Comparison of allopatric cutthroat trout stocks with those sympatric with coho salmon and sculpins in small streams

Synopsis Juvenile stocks of allopatric (upstream of barrier falls) cutthroat troutSalmo clarki and those sympatric (downstream of barrier falls) with coho salmonOncorhynchus kisutch and sculpinsCottus spp., were sampled during the late summer period of low flows in six small coastal streams in British Columbia. The objective was to obtain and compare information on pattern of habitat use and fish size distribution of these two trout types. In most instances, density (n m^–2; g m^–2) of cutthroat trout was considerably greater in pools and glides in the allopatric than in the sympatric stocks. The sympatric salmonids were dominated by juvenile coho salmon in pools and cutthroat trout in riffles. Sympatric cutthroat trout constituted from 7 to 45 % of the salmonids present in pools and from 50 to 90% in riffles. Glides were areas of intermediate densities for both salmonids, although coho salmon was the more abundant species in most instances. The density of sculpins was high in all three habitat types, and frequently it exceeded that of coho salmon and cutthroat trout combined. Sympatric cutthroat trout consisted primarily of underyearling fish, whereas allopatric cutthroat trout consisted primarily of two or more age classes with a large proportion of them living in pools. When tested in a laboratory stream both types of cutthroat trout had similar habitat preferences and agonistic behaviours, with the exception that allopatric trout made greater use of cover and defended pools more intensely than sympatric trout when the flow was increased. The results of this study provide insight of potential impact of coho salmon juvenile transplants into stream segments supporting allopatric cutthroat trout.     

Spatial patterns of hybridization between bull trout, <Emphasis Type="Italic">Salvelinus confluentus</Emphasis>, and brook trout, <Emphasis Type="Italic">Salvelinus fontinalis</Emphasis> in an Oregon stream network

Hybridization with introduced species represents a serious threat to the persistence of many native fish populations. Brook trout (Salvelinus fontinalis) have been introduced extensively throughout the native range of bull trout (S. confluentus) and hybridization has been documented in several systems where they co-exist and is seen as a significant threat to the persistence of bull trout populations. We identified a group of diagnostic microsatellite loci to differentiate bull trout and brook trout and then used these loci to examine the spatial distribution of hybrids in the Malheur River basin, Oregon USA. In random samples of approximately 100 fish from each of three creeks we identified 181 brook trout, 112 bull trout and 14 hybrids. Although bull trout, brook trout and hybrids were found in all three creeks, they were not evenly distributed; brook trout were primarily found in the lower sections of the creeks, bull trout further upstream, and hybrids in the areas of the greatest overlap. One creek with a population of brook trout in a headwater lake provided an exception to this pattern; brook trout were found distributed throughout the creek downstream of the lake. Several post-F1 hybrids were identified suggesting that hybrids are reproducing in the Malher River Basin. Mitochondrial DNA analysis indicated that both female bull trout and brook trout are involved in hybridization events. Analysis of population structure suggested that brook trout have established multiple spawning populations within the Malheur system. Data presented in this study suggest that relative abundance of brook trout and habitat quality are important factors to consider when evaluating the threat of hybridization to bull trout populations.     

