314). Position in stream Minimal differences were found on days 3–7 (all p > 0.21) (Fig. 3 ). Day 2 exhibited a significant overall effect (χ² = 6.26, df = 2, p = 0.0438), but pairwise Wilcoxon rank-sum tests with Bonferroni correction revealed no significant differences between any specific group pairs (all adjusted p > 0.12). By day 8 (after removal of boldest fish), significant differences emerged in positioning between groups (χ² = 11.26, df = 2, p = 0.0036). The most pronounced difference was observed on day 10 (χ² = 28.53, df = 2, p < 0.001), with highly significant differences between brook trout and brown trout in allopatry (p < 0.0001) and between brook trout and brown trout in sympatry (p < 0.0001). In summary it appears that after removal of the boldest fish on day 7 significant differences arose driven primarily by brook trout establishing a distinctly higher position compared to brown trout in sympatric groups. This divergence became even more pronounced by day 10 (Fig. 4 ). Change in water level did not appear to affect the outcome. Shelter use The fixed effects indicated a significant baseline intercept (p < 0.001) reflecting higher shelter use for brook trout on day 2. The main effect of species indicated that brown trout had significantly lower overall shelter use than brook trout (p = 0.001). Day-specific effects showed that, relative to day 2, there were no differences on days 3 and 4. However, on day 5, after lowering the water level, there was a significant increase in shelter use for brook trout (p = 0.00111). Following the removal of the boldest brook trout on day 7, no significant differences were observed. On day 8, the interaction indicated a significant increase for brown trout relative to brook trout (p = 0.018), with a similar pattern on day 10 (p = 0.016). Activity Brook trout exhibited higher activity than sympatric brown trout on multiple days, with significant differences on days 6–8 (all p.adj < 0.01) (Fig. 6 ). Allopatric and sympatric brown trout did not differ significantly on any individual day. Brook trout were more active than sympatric brown trout at sunset and at night (both p.adj < 0.001). Allopatric brown trout were more active than sympatric brown trout during the day (p.adj < 0.001) (Fig. 7 ). Across days day mean activity ranged from approximately 1.72 to 2.82 for brook trout and 1.04 to 2.80 for sympatric brown trout. Discussion In allopatric populations, we observed the expected relationship (i) between dominance status and growth rate; the most dominant fish clearly gain the benefit of enhanced mass growth compared to the sub-dominant fish. Hence, these individuals were better at monopolizing resources, consistent with previous findings that link social status to enhanced growth opportunities in salmonids (Metcalfe, 1986; Alanärä & Brännäs, 1996). However, and in agreement with our second hypothesis (ii), this fundamental relationship was completely diminished in sympatric populations, where dominance status showed no predictive power for growth outcomes for brown trout. Furthermore, sympatric brown trout showed differences in their activity patterns compared to their conspecifics in allopatry which was our third (iii) prediction, which may indicate that the cost of maintaining dominance for brown trout in the presence of brook trout is greater than in only intraspecific competition conditions. However, we did not find that sympatric brown trout overall were positioned further downstream or that they were more active during the day (iii). The average position of all groups of fish in the stream instead remained relatively stable throughout most of the experiment, with significant changes emerging in the final days following two key environmental perturbations: increase of water level after a period of low flow and the removal of the fish monopolizing the furthest upstream shelter. In contrast to our hypothesis (iv), removal of brook trout did not increase the use of the more profitable habitats for the brown trout. Interestingly, despite the removal of a conspecific, brook trout appeared to instead benefit more from these environmental changes than brown trout, potentially suggesting superior behavioural plasticity in response to environmental fluctuations. This rapid exploitation of “newly” available habitat space aligns with previous observations that brook trout are particularly effective at colonizing and utilizing habitats following environmental disturbances such as flooding or shifts in water flows (Fausch, 2008). Their ability to thrive in marginal and diverse habitats, especially during periods of environmental change, is considered a key factor in their success as invasive species (Korsu et al., 2012). Additionally, brown trout consistently showed lower probability of shelter use compared to brook trout for most days, although the magnitude of differences varied. Notably, during the initial low water phase on day 5, the differences were pronounced with a significantly higher probability of shelter use for brook trout when habitat availability suddenly decreased for the fish, again suggesting a competitive benefit for brook trout following an environmental change, under the assumption that the limited shelters of the flumes are subject to competition. The observed pattern of spatial reorganization also raises important implications for management strategies; the selective removal of bold individuals may inadvertently benefit the remaining brook trout by releasing them from intraspecific competition, potentially enabling them to become stronger interspecific competitors with brown trout. Our finding suggests that partial removal efforts, particularly those targeting bold individuals, might not effectively control invasive brook trout populations and could potentially strengthen their impact on native communities through more intensive exploitation of the micro-habitats suitable for the invasive species. Counterproductive effects of selective or partial removal have been documented previously. For example, Côté et al. (2014) found that lionfish on repeatedly culled coral reefs were less active and exploratory, displayed increased concealment and shyness compared to those on unculled reefs. In the short term, selective removal of individuals utilizing certain stream microhabitats can reduce intraspecific competition, potentially benefiting the remaining individuals. This may be the case because the fish occupying the stream’s preferred spots are typically most competitively dominant or bold. Eliminating these better competitors may immediately relax competition, allowing subordinates to redistribute into the newly vacated patches. In the long term, partial removal of an invasive species may lead to overcompensation, where the targeted species increases in abundance due to density-dependent recruitment processes, particularly in high-fecundity species when removal targets reproductive adults rather than juveniles (Zipkin et al., 2009). Consequently, selective removal presents a possible challenge with counterproductive effects both in the long and short term especially in the context of removing invasive species. Sympatric brown trout showed significantly lower daytime activity than allopatric brown trout which was opposite to what we predicted (iii), yet their overall activity rate was equal. This indicates compensation via increased activity at other times, including during the night. These diel activity variations emerge from individuals balancing the growth and survival trade-off under different competitive conditions (Railsback et al., 2005). Furthermore, brook trout were significantly more active than brown trout during the night in sympatry. It thus appears that sympatric brown trout change their diurnal activity in a maladaptive way, whereas brook trout do not. In sympatry, brook trout were significantly more active than brown trout during the night, meaning brown trout’s compensatory activity occurs when competitors are already highly active. Taken together, these results suggest that brown trout shift their activity in a way that is likely maladaptive, trading optimal foraging periods for times with higher competitive