Native Fish Abundance Research Articles

Evaluation of extinction risk for stream fishes within an urban riverscape using population viability analysis