Brown trout and food web interactions in a Minnesota stream

1. We examined indirect, community‐level interactions in a stream that contained non‐native brown trout (Salmo trutta Linnaeus), native brook trout (Salvelinus fontinalis Mitchill) and native slimy sculpin (Cottus cognatus Richardson). Our objectives were to examine benthic invertebrate composition and prey selection of fishes (measured by total invertebrate dry mass, dry mass of individual invertebrate taxa and relative proportion of invertebrate taxa in the benthos and diet) among treatments (no fish, juvenile brook trout alone, juvenile brown trout alone, sculpin with brook trout and sculpin with brown trout). 2. We assigned treatments to 1 m² enclosures/exclosures placed in riffles in Valley Creek, Minnesota, and conducted six experimental trials. We used three designs of fish densities (addition of trout to a constant number of sculpin with unequal numbers of trout and sculpin; addition of trout to a constant number of sculpin with equal numbers of trout and sculpin; and replacement of half the sculpin with an equal number of trout) to investigate the relative strength of interspecific versus intraspecific interactions. 3. Presence of fish (all three species, alone or in combined‐species treatments) was not associated with changes in total dry mass of benthic invertebrates or shifts in relative abundance of benthic invertebrate taxa, regardless of fish density design. 4. Brook trout and sculpin diets did not change when each species was alone compared with treatments of both species together. Likewise, we did not find evidence for shifts in brown trout or sculpin diets when each species was alone or together. 5. We suggest that native brook trout and non‐native brown trout fill similar niches in Valley Creek. We did not find evidence that either species had an effect on stream communities, potentially due to high invertebrate productivity in Valley Creek.     

Interspecific competition between stream-dwelling brown trout and Alpine bullhead

Density and composition of benthic invertebrates and the diet of brown trout Salmo trutta and Alpine bullhead Cottus poecilopus were studied at two sites in one Norwegian stream. The sites were separated by an impassable waterfall, and brown trout density was five to 10 times higher at the upper, allopatric site than downstream where it lived in sympatry with the Alpine bullhead. Benthic invertebrate communities did not differ between sites; however, the size distribution of chironomids and trichopterans were skewed towards lighter individuals at the sympatric site. Diet composition suggested that sympatric brown trout foraged more on invertebrate drift and from the surface than allopatric brown trout. Alpine bullhead diet did not differ significantly from brown trout diet, except that the Alpine bullhead fed on heavier individual prey within a few taxa and did not consume chironomid pupae or surface insects. The collected data support the hypothesis that brown trout living in sympatry with Alpine bullhead feed at locations with higher predation risk, which is a probable explanation for their lower population density.     

Food partitioning between coexisting Atlantic salmon and brook trout in the Sainte‐Marguerite River ecosystem, Quebec

Food resource partitioning between similar‐sized, sympatric Atlantic salmon Salmo salar and brook trout Salvelinus fontinalis was examined as a possible mechanism enabling their coexistence in a stream (Allaire) of the Sainte‐Marguerite River ecosystem, Quebec, Canada. Fish stomach contents and invertebrate drift were collected concurrently during three diel cycles in August to September 1996. The food and feeding habits of an allopatric brook trout population in a nearby stream (Epinette) were studied for comparison. The diel feeding rhythms of the two coexisting fish species were similar. The composition of their diet, however, showed significant differences. Atlantic salmon predominantly (60–90%) fed on aquatic insects, mainly Ephemeroptera (35–60% of the diet). The brook trout mostly (50–80%) fed upon the allochthonous terrestrial insects (mainly adults of Coleoptera, Hymenoptera and Diptera) which comprised 5–40% of the stream drift. The allopatric brook trout fed opportunistically on the more abundant aquatic insects and terrestrial insects rarely formed 25% of its diet. The allopatric trout fed nearly twice as much as the sympatric brook trout during a day. The results suggest that the differences in feeding by brook trout in the two streams (with and without Atlantic salmon) are the result of inter‐specific interaction with Atlantic salmon and are not related to the differences in food availability between the two streams. Food resource partitioning between Atlantic salmon and brook trout may be viewed as an adaptive response resulting in a greater exploitation of available resources and coexistence.     