pressure, whereas brook trout do not. This conclusion is reasonable to draw because brook trout are better adapted to nighttime activity and have previously found to display a consistent primarily nocturnal activity pattern (Toobaie et al., 2013). This in spite of both species having very similar scotopic sensitivity levels and are in that way similarly visually equipped to low light levels. The reduced daytime activity could propose a problem for species more adapted to daytime foraging, and particularly for drift-feeding salmonids that rely heavily on visual detection of prey items. Elliot et al. (2011) experimentally demonstrated brown trout’s vision to optimized for daylight conditions, showing a superior feeding ability compared to arctic char ( Salvelinus alpinus ) during the day, but with an exponential decrease at light levels below 0.04 lx, and complete inability to feed in total darkness. Switching foraging strategy when adapted to daytime could come at a great cost to foraging efficiency. As an example, Fraser and Metcalfe (1997) found foraging efficiency for Atlantic salmon to be reduced to 10–35% of that of daytime during twilight and night. However, brown trout have well developed low-light vision and show a great deal of plasticity in their diurnal activity, which could be greatly influenced by social cues and may in that way explain why brown trout are decreasing rather than increasing their daytime activity in a presence of a more nocturnal species (Reebs, 2002). On the other hand they could be forced to make this change in order to maintain dominance status and guarding territory. Either way this could potentially help explain why the dominant fish are not displaying the enhanced growth in sympatry like they are in allopatry. The complete disruption of dominance-growth relationships, coupled with significant shifts in temporal activity patterns and spatial habitat use demonstrated in this study suggests that brook trout invasion may, in fact, trigger a cascade of behavioural and ecological changes in native brown trout populations negatively affecting fitness. These findings underscore the importance of considering behavioural plasticity and indirect effects when assessing invasion impacts. Understanding these complex behavioural responses will be crucial for predicting ecosystem outcomes and developing effective conservation strategies for native species and communities. Sympatric brown trout showed significantly lower daytime activity than allopatric brown trout which was opposite to what we predicted (iii), yet their overall activity rate was equal. This indicates compensation via increased activity at other times, potentially during the night. These diel activity variations emerge in the wild from individuals balancing the growth and survival trade-off under different competitive conditions (Railsback et al., 2005). Furthermore, brook trout were significantly more active than brown trout during the night in sympatry. It thus appears that sympatric brown trout change their diurnal activity in a maladaptive way, whereas brook trout do not. In sympatry, brook trout were significantly more active than brown trout during the night, meaning brown trout’s compensatory activity occurs when competitors are already highly active. Taken together, these results suggest that brown trout shift their activity in a way that is likely maladaptive, trading optimal foraging periods for times with higher competitive pressure, whereas brook trout do not. This conclusion is reasonable to draw because brook trout are better adapted to nighttime activity and have previously found to display a consistent primarily nocturnal activity pattern (Toobaie et al., 2013) in spite of both species having very similar scotopic sensitivity levels and are similarly visually equipped to low light levels. The reduced daytime activity could be a problem for species more adapted to daytime foraging, and particularly for drift-feeding salmonids that rely heavily on visual detection of prey items. Elliot et al. (2011) experimentally demonstrated brown trout’s vision to optimized for daylight conditions, showing a superior feeding ability compared to arctic char ( Salvelinus alpinus ) during the day, but with an exponential decrease at light levels below 0.04 lx, and complete inability to feed in total darkness. Switching foraging strategy when adapted to daytime could come at a great cost to foraging efficiency. As an example, Fraser and Metcalfe (1997) found foraging efficiency for Atlantic salmon to be reduced to 10–35% of that of daytime during twilight and night. However, brown trout have well developed low-light vision and show a great deal of plasticity in their diurnal activity, which could be greatly influenced by social cues and may in that way explain why brown trout are decreasing rather than increasing their daytime activity in a presence of a more nocturnal species (Reebs, 2002). On the other hand they could be forced to make this change in order to maintain dominance status and guarding territory. Either way this could potentially help explain why the dominant fish are not displaying the enhanced growth in sympatry like they are in allopatry. The complete disruption of dominance-growth relationships, coupled with significant shifts in temporal activity patterns and spatial habitat use demonstrated in this study suggests that brook trout invasion may, in fact, trigger a cascade of behavioural and ecological changes in native brown trout populations negatively affecting fitness. These findings underscore the importance of considering behavioural plasticity and indirect effects when assessing invasion impacts. Furthermore, our results indicate that partial removal of brook trout may fail to restore native social structures and can even strengthen invasive advantages. Understanding these complex behavioural responses will be crucial for predicting ecosystem outcomes and developing effective conservation strategies for native species and communities. Declarations Author contributions BA, LZ, SM and JH conceived and designed the study. BA, LZ, SM carried out the experiments and collected the data. LZ, SA and SM prepared study facilities. BA led the writing of the manuscript. BA, LZ, SM, SA and JH contributed to data interpretation and manuscript revisions. All authors contributed to the final text and approved the submitted version. Acknowledgements We thank following colleagues and students for their assistance: Pernilla Hansson, Simon Lafont, Lukas Wimmer, Johanna Harm, Evelina Olsen and Simon Vitecek. This project (grant 2020-00048) was funded by the Swedish Environmental Protection Agency (Naturvårdsverket) and the Swedish Research Council Formas to improve the management of invasive species in a joint effort with the Swedish Agency for Marine and Water Management (Havs- och vattenmyndigheten) and the Swedish Transport Administration (Trafikverket). The experiment was approved by the Ethical Committee for Animal Research in Göteborg (ID nr: 004882, D 408 nr 5.8.18–04990/2023) and compiled with Swedish and Austrian law. 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Journal of Fish Biology, 70 (4), 1095–1108. https://doi.org/10.1111/j.1095-8649.2007.01370.x Supplementary Files Appendix.docx Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 05 Feb, 2026 Reviewers invited by journal 03 Feb, 2026 Editor invited by journal 19 Jan, 2026 Editor assigned by journal 13 Jan, 2026 First submitted to journal 12 Jan, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8588818","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":585157329,"identity":"8959c716-4ab5-4817-8438-69adb16114e2","order_by":0,"name":"Benedikte Austad","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0002-1779-2396","institution":"University of Gothenburg Faculty of Science: Goteborgs universitet Naturvetenskapliga Fakulteten","correspondingAuthor":true,"prefix":"","firstName":"Benedikte","middleName":"","lastName":"Austad","suffix":""},{"id":585157330,"identity":"9e628c47-c79d-41fd-9c0f-bcc060979e02","order_by":1,"name":"Libor Závorka","email":"","orcid":"","institution":"WasserClsuter - Biologische Station Lunz, Inter-university Centre for Aquatic Ecosystem Research","correspondingAuthor":false,"prefix":"","firstName":"Libor","middleName":"","lastName":"Závorka","suffix":""},{"id":585157331,"identity":"43aad8c7-796d-4d7f-8f65-3fc6d3625808","order_by":2,"name":"Stefano Mari","email":"","orcid":"","institution":"WasserCluster, Biologische Station Lunz, Inter-Univeristy Centre for Aquatic Ecosystem Research","correspondingAuthor":false,"prefix":"","firstName":"Stefano","middleName":"","lastName":"Mari","suffix":""},{"id":585157332,"identity":"1d60de1f-78c1-43da-a694-995f11c54596","order_by":3,"name":"Stefan Auer","email":"","orcid":"","institution":"Institute of Hydrobiology and Aquatic Ecosystem Management, BOKU Univeristy","correspondingAuthor":false,"prefix":"","firstName":"Stefan","middleName":"","lastName":"Auer","suffix":""},{"id":585157335,"identity":"09bf2447-2c56-45ed-8be4-2c0d03763b0b","order_by":4,"name":"Johan Höjesjö","email":"","orcid":"","institution":"University of Gothenburg Faculty of Science: Goteborgs universitet Naturvetenskapliga Fakulteten","correspondingAuthor":false,"prefix":"","firstName":"Johan","middleName":"","lastName":"Höjesjö","suffix":""}],"badges":[],"createdAt":"2026-01-13 07:51:08","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8588818/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8588818/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":102040169,"identity":"7c54a3f7-072b-4fcc-8f75-5b5d23fe49a1","added_by":"auto","created_at":"2026-02-06 12:48:18","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":66947,"visible":true,"origin":"","legend":"\u003cp\u003ea) Schematic of the two channels at the HyTEC facility viewed from above and the experimental enclosures (1-8), and b) schematic displaying an individual experimental enclosure. The black arrow indicates direction of habitat quality improvement within the enclosure.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8588818/v1/c8b54eb1191c5e4c974e0910.png"},{"id":102295639,"identity":"5c97ae3d-ed30-4542-abff-d3c50ed58615","added_by":"auto","created_at":"2026-02-10 10:13:30","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":48641,"visible":true,"origin":"","legend":"\u003cp\u003eSpecific growth rate for mass plotted against dominance status for brown trout in allopatry and sympatry.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8588818/v1/dc9bfbb60e99430887c53ae8.png"},{"id":102040170,"identity":"bd66a062-f93c-4ea9-b537-311340fbc5be","added_by":"auto","created_at":"2026-02-06 12:48:18","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":340288,"visible":true,"origin":"","legend":"\u003cp\u003eOverview of where in the stream channel fish were detected over the different days of the experiment for brook trout, sympatric brown trout and allopatric brown trout for control. Bigger points indicate group averages with standard deviation lines.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8588818/v1/6e510b780062f33750f3b803.png"},{"id":102295881,"identity":"afa0e5f6-1252-450d-b7a1-f3cd3dc1ceaa","added_by":"auto","created_at":"2026-02-10 10:15:47","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":65313,"visible":true,"origin":"","legend":"\u003cp\u003ePlot visualising the spatial distribution of fish in the stream at day 10 of the experiment.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8588818/v1/c6808c21046ebd17a0e08b0d.png"},{"id":102040173,"identity":"9fdfbb60-9cb1-47bb-8e0d-70cf5124b212","added_by":"auto","created_at":"2026-02-06 12:48:18","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":47485,"visible":true,"origin":"","legend":"\u003cp\u003eProbability of detected fish being detected in a shelter across days for brook trout and brown trout in sympatry. Vertical dashed lines indicate periods of shifting water levels or fish removal.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8588818/v1/193738585dcf70bda98a36fd.png"},{"id":102295777,"identity":"24130324-a17f-491d-8d11-50689a9076f6","added_by":"auto","created_at":"2026-02-10 10:14:59","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":51356,"visible":true,"origin":"","legend":"\u003cp\u003eActivity rates per group each experimental day, compared between brook trout and sympatric brown trout, and sympatric and allopatric brown trout. Statistical significance in the figures is indicated by asterisks: *** denotes p \u0026lt; 0.001, ** denotes p \u0026lt; 0.01, and * denotes p \u0026lt; 0.05. If no brackets are present the differences are non-significant.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8588818/v1/9018efa5ee545f06420edcd1.png"},{"id":102040175,"identity":"deba5958-1bc8-417f-ab5a-9c253fe23b02","added_by":"auto","created_at":"2026-02-06 12:48:18","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":43736,"visible":true,"origin":"","legend":"\u003cp\u003eActivity rates per group during four intervals of the day, compared between brook trout and sympatric brown trout, and sympatric and allopatric brown trout. Statistical significance in the figures is indicated by asterisks: *** denotes p \u0026lt; 0.001, ** denotes p \u0026lt; 0.01, and * denotes p \u0026lt; 0.05. “Ns” indicates non-significant differences.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8588818/v1/a4eda623e797288cf7752934.png"},{"id":102299069,"identity":"d42f379a-dd1f-4aba-897f-8e88151d4245","added_by":"auto","created_at":"2026-02-10 11:02:47","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1218993,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8588818/v1/da15e256-3a35-4ef8-aea2-54a95997ca54.pdf"},{"id":102295576,"identity":"e8a4d88d-ea81-43b5-9f29-e4a5d5d3bcf6","added_by":"auto","created_at":"2026-02-10 10:12:46","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":182016,"visible":true,"origin":"","legend":"","description":"","filename":"Appendix.docx","url":"https://assets-eu.researchsquare.com/files/rs-8588818/v1/9e53cd2f3f14af7055138741.docx"}],"financialInterests":"","formattedTitle":"\u003cp\u003eInvasive Brook trout (\u003cem\u003eSalvelinus fontinalis\u003c/em\u003e) presence may reduce dominance-linked growth and shift diel activity in brown trout \u003cem\u003e(Salmo trutta\u003c/em\u003e)\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eTo increase foraging and competitive success, and reduce the risk of predation, animals generally optimize habitat use both in time and space (for example: Suselbeek et al., 2014; Kohl et al., 2018; Moiron et al., 2018; Sells \u0026amp; Mitchell, 2020), where decisions reflect an apparent trade-off between the risk of predation/injury and the benefit to be gained from engaging in said activity (Lima \u0026amp; Dill, 1990). Even closely related species, such as different salmonid fishes, demonstrate variation in activity patterns reflecting the species-specific adaptations to the trade-offs between foraging, competitive ability and predator avoidance (Johnson \u0026amp; McKenna, 2015; David et al., 2007). Social hierarchies are common among territorial fish and allow them to reduce the cost of aggressive interactions and facilitate access to resources (Eaton, 2011). Under a stable social hierarchy, subordinate individuals are excluded from the most profitable positions in the stream, but they may still maintain high fitness by avoiding costly interaction while feeding at profitable sites downstream (Nilsson et al., 2004). Alternatively, fish might adjust their activity temporally to avoid costly interactions with familiar dominant individuals (Johnsson, 2010). For example, Alan\u0026auml;r\u0026auml; et al. (2001) demonstrated that dominant brown trout (\u003cem\u003eSalmo trutta\u003c/em\u003e) fed predominantly at the most beneficial times from dusk until early night, when food was abundant and predation risk lower, while subdominant brown trout fed more during the day. Variation in activity, boldness, and aggressiveness within and between the species may be critical for the formation and stability of social hierarchy (Coll\u0026eacute;ter \u0026amp; Brown, 2011, Railsback et al., 2005). For example, Z\u0026aacute;vorka et al. (2016) showed that shy and possibly subdominant brown trout display higher changes in the habitat use between day and night, being more active during the night, while bold and likely more dominant individuals maintained high activity throughout the whole daily cycle.