The effects of population density and lake characteristics on growth and size structure of brook trout Salvelinus fontinalis (Mitchill, 1815) in boreal forest lakes in Canada

This paper examines the effect of lake characteristics on population density and how this variation affects growth, mortality and population size structure of brook trout, Salvelinus fontinalis. The study was conducted on 17 recreationally fished, reproductively isolated boreal forest lakes in Newfoundland, Canada from 1993 to 2000. A standardized sampling program, the Fyke Littoral Index Netting program (FLIN) was used to collected data that describes brook trout population parameters and life history attributes. Regression analyses showed significant relationships between fish density and biomass and characteristics of the lakes. Variation in fish density and biomass was explained by lake surface area and littoral habitat area. Significant relationships were found when growth, mortality and size structure were regressed against density. The proportional stock distribution and theoretical maximum size of brook trout were negatively related to density, and natural mortality was positively related to density. The largest maximum length and highest proportional stock densities occurred at brook trout densities of less than 30 fish/ha. In general, the higher the proportion of littoral habitat area the higher the densities of brook trout, which correspondingly had important effects on growth, natural mortality and size structure of the brook trout populations. This information is critical to the development of management strategies aimed at altering size distribution to produce specific fisheries management outcomes.     

Individual condition indicators of thermal habitat quality in field populations of redband trout (Oncorhynchus mykiss gairdneri)

We monitored redband trout (Oncorhynchus mykiss gairdneri) physiological status in multiple streams across eastern Oregon, USA that exhibit various summer thermal profiles. Summer river temperatures were higher overall in 2009 than in 2010. Heat shock protein 70 (hsp70) levels in both liver and fin tissue from redband trout increased with temperature regardless of location. Lipid levels were lower in fish from waters above 23 °C (in terms of degree-day exposure), and lipids were also related to food availability in 2010. Overall, fish ate more in 2009 than in 2010. Fish size increased more rapidly, and lipids were higher and hsp70 was lower in the cooler Crooked River relative to streams in the John Day River system, though the Crooked River did not have the highest food available. While food availability plays a role in physiological health, our data suggest that temperature is the most influential factor driving growth and energy reserves in stream salmonids during high temperature months; we argue that monitoring physiological indicators in individual fish can serve as a useful proxy for overall thermal habitat quality and population status.     

Habitat shifts in rainbow trout: competitive influences of brown trout

Summary We compared habitat use by rainbow trout sympatric (three streams) and allopatric (two streams) with brown trout to determine whether competition occurred between these two species in the southern Appalachian Mountains. We measured water depth, water velocity, substrate, distance to overhead vegetation, sunlight, and surface turbulence both where we collected trout and for the streams in general. This enabled us to separate the effects of habitat availability from possible competitive effects. The results provided strong evidence for asymmetrical interspecific competition. Habitat use varied significantly between allopatric and sympatric rainbow trout in 68% of the comparisons made. Portions of some differences refelected differences in habitats available in the several streams. However, for all habitat variables measured except sunlight, rainbow trout used their preferred habitats less in sympatry with brown trout than in allopatry if brown trout also preferred the same habitats. Multivariate analysis indicated that water velocity and its correlates (substrate particle size and surface turbulence) were the most critical habitat variables in the interaction between the species, cover in the form of shade and close overhead vegetation was second most important, and water depth was least important.     

Non-native species limit stream restoration benefits for brook trout

Success of stream restoration can be difficult to define because many interacting abiotic and biotic factors across spatio-temporal scales can have measurable effects. Consequently, failure in habitat restoration to achieve targeted biological goals may reflect interactions of habitat restoration with unaccounted risks that have yet to be addressed on the landscape. This is particularly true within invaded landscapes, where habitat restoration can benefit non-native competitors as much as the native fishes for which restoration is designed. We tested for interacting effects of a reach scale habitat restoration effort and non-native trout competition on habitat use by a brook trout (Salvelinus fontinalis) metapopulation within a productive main stem corridor of the Shavers Fork watershed, West Virginia. We used a joint species occupancy model within a BACI sampling design to show that brook trout occupancy of main stem habitat was highest post-restoration within restored sampling reaches, but this benefit to native brook trout was conditional on brown trout (Salmo trutta) not being present within the main stem habitat. Collectively these results indicate that habitat restoration was only beneficial for native brook trout when non-native trout were absent from the restored sampling area. Proactive approaches to restoration will be integral for supporting resilient ecosystems in response to future anthropogenic threats (e.g. climate change), and we have shown that such actions will only be successful if non-native competitors do not also benefit from the restoration actions.     