\u003c/p\u003e \u003cp\u003eThe competition and habitat use of riverine fish is also strongly influenced by the hydrological regime as this defines the availability and distribution of suitable habitats. Increasing rapid changes in water level of streams are expected, both because of the more frequent severe weather events (Benetti et al., 2024) and habitat degradation due to fragmentation of rivers and short-term water regulations. This may force stream fish into suboptimal habitats, locally increase the density of fish and produce unexpected outcomes for their diel activity patterns and competitive interactions. For example, Stradmeyer et al. (2008) found that during rapid dewatering events, juvenile Atlantic salmon in sympatry with brown trout were able to maintain access to pool refuges. This was observed even though brown trout are typically the more dominant and aggressive species and occupied pools during normal flow conditions to a larger degree than the salmon did and were thereby predicted to exclude salmon from these habitats. Hence, salmonids adjust their diel patterns and habitat use depending on a whole suite of internal and external factors (Metcalfe et al., 1998; Railsback et al., 2005; Larranaga \u0026amp; Steingr\u0026iacute;msson, 2015); flexibility in this trait may be especially beneficial in dynamic environments typical for salmonid nursery streams, where available habitat and density of conspecifics may change rapidly (Metcalfe et al., 1998).\u003c/p\u003e \u003cp\u003eAdditionally, competition with closely related non-native species has been shown to hamper optimal habitat use and fitness of native fishes (Cucherousset \u0026amp; Olden, 2011; Z\u0026aacute;vorka et al., 2017), which may lead to decrease of population size and local extinction of the native fishes (Houde et al., 2016). Hence, benefits of stable social hierarchy for native fish may be disrupted by presence of non-native species with different behavioural and social traits. For example, Roberge et al. (2008) found that the introduction of rainbow trout (\u003cem\u003eOncorhynchus mykiss\u003c/em\u003e) disrupted social hierarchies in Atlantic salmon (\u003cem\u003eSalmo salar\u003c/em\u003e). Similarly, it has been shown that introduction of brook trout (\u003cem\u003eSalvelinus fontinalis\u003c/em\u003e) can destabilize the social hierarchy of brown trout (Lov\u0026eacute;n Wallerius et al., 2022), with impacts on its daily activity patterns (Larranaga et al., 2019) and home range size (Z\u0026aacute;vorka et al., 2017). Consequently, considerable effort is being made to remove non-native fish species from ecosystems (Simberloff, 2009). However, complete eradication of invasive species is frequently not achieved (Britton et al., 2010). Therefore, an important but often overlooked question is how the partial removal of invasive fish species may affect habitat use and fitness of native fishes in the dynamic habitat of headwater streams. This question is even more pressing because removal techniques for invasive fish, such as electrofishing, netting (Biro \u0026amp; Dingemanse, 2009) or spearfishing (C\u0026ocirc;t\u0026eacute; et al., 2014), may be biased towards bolder and more active behavioural phenotypes. Therefore, it is reasonable to predict that removal effort of invasive species will change the proportion of certain phenotypes which may influence competitive interactions and fitness consequences in the native fish population.\u003c/p\u003e \u003cp\u003eGiven the potential for invasive species to alter social hierarchies and the distribution of behavioural phenotypes in native fish populations, the aim of the current study is to test the effect of competition between brook and brown trout in different water regimes and after selective removal of invasive brook trout. Using an artificial flume, we were able to obtain detailed information on habitat use, activity and fitness of native brown trout in various water flows with (sympatric conditions) and without (allopatric conditions) the presence of brook trout.\u003c/p\u003e \u003cp\u003eSpecifically, we predict that:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eDominant brown trout in allopatry will display faster growth rates compared to subdominant conspecifics.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eCompetition with invasive brook trout will hamper the fitness benefits for the dominant brown trout, reflected in their growth.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eCompetition with invasive brook trout will prevent brown trout monopolizing profitable habitats and optimize their diurnal activity. Specifically, we predict sympatric brown trout to stay further downstream in less optimal habitats and to be more active during the day.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003ePartial removal of invasive brook trout from the most profitable habitat will allow the sympatric brown trout to monopolize the most profitable habitats.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eHyTEC flumes\u003c/h2\u003e \u003cp\u003eIn this study, we investigated the effects of brook trout invasion on brown trout by comparing habitat use, activity pattern and growth between allopatric groups of brown trout with sympatric groups of brown trout and brook trout. The research was conducted at the so called HyTEC facility (Hydromorphological and Temperature Experimental Channels) in Lunz am See (Austria), which consists of two 40 m long artificial stream channels designed to simulate natural stream habitat conditions. The water is supplied by Lake Lunz and discharged back to the natural outflow of the lake (see: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://hydropeaking.boku.ac.at\u003c/span\u003e\u003cspan address=\"https://hydropeaking.boku.ac.at\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). Each channel was subdivided longitudinally into four 7 m long enclosures,1.5 m in width, separated by 2 m buffer zones. The cross-sectional profile of each enclosure included a deep and a shallow part. During high water levels, the water depth was ~\u0026thinsp;30 cm and ~\u0026thinsp;15 cm in the deep and shallow part, respectively, and the average water flow was ~\u0026thinsp;7 cm/s. During low water levels, the water depth was ~\u0026thinsp;15 cm and ~\u0026thinsp;0 cm in the deep and shallow part, respectively, with an average water flow of ~\u0026thinsp;16 cm/s.\u003c/p\u003e \u003cp\u003eThe enclosures were partitioned using fencing with approximately 0.5 cm\u0026sup2; mesh size, which allowed continuous water flow while preventing fish movement between sections (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Water discharge was maintained at a constant rate of 25 L s⁻\u0026sup1;, with water temperatures ranging from 8\u0026deg;C to 12\u0026deg;C throughout the study period (\u003cb\u003eAppendix A\u003c/b\u003e, Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003eA1\u003c/span\u003e). Water levels within the flumes were regulated using adjustable wooden panels positioned at the downstream end of each enclosure, allowing for both low and high-water experimental conditions.\u003c/p\u003e \u003cp\u003eTo establish natural prey conditions, each enclosure was initially stocked with benthic invertebrates collected from a nearby stream (\u003cb\u003eAppendix A\u003c/b\u003e, Table \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003eA2\u003c/span\u003e). Approximately 60 000 macroinvertebrates were introduced according to the method described in Mari et al. (2025), establishing a density of 5000\u0026ndash;6000 individuals per m\u0026sup2;. This density was maintained throughout the experimental period by reinoculating the enclosures (three times in total).\u003c/p\u003e \u003cp\u003eEach enclosure was divided into three distinct habitat sections. The upstream section, considered the \"best\" habitat, had a substrate composed of gravel and pebbles and a shelter consisting of a half-buried flowerpot with a piece of wood mounted to represent a more complex structure preferred by salmonids (Bjornn \u0026amp; Reiser, 1991; Whiteway et al., 2010). The \"intermediate\" habitat had a gravel substrate and two shelters without additional wood enrichment. The downstream \"poorest\" habitat consisted of a sandy substrate with a single shelter. Additionally, the upstream buffer zone was inoculated with invertebrates and regularly moved to promote the drift of food items into the enclosure. This design created a continuous gradient within the enclosure, from the most to the least favourable habitat (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eEach enclosure was equipped with two stationary RFID antennas (Multiple Antenna PIT tag reader, Oregon RFID, USA) and an IR-sensitive camera (RLC-810A, Reolink, China) mounted on an overhead metal frame, providing complete video coverage of the enclosure area. This allowed for tracking of fish to be used for determination of dominant fish and activity rates. The cameras were programmed to record 30-minute intervals throughout both day and night, specifically during periods when the experimental area was undisturbed by human activity. To allow for nighttime observations, infrared lights were installed on the metal frames.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eExperimental design\u003c/h3\u003e\n\u003cp\u003eEach enclosure contained six size-matched fish: either six brown trout (allopatric treatment) or three brown trout and three brook trout (sympatric treatment). The brown trout were wild-caught or hatchery-reared from wild parental broodstock, with the latter having been exposed to long-term laboratory captivity before the experiment (Z\u0026aacute;vorka et al. 2025, Mari et al. 2025). This experiment was not specifically designed to compare hatchery-reared and wild fish; however, hatchery fish were included in accordance with the 3R\u0026rsquo;s principle of animal research (Russell and Burch, 1959). This approach exemplifies the principle of reduction by avoiding the use of additional wild fish to increase statistical power, thereby minimizing the number of research animals used. We argue that pooling these two groups is appropriate as the patterns we observed did not differ between the hatchery and wild fish. All brook trout were wild caught. For transfer to and from the enclosures the fish were collected using electrofishing (EFKO 1500, Germany).\u003c/p\u003e \u003cp\u003eEach experimental replicate utilized new brown trout individuals; however, brook trout were reused multiple rounds of the experiment to further reduce the use of wild fish. The brook trout were redistributed among the enclosures before each experimental round to minimize their familiarity with conspecifics and to reduce any advantage they might gain over brown trout due to the prior resident effect. Prior to each experimental run, fish were held in an acclimation zone at the downstream end of the channels for approximately one week. Individual fish were measured, weighed, and then placed in one of the eight enclosures. Each experimental runs lasted 10 days, during which daily monitoring was conducted using a portable RFID antenna (HPR Plus reader with BP Plus Portable Antenna, Biomark, USA) to record each fish's precise location within the enclosure (measured in meters from downstream to upstream, including shelter usage). This scanning procedure was performed three times a day always in the morning, with the consecutive runs divided by ~\u0026thinsp;15 minutes intervals between them. The scanning was always conducted from down- to upstream direction.\u003c/p\u003e \u003cp\u003eWater levels were systematically manipulated throughout the experimental run following this schedule:\u003c/p\u003e\n\u003ch3\u003eDays 1–4: High water level\u003c/h3\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eDays 5\u0026ndash;6: Lowered water level\u003c/h2\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003eDays 7\u0026ndash;10: Return to high water level\u003c/h2\u003e \u003cp\u003eOn day 7, one fish was removed from each enclosure. The selection was based on behavioural data collected from both stationary and portable antennas (activity rate and time spent upstream) that were continually assessed throughout the experimental runs, targeting the individual assumed to be the boldest; in the sympatric enclosures the boldest brook trout was selected. During the experiment a subset of the fish was subjected to an emergence test in the laboratory, where latency to leave a shelter was used as the measure of boldness. Although the results were only available after the experiments were finished, we used them to evaluate the legitimacy of our removal selection specifically for brook trout (\u003cb\u003eAppendix A\u003c/b\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). On day 10 all fish were collected, weighed and length measured to determine individual growth. Brown trout were euthanized with an overdose of anesthetics (0.5 ml\u0026middot;L⁻\u0026sup1; 2‑phenoxyethanol) to take samples of brain, muscle and liver for a different study (Z\u0026aacute;vorka et al., 2025).\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eAnalysis\u003c/h2\u003e \u003cp\u003eThe single, most dominant individual in each enclosure was identified by analyzing videos and cross-referencing the observations with data from antenna passings. To ensure accuracy, the fish were given a settling period before dominance was assessed. In most cases, the social hierarchy appeared to be established by day 8, to determine the most dominant individual displaying antagonistic behaviour (i.e. chasing, biting and guarding of territory) towards other fish. It is reasonable to believe the group structure would have settled on a hierarchy within this period (Ejike \u0026amp; Schreck, 1980). As we were identifying only the most dominant a fish this was a sufficient method and the additional measurable metrics usually required in observational studies (Metcalfe, 1989) were not necessary. The fish were thus categorized as either dominant (1) or non-dominant (0). We compared brown trout in allopatry and in sympatry with brook trout to determine whether an eventual growth benefit would be gained from being most dominant in both treatments. The specific mass growth rate of mass was used as the growth proxy.\u003c/p\u003e \u003cp\u003ePosition in the stream were measured in meters from the poorest habitat furthest downstream, resulting in individual detections receiving a score ranging from 0 (poorest) to 7 (best habitat). The average position score per experimental day, across all the experimental runs, could then be compared between individuals and species both in sympatric and allopatric groups. Similarly, shelter use was quantified from the data from active telemetry with emphasis on the later days of the experimental runs. Fish utilizing a shelter were noted daily when scanning the enclosures.\u003c/p\u003e \u003cp\u003eFish activity was quantified as detected movements between both antennas (if a fish is detected at antenna A and then at antenna B). This metric was used, as opposed to a pure count of antenna detections, to avoid mischaracterizing a stationary fish sitting within detection vicinity of one antenna as active.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eStatistics\u003c/h3\u003e\n\u003cp\u003eStatistical analyses were performed using R version 4.3.3 (R Core Team, 2023) and the package ggplot2 (Wickam, 2016) was used for visualizations. To assess the dominance-growth relationship, we performed linear regression analyses separately for allopatric and sympatric treatment groups. Model fit and normality assumptions were assessed by checking the distribution of residuals using histograms. The response variable used was mass growth rate, with dominance status and size rank as fixed effects.\u003c/p\u003e \u003cp\u003eFor the analysis of position within the enclosure, Kruskal-Wallis tests were performed, with pairwise Wilcoxon rank-sum tests. Bonferroni correction for multiple comparisons were used to identify which specific groups differed. Fish position (meters from downstream) served as the response variable and were compared between treatment groups and species.