Non‐native trout limit native brook trout access to space and thermal refugia in a restored large‐river system

We used direct observation via snorkeling surveys to quantify microhabitat use by native brook (Salvelinus fontinalis) and non‐native brown (Salmo trutta) and rainbow (Onchorynchus mykiss) trout occupying natural and restored pool habitats within a large, high‐elevation Appalachian river, United States. Permutational multivariate analysis of variance (PERMANOVA) and subsequent two‐way analysis of variance (ANOVA) indicated a significant difference in microhabitat use by brook and non‐native trout within restored pools. We also detected a significant difference in microhabitat use by brook trout occupying pools in allopatry versus those occupying pools in sympatry with non‐native trout—a pattern that appears to be modulated by size. Smaller brook trout often occupied pools in the absence of non‐native species, where they used shallower and faster focal habitats. Larger brook trout occupied pools with, and utilized similar focal habitats (i.e. deeper, slower velocity) as, non‐native trout. Non‐native trout consistently occupied more thermally suitable microhabitats closer to cover as compared to brook trout, including the use of thermal refugia (i.e. ambient–focal temperature >2°C). These results suggest that non‐native trout influence brook trout use of restored habitats by: (1) displacing smaller brook trout from restored pools, and (2) displacing small and large brook trout from optimal microhabitats (cooler, deeper, and lower velocity). Consequently, benefits of habitat restoration in large rivers may only be fully realized by brook trout in the absence of non‐native species. Future research within this and other large river systems should characterize brook trout response to stream restoration following removal of non‐native species.     

Competition for space between introduced brown trout (Salmo trutta L.) and a native galaxiid (Galaxias vulgaris Stokell) in a New Zealand stream

Increasing circumstantial evidence indicates that the introduction of brown trout ( Salmo trutta L.) to New Zealand has caused a widespread decline in native fish populations but few of the underlying mechanisms have been investigated. The possibility of spatial competition was investigated by comparing the microhabitat used by native Galaxias vulgaris Stokell (Family Galaxiidae) that were sympatric and allopatric with brown trout. A range of microhabitat variables was measured from random locations where G. vulgaris were present in the Shag River during the day. G. vulgaris preferred coarse substrates, using them as resting places, but showed no other microhabitat preferences. This pattern of microhabitat use did not change in the presence of brown trout although galaxiid densities were considerably lower. Experiments in in situ stream channels confirmed that competition for space does not occur during the day even at high galaxiid densities. This situation changed dramatically at night, however, with G. vulgaris spending significantly more time in slower areas when trout were present. G. vulgaris feeds on drifting invertebrates, so brown trout could affect the galaxiids deleteriously by forcing them to occupy less profitable feeding positions. Interspecific competition for space, perhaps combined with competition for food and predation by trout, could explain declines in G. vulgaris populations.     

Species replacement by a nonnative salmonid alters ecosystem function by reducing prey subsidies that support riparian spiders

Replacement of a native species by a nonnative can have strong effects on ecosystem function, such as altering nutrient cycling or disturbance frequency. Replacements may cause shifts in ecosystem function because nonnatives establish at different biomass, or because they differ from native species in traits like foraging behavior. However, no studies have compared effects of wholesale replacement of a native by a nonnative species on subsidies that support consumers in adjacent habitats, nor quantified the magnitude of these effects. We examined whether streams invaded by nonnative brook trout (Salvelinus fontinalis) in two regions of the Rocky Mountains, USA, produced fewer emerging adult aquatic insects compared to paired streams with native cutthroat trout (Oncorhynchus clarkii), and whether riparian spiders that depend on these prey were less abundant along streams with lower total insect emergence. As predicted, emergence density was 36% lower from streams with the nonnative fish. Biomass of brook trout was higher than the cutthroat trout they replaced, but even after accounting for this difference, emergence was 24% lower from brook trout streams. More riparian spiders were counted along streams with greater total emergence across the water surface. Based on these results, we predicted that brook trout replacement would result in 6–20% fewer spiders in the two regions. When brook trout replace cutthroat trout, they reduce cross-habitat resource subsidies and alter ecosystem function in stream-riparian food webs, not only owing to increased biomass but also because traits apparently differ from native cutthroat trout.     