\u003c/p\u003e \u003cp\u003eShelter use were analyzed using a generalized linear mixed model (GLMM) with binomial distribution to examine the probability of detected fish being found in shelter locations. The model included species and experimental day as fixed effects, with day 2 as reference level. Model fitting was performed using maximum likelihood estimation. The analysis examined both main effects and interaction terms. The model structure allowed for examination of baseline shelter use probabilities, species and treatment differences in overall shelter use patterns, and temporal changes in shelter use across experimental days.\u003c/p\u003e \u003cp\u003eFish activity was analyzed using position changes per hour as a proxy for activity. The analysis included both daily activity comparisons and diel activity patterns. Prior to analysis, outliers were identified and removed using the interquartile range method applied separately within each species \u0026times; treatment to ensure robust inference (\u003cb\u003eAppendix A\u003c/b\u003e, Fig. \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003eA2\u003c/span\u003e). Daily activity differences between groups were tested using Wilcoxon rank-sum tests with Bonferroni correction for multiple comparisons. Diel activity was analysed by categorizing observations into four periods (sunrise at 0400\u0026ndash;1000, day at 1000\u0026ndash;1600, sunset at 1600\u0026ndash;2200, night at 2200\u0026thinsp;\u0026minus;\u0026thinsp;0400) and comparing activity between groups within each period using Wilcoxon rank-sum tests with Bonferroni correction.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eDominance and growth\u003c/h2\u003e \u003cp\u003eFor allopatric brown trout, dominance was a significant positive predictor of specific growth rate (t\u0026thinsp;=\u0026thinsp;2.268, p\u0026thinsp;=\u0026thinsp;0.025) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), whereas size did not have a significant effect (t = -0.126, p\u0026thinsp;=\u0026thinsp;0.900). In stark contrast, for sympatric brown trout dominance was not a significant predictor of growth, the model showing effectively no relationship at all (t\u0026thinsp;=\u0026thinsp;0.000, p\u0026thinsp;=\u0026thinsp;1.000) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Effect of size was not significant either (t = -0.865, p\u0026thinsp;=\u0026thinsp;0.390). In sympatric groups, brown trout was the most dominant fish in 66.7% of the experimental trails. Overall, specific growth rates did not differ between treatment groups.\u003c/p\u003e \u003cp\u003eFor specific growth rate in mass, sympatry fish showed slightly higher values than allopatry fish (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD: -0.035\u0026thinsp;\u0026plusmn;\u0026thinsp;0.475 vs -0.075\u0026thinsp;\u0026plusmn;\u0026thinsp;0.522, respectively; t\u0026thinsp;=\u0026thinsp;0.557, p\u0026thinsp;=\u0026thinsp;0.578). Similarly, for specific growth rate in length, allopatric fish had marginally higher values than sympatry fish (0.025\u0026thinsp;\u0026plusmn;\u0026thinsp;0.130 vs 0.008\u0026thinsp;\u0026plusmn;\u0026thinsp;0.142, respectively; t = -1.010, p\u0026thinsp;=\u0026thinsp;0.314).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003ePosition in stream\u003c/h2\u003e \u003cp\u003eMinimal differences were found on days 3\u0026ndash;7 (all p\u0026thinsp;\u0026gt;\u0026thinsp;0.21) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Day 2 exhibited a significant overall effect (χ\u0026sup2; = 6.26, df\u0026thinsp;=\u0026thinsp;2, p\u0026thinsp;=\u0026thinsp;0.0438), but pairwise Wilcoxon rank-sum tests with Bonferroni correction revealed no significant differences between any specific group pairs (all adjusted p\u0026thinsp;\u0026gt;\u0026thinsp;0.12).\u003c/p\u003e \u003cp\u003eBy day 8 (after removal of boldest fish), significant differences emerged in positioning between groups (χ\u0026sup2; = 11.26, df\u0026thinsp;=\u0026thinsp;2, p\u0026thinsp;=\u0026thinsp;0.0036). The most pronounced difference was observed on day 10 (χ\u0026sup2; = 28.53, df\u0026thinsp;=\u0026thinsp;2, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), with highly significant differences between brook trout and brown trout in allopatry (p\u0026thinsp;\u0026lt;\u0026thinsp;0.0001) and between brook trout and brown trout in sympatry (p\u0026thinsp;\u0026lt;\u0026thinsp;0.0001). In summary it appears that after removal of the boldest fish on day 7 significant differences arose driven primarily by brook trout establishing a distinctly higher position compared to brown trout in sympatric groups. This divergence became even more pronounced by day 10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Change in water level did not appear to affect the outcome.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eShelter use\u003c/h2\u003e \u003cp\u003eThe fixed effects indicated a significant baseline intercept (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) reflecting higher shelter use for brook trout on day 2. The main effect of species indicated that brown trout had significantly lower overall shelter use than brook trout (p\u0026thinsp;=\u0026thinsp;0.001). Day-specific effects showed that, relative to day 2, there were no differences on days 3 and 4. However, on day 5, after lowering the water level, there was a significant increase in shelter use for brook trout (p\u0026thinsp;=\u0026thinsp;0.00111). Following the removal of the boldest brook trout on day 7, no significant differences were observed. On day 8, the interaction indicated a significant increase for brown trout relative to brook trout (p\u0026thinsp;=\u0026thinsp;0.018), with a similar pattern on day 10 (p\u0026thinsp;=\u0026thinsp;0.016).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eActivity\u003c/h2\u003e \u003cp\u003eBrook trout exhibited higher activity than sympatric brown trout on multiple days, with significant differences on days 6\u0026ndash;8 (all p.adj\u0026thinsp;\u0026lt;\u0026thinsp;0.01) (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Allopatric and sympatric brown trout did not differ significantly on any individual day. Brook trout were more active than sympatric brown trout at sunset and at night (both p.adj\u0026thinsp;\u0026lt;\u0026thinsp;0.001). Allopatric brown trout were more active than sympatric brown trout during the day (p.adj\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). Across days day mean activity ranged from approximately 1.72 to 2.82 for brook trout and 1.04 to 2.80 for sympatric brown trout.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn allopatric populations, we observed the expected relationship (i) between dominance status and growth rate; the most dominant fish clearly gain the benefit of enhanced mass growth compared to the sub-dominant fish. Hence, these individuals were better at monopolizing resources, consistent with previous findings that link social status to enhanced growth opportunities in salmonids (Metcalfe, 1986; Alan\u0026auml;r\u0026auml; \u0026amp; Br\u0026auml;nn\u0026auml;s, 1996). However, and in agreement with our second hypothesis (ii), this fundamental relationship was completely diminished in sympatric populations, where dominance status showed no predictive power for growth outcomes for brown trout. Furthermore, sympatric brown trout showed differences in their activity patterns compared to their conspecifics in allopatry which was our third (iii) prediction, which may indicate that the cost of maintaining dominance for brown trout in the presence of brook trout is greater than in only intraspecific competition conditions. However, we did not find that sympatric brown trout overall were positioned further downstream or that they were more active during the day (iii).\u003c/p\u003e \u003cp\u003eThe average position of all groups of fish in the stream instead remained relatively stable throughout most of the experiment, with significant changes emerging in the final days following two key environmental perturbations: increase of water level after a period of low flow and the removal of the fish monopolizing the furthest upstream shelter. In contrast to our hypothesis (iv), removal of brook trout did not increase the use of the more profitable habitats for the brown trout. Interestingly, despite the removal of a conspecific, brook trout appeared to instead benefit more from these environmental changes than brown trout, potentially suggesting superior behavioural plasticity in response to environmental fluctuations. This rapid exploitation of \u0026ldquo;newly\u0026rdquo; available habitat space aligns with previous observations that brook trout are particularly effective at colonizing and utilizing habitats following environmental disturbances such as flooding or shifts in water flows (Fausch, 2008). Their ability to thrive in marginal and diverse habitats, especially during periods of environmental change, is considered a key factor in their success as invasive species (Korsu et al., 2012). Additionally, brown trout consistently showed lower probability of shelter use compared to brook trout for most days, although the magnitude of differences varied. Notably, during the initial low water phase on day 5, the differences were pronounced with a significantly higher probability of shelter use for brook trout when habitat availability suddenly decreased for the fish, again suggesting a competitive benefit for brook trout following an environmental change, under the assumption that the limited shelters of the flumes are subject to competition.\u003c/p\u003e \u003cp\u003eThe observed pattern of spatial reorganization also raises important implications for management strategies; the selective removal of bold individuals may inadvertently benefit the remaining brook trout by releasing them from intraspecific competition, potentially enabling them to become stronger interspecific competitors with brown trout. Our finding suggests that partial removal efforts, particularly those targeting bold individuals, might not effectively control invasive brook trout populations and could potentially strengthen their impact on native communities through more intensive exploitation of the micro-habitats suitable for the invasive species. Counterproductive effects of selective or partial removal have been documented previously. For example, C\u0026ocirc;t\u0026eacute; et al. (2014) found that lionfish on repeatedly culled coral reefs were less active and exploratory, displayed increased concealment and shyness compared to those on unculled reefs. In the short term, selective removal of individuals utilizing certain stream microhabitats can reduce intraspecific competition, potentially benefiting the remaining individuals. This may be the case because the fish occupying the stream\u0026rsquo;s preferred spots are typically most competitively dominant or bold. Eliminating these better competitors may immediately relax competition, allowing subordinates to redistribute into the newly vacated patches. In the long term, partial removal of an invasive species may lead to overcompensation, where the targeted species increases in abundance due to density-dependent recruitment processes, particularly in high-fecundity species when removal targets reproductive adults rather than juveniles (Zipkin et al., 2009). Consequently, selective removal presents a possible challenge with counterproductive effects both in the long and short term especially in the context of removing invasive species.\u003c/p\u003e \u003cp\u003eSympatric brown trout showed significantly lower daytime activity than allopatric brown trout which was opposite to what we predicted (iii), yet their overall activity rate was equal. This indicates compensation via increased activity at other times, including during the night. These diel activity variations emerge from individuals balancing the growth and survival trade-off under different competitive conditions (Railsback et al., 2005). Furthermore, brook trout were significantly more active than brown trout during the night in sympatry. It thus appears that sympatric brown trout change their diurnal activity in a maladaptive way, whereas brook trout do not.\u003c/p\u003e \u003cp\u003eIn sympatry, brook trout were significantly more active than brown trout during the night, meaning brown trout\u0026rsquo;s compensatory activity occurs when competitors are already highly active. Taken together, these results suggest that brown trout shift their activity in a way that is likely maladaptive, trading optimal foraging periods for times with higher competitive pressure, whereas brook trout do not.\u003c/p\u003e \u003cp\u003eThis conclusion is reasonable to draw because brook trout are better adapted to nighttime activity and have previously found to display a consistent primarily nocturnal activity pattern (Toobaie et al., 2013). This in spite of both species having very similar scotopic sensitivity levels and are in that way similarly visually equipped to low light levels. The reduced daytime activity could propose a problem for species more adapted to daytime foraging, and particularly for drift-feeding salmonids that rely heavily on visual detection of prey items. Elliot et al. (2011) experimentally demonstrated brown trout\u0026rsquo;s vision to optimized for daylight conditions, showing a superior feeding ability compared to arctic char (\u003cem\u003eSalvelinus alpinus\u003c/em\u003e) during the day, but with an exponential decrease at light levels below 0.04 lx, and complete inability to feed in total darkness. Switching foraging strategy when adapted to daytime could come at a great cost to foraging efficiency. As an example, Fraser and Metcalfe (1997) found foraging efficiency for Atlantic salmon to be reduced to 10\u0026ndash;35% of that of daytime during twilight and night. However, brown trout have well developed low-light vision and show a great deal of plasticity in their diurnal activity, which could be greatly influenced by social cues and may in that way explain why brown trout are decreasing rather than increasing their daytime activity in a presence of a more nocturnal species (Reebs, 2002). On the other hand they could be forced to make this change in order to maintain dominance status and guarding territory. Either way this could potentially help explain why the dominant fish are not displaying the enhanced growth in sympatry like they are in allopatry.\u003c/p\u003e \u003cp\u003eThe complete disruption of dominance-growth relationships, coupled with significant shifts in temporal activity patterns and spatial habitat use demonstrated in this study suggests that brook trout invasion may, in fact, trigger a cascade of behavioural and ecological changes in native brown trout populations negatively affecting fitness. These findings underscore the importance of considering behavioural plasticity and indirect effects when assessing invasion impacts. Understanding these complex behavioural responses will be crucial for predicting ecosystem outcomes and developing effective conservation strategies for native species and communities.\u003c/p\u003e \u003cp\u003eSympatric brown trout showed significantly lower daytime activity than allopatric brown trout which was opposite to what we predicted (iii), yet their overall activity rate was equal. This indicates compensation via increased activity at other times, potentially during the night. These diel activity variations emerge in the wild from individuals balancing the growth and survival trade-off under different competitive conditions (Railsback et al., 2005). Furthermore, brook trout were significantly more active than brown trout during the night in sympatry. It thus appears that sympatric brown trout change their diurnal activity in a maladaptive way, whereas brook trout do not.