Habitat use by Nonnative Rainbow Trout, Oncorhynchus mykiss, and Native Little Colorado spinedace, Lepidomeda vittata

We evaluated overlap in microhabitat use between nonnative rainbow trout, Oncorhynchus mykiss, and native Little Colorado spinedace, Lepidomeda vittata, a federally threatened cyprinid, in natural and experimental settings. In natural settings, we also examined occurrence and microhabitat use of two other native fishes, speckled dace, Rhinichthys osculus, and bluehead sucker, Catostomus discobolus. Native species co-occurred, as did rainbow trout and bluehead sucker. However, occurrences of Little Colorado spinedace and speckled dace were not significantly correlated with occurrence of rainbow trout. Total lengths of all three native species were significantly smaller at allopatric sites than at sites sympatric with rainbow trout. Microhabitat characteristics at sites with rainbow trout did not differ from those where the other three species were found, but did differ among the native species. In laboratory experiments with Little Colorado spinedace and rainbow trout, rainbow trout used the lower depth zone most, and spinedace increased use of the lower depth zone upon addition of rainbow trout. In addition, species tended to co-occur in zones, but used cover independently of one-another, suggesting a low level of agonistic interactions. However, after addition of a high density of rainbow trout, spinedace tended to use cover less than before. We suggest that the species can coexist at low rainbow trout densities. Potential negative effects of rainbow trout on Little Colorado spinedace likely increase with increasing densities of rainbow trout, and rainbow trout likely affect smaller size classes of Little Colorado spinedace more than larger ones.     

Effects of predation, trout density and discharge on habitat use by brown trout, Salmo trutta, in artificial streams

1. The effects of predation risk, fish density and discharge on habitat use by juvenile brown trout, Salmo trutta, in four artificial streams were studied. Each stream contained three habitats, riffles, runs and pools, the latter two each being further divided into shallow margins and deeper mid-regions. 2. The presence of northern pike, Esox Indus, caused trout to decrease use of pool midregions, where pike also occurred, and to increase use of other habitats. Increasing the number of trout caused trout to increase use of pools and the shallow margins of runs. Decreasing discharge reduced the area of the run and pool margins covered by water, thereby reducing use of these areas by trout. 3. Habitat selection indices for the different treatments were calculated. The data indicated that riffles and the mid-regions of runs were preferred habitats, whereas run margins and pools were inferior habitats used when intraspecific fish densities were high. 4. Despite density- and discharge-dependent habitat use by trout, the number of trout consumed by pike was independent of trout density and discharge. 5. The results reveal the flexibility of habitat use by trout and illustrate the potential danger of applying data on habitat use in one stream to others where habitat availability and bioric interactions may differ.     

Selection on Arctic charr generated by competition from brown trout

We experimentally explored population‐ and individual‐level effects on Arctic charr (Salvelinus alpinus) resulting from resource competition with its common European competitor, the brown trout (Salmo trutta). At the population level, we compared performance of the two species in their natural sympatric state with that of Arctic charr in allopatry. At the individual level, we established selection gradients for morphological traits of Arctic charr in allopatric and in sympatric conditions. We found evidence for interspecific competition likely by interference at the population level when comparing differences in average performance between treatments. The growth and feeding rates did not differ significantly between allopatric and sympatric Arctic charr despite lower charr densities (substitutive design) in sympatric enclosures indicating that inter‐ and intraspecific competition are of similar strength. The two species showed distinct niche segregation in sympatry, and brown trout grew faster than Arctic charr. Arctic charr did not expand their niche in allopatry, indicating that the two species compete to a limited degree for the same resources and that interference may suppress the growth of charr in sympatric enclosures. At the individual level, however, we found directional selection in sympatric enclosures against individual Arctic charr with large head and long fins and against individuals feeding on zoobenthos rather than zooplankton indicating competition for common resources (possibly exploitative) between trout and these charr individuals. In allopatric enclosures these relations were not significant. Diets were correlated to the morphology supporting selection against the benthic‐feeding type, i.e. individuals with morphology and feeding behaviour most similar to their competitor, the benthic feeding brown trout. Thus, this study lends support to the hypothesis that Arctic charr have evolved in competition with brown trout, and through ecological character displacement adapted to their present niche.     

Can replacement of native by non‐native trout alter stream‐riparian food webs?

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Thermal tolerance and metabolic physiology among redband trout populations in south‐eastern Oregon

Streamside measurements of critical thermal maxima (T_crit), swimming performance (U_crit), and routine (R_r) and maximum (R_max) metabolic rates were performed on three populations of genetically distinct redband trout Oncorhynchus mykiss in the high‐desert region of south‐eastern Oregon. The T_crit values (29·4 ± 0·1° C) for small (40–140 g) redband trout from the three streams, and large (400–1400 g) redband trout at Bridge Creek were not different, and were comparable to published values for other salmonids. At high water temperatures (24–28° C), large fish incurred higher metabolic costs and were more thermally sensitive than small fish. U_crit(3·6 ± 0·1 L_F s^?1), R_r(200 ± 13 mg O₂ kg^?0·830 h^?1) and metabolic power (533 ± 22 mg O₂ kg^?0·882 h^?1) were not significantly different between populations of small redband trout at 24° C. R_max and metabolic power, however, were higher than previous measurements for rainbow trout at these temperatures. Fish from Bridge Creek had a 30% lower minimum total cost of transport (C_min), exhibited a lower refusal rate, and had smaller hearts than fish at 12‐mile or Rock Creeks. In contrast, no differences in U_crit or metabolism were observed between the two size classes of redband trout, although C_min was significantly lower for large fish at all swimming speeds. Biochemical analyses revealed that fish from 12‐mile Creek, which had the highest refusal rate (36%), were moderately hyperkalemic and had substantially lower circulating levels of free fatty acids, triglycerides and albumin. Aerobic and anaerobic enzyme activities in axial white muscle, however, were not different between populations, and morphological features were similar. Results of this study: 1) suggest that the physiological mechanisms that determine T_crit in salmonids are highly conserved; 2) show that adult (large) redband trout are more susceptible to the negative affects of elevated temperatures than small redband trout; 3) demonstrate that swimming efficiency can vary considerably between redband trout populations; 4) suggest that metabolic energy stores correlate positively with swimming behaviour of redband trout at high water temperatures; 5) question the use of T_crit for assessing physiological function and defining thermal habitat requirements of stream‐dwelling salmonids like the redband trout.     

More than a corridor: use of a main stem stream as supplemental foraging habitat by a brook trout metapopulation

Coldwater fishes in streams, such as brook trout (Salvelinus fontinalis), typically are headwater specialists that occasionally expand distributions downstream to larger water bodies. It is unclear, however, whether larger streams function simply as dispersal corridors connecting headwater subpopulations, or as critical foraging habitat needed to sustain large mobile brook trout. Stable isotopes (δ¹³C and δ¹⁵N) and a hierarchical Bayesian mixing model analysis was used to identify brook trout that foraged in main stem versus headwater streams of the Shavers Fork watershed, West Virginia. Headwater subpopulations were composed of headwater and to a lesser extent main stem foraging individuals. However, there was a strong relationship between brook trout size and main stem prey contributions. The average brook trout foraging on headwater prey were limited to 126 mm standard length. This size was identified by mixing models as a point where productivity support switched from headwater to main stem dependency. These results, similar to other studies conducted in this watershed, support the hypothesis that productive main stem habitat maintain large brook trout and potentially facilitates dispersal among headwater subpopulations. Consequently, loss of supplementary main stem foraging habitats may explain loss of large, mobile fish and subsequent isolation of headwater subpopulations in other central Appalachian watersheds.     