\u003c/p\u003e \u003cp\u003eIn sympatry, brook trout were significantly more active than brown trout during the night, meaning brown trout\u0026rsquo;s compensatory activity occurs when competitors are already highly active. Taken together, these results suggest that brown trout shift their activity in a way that is likely maladaptive, trading optimal foraging periods for times with higher competitive pressure, whereas brook trout do not. This conclusion is reasonable to draw because brook trout are better adapted to nighttime activity and have previously found to display a consistent primarily nocturnal activity pattern (Toobaie et al., 2013) in spite of both species having very similar scotopic sensitivity levels and are similarly visually equipped to low light levels. The reduced daytime activity could be a problem for species more adapted to daytime foraging, and particularly for drift-feeding salmonids that rely heavily on visual detection of prey items. Elliot et al. (2011) experimentally demonstrated brown trout\u0026rsquo;s vision to optimized for daylight conditions, showing a superior feeding ability compared to arctic char (\u003cem\u003eSalvelinus alpinus\u003c/em\u003e) during the day, but with an exponential decrease at light levels below 0.04 lx, and complete inability to feed in total darkness. Switching foraging strategy when adapted to daytime could come at a great cost to foraging efficiency. As an example, Fraser and Metcalfe (1997) found foraging efficiency for Atlantic salmon to be reduced to 10\u0026ndash;35% of that of daytime during twilight and night. However, brown trout have well developed low-light vision and show a great deal of plasticity in their diurnal activity, which could be greatly influenced by social cues and may in that way explain why brown trout are decreasing rather than increasing their daytime activity in a presence of a more nocturnal species (Reebs, 2002). On the other hand they could be forced to make this change in order to maintain dominance status and guarding territory. Either way this could potentially help explain why the dominant fish are not displaying the enhanced growth in sympatry like they are in allopatry.\u003c/p\u003e \u003cp\u003eThe complete disruption of dominance-growth relationships, coupled with significant shifts in temporal activity patterns and spatial habitat use demonstrated in this study suggests that brook trout invasion may, in fact, trigger a cascade of behavioural and ecological changes in native brown trout populations negatively affecting fitness. These findings underscore the importance of considering behavioural plasticity and indirect effects when assessing invasion impacts. Furthermore, our results indicate that partial removal of brook trout may fail to restore native social structures and can even strengthen invasive advantages. Understanding these complex behavioural responses will be crucial for predicting ecosystem outcomes and developing effective conservation strategies for native species and communities.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eBA, LZ, SM and JH conceived and designed the study. BA, LZ, SM carried out the experiments and collected the data. LZ, SA and SM prepared study facilities. BA led the writing of the manuscript. BA, LZ, SM, SA and JH contributed to data interpretation and manuscript revisions. All authors contributed to the final text and approved the submitted version.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eWe thank following colleagues and students for their assistance: Pernilla Hansson, Simon Lafont, Lukas Wimmer, Johanna Harm, Evelina Olsen and Simon Vitecek. This project (grant 2020-00048) was funded by the Swedish Environmental Protection Agency (Naturv\u0026aring;rdsverket) and the Swedish Research Council Formas to improve the management of invasive species in a joint effort with the Swedish Agency for Marine and Water Management (Havs- och vattenmyndigheten) and the Swedish Transport Administration (Trafikverket). The experiment was approved by the Ethical Committee for Animal Research in G\u0026ouml;teborg (ID nr: 004882, D 408 nr 5.8.18\u0026ndash;04990/2023) and compiled with Swedish and Austrian law.\u003c/p\u003e\u003ch2\u003eData availability statement\u003c/h2\u003e \u003cp\u003eData will be made available upon acceptance of the manuscript\u003c/p\u003e\u003ch2\u003eData availability statement\u003c/h2\u003e \u003cp\u003eData will be made available upon acceptance of the manuscript\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAlan\u0026auml;r\u0026auml;, A., \u0026amp; Br\u0026auml;nn\u0026auml;s, E. (1996). 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Life history and large-scale habitat use of brown trout (\u003cem\u003eSalmo trutta\u003c/em\u003e) and brook trout (\u003cem\u003eSalvelinus fontinalis\u003c/em\u003e) - Implications for species replacement patterns. \u003cem\u003eJournal of Fish Biology, 70\u003c/em\u003e(4), 1095\u0026ndash;1108. https://doi.org/10.1111/j.1095-8649.2007.01370.x\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"biological-invasions","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"binv","sideBox":"Learn more about [Biological Invasions](https://www.springer.com/journal/10530)","snPcode":"10530","submissionUrl":"https://submission.nature.com/new-submission/10530/3","title":"Biological Invasions","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Invasive species, diel activity, social dominance, salmonids, brook trout (Salvelinus fontinalis), brown trout (Salmo trutta)","lastPublishedDoi":"10.21203/rs.3.rs-8588818/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8588818/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSalmonid fishes optimize their habitat use through temporal and spatial activity patterns to balance foraging success, competitive ability, and predator avoidance. Under stable social hierarchies, dominant individuals typically occupy profitable stream positions and achieve higher growth rates. However, the introduction of non-native salmonid species can disrupt these established social hierarchies and alter optimal habitat use patterns. Moreover, the removal of invasive species is often difficult to fully accomplish and may disproportionately target certain behavioural phenotypes, potentially exacerbating the impact of invasive species. Using seminatural stream flumes, this study compared dominance-growth relationship, habitat use, and diel activity patterns between allopatric brown trout groups and sympatric groups of brown trout and invasive brook trout, under varying flow conditions and brook trout removal scenarios. Our results revealed that brook trout invasion disrupted the fundamental dominance-growth relationship observed in allopatric brown trout, indicating that the costs of maintaining dominance in the presence of brook trout outweigh the benefits. Additionally, brook trout invasion significantly altered brown trout diel activity patterns and spatial habitat distribution, even after partial removal of brook trout. These findings demonstrate that invasive species can disrupt established link between dominance, growth and habitat use in native fish communities.\u003c/p\u003e","manuscriptTitle":"Invasive Brook trout (Salvelinus fontinalis) presence may reduce dominance-linked growth and shift diel activity in brown trout (Salmo trutta)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-06 12:48:09","doi":"10.21203/rs.3.rs-8588818/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2026-02-05T11:52:07+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-03T15:38:53+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Biological Invasions","date":"2026-01-19T14:05:31+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-13T12:23:12+00:00","index":"","fulltext":""},{"type":"submitted","content":"Biological Invasions","date":"2026-01-13T02:50:56+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"biological-invasions","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"binv","sideBox":"Learn more about [Biological Invasions](https://www.springer.com/journal/10530)","snPcode":"10530","submissionUrl":"https://submission.nature.com/new-submission/10530/3","title":"Biological Invasions","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"4acb8b9e-603f-44e5-b9c0-902aa062f37f","owner":[],"postedDate":"February 6th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-02-06T12:48:09+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-06 12:48:09","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8588818","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8588818","identity":"rs-8588818","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}