Disparate infection patterns of Ceratomyxa shasta (Myxozoa) in rainbow trout (Oncorhynchus mykiss) and Chinook salmon (Oncorhynchus tshawytscha) correlate with internal transcribed spacer-1 sequence variation in the parasite

Ceratomyxa shasta is a virulent myxosporean parasite of salmon and trout in the Pacific Northwest of North America. The parasite is endemic in the Klamath River, Oregon/California, where a series of dams prevent movement of fish hosts between the upper and lower parts of the basin. Ceratomyxa shasta exhibits a range of infection patterns in different fish species above and below the dams. We hypothesised that the variations in infection and disease are indicators that different strains of the parasite exist, each with distinct host associations. Accordingly, we sought to identify strain-specific genetic markers in the ssrRNA and internal transcribed spacer region 1 (ITS-1). We examined 46 C. shasta isolates from water samples and two fish hosts, from June 2007 field exposures at upper and lower Klamath River sites with similarly high parasite densities. We found 100% of non-native rainbow trout became infected and died at both locations. In contrast, mortality in native Chinook salmon was <10% in the upper basin, compared with up to 40% in the lower basin. Parasite ssrRNA sequences were identical from all fish. However, ITS-1 sequences contained multiple polymorphic loci and a trinucleotide repeat (ATC)_0-3 from which we defined four genotypes: 0, I, II and III. Non-native rainbow trout at both sites were infected with genotype II and with a low level of genotype III. Chinook salmon in the upper basin had genotypes II and III, whereas in the lower basin genotype I predominated. Genotype I was not detected in water from the upper basin, a finding consistent with the lack of anadromous Chinook salmon there. Genotype O was only detected in water from the upper basin. Resolution of C. shasta into sympatric, host-specific genotypes has implications for taxonomy, monitoring and management of this significant parasite.     

Shifting thermal regimes influence competitive feeding and aggression dynamics of brook trout (Salvelinus fontinalis) and creek chub (Semotilus atromaculatus)

The natural distributions of freshwater fish species are limited by their thermal tolerances via physiological constraints and increased interspecific competition as temperatures shift toward the thermal optima of other syntopic species. Species may mediate stress from temperature change physiologically, behaviorally, or both; but these changes may compromise competitive advantages through effects on feeding and social behavior. In the Appalachian Mountains of North America, creek chub (Semotilus atromaculatus) are found in warm‐water and cold‐water streams and overlap in range with brook trout (Salvelinus fontinalis) across lower thermal maxima, where they compete for food and space. As stream temperatures continue to increase due to climate change, brook trout are under increasing thermal stress which may negatively affect their ability to compete with creek chub. To examine the influences of temperature on competitive interactions between these species, we observed feeding behavior, aggression, and habitat use differences at three temperatures approaching brook trout thermal maxima (18°C, 20°C, and 22°C) among dyad pairs for all combinations of species in experimental flow‐through tanks. We also examined feeding and habitat use of both species under solitary conditions. We found as temperature increased, feeding and aggression of brook trout were significantly reduced in the presence of creek chub. Creek chub pairs were more likely to occupy benthic areas and refugia while brook trout pairs used surface water more. Space use patterns significantly changed by pairing treatment. Aggression and space use shifts allowed increased exploitative and interference competition from creek chub when paired with brook trout that was not present in conspecific pairs. The decreased dominance of a top predator may lead to diverse impacts on stream community dynamics with implications for the future range restriction of brook trout and demonstrate possible mechanisms to facilitate competitive advantages of warm water generalist species under thermal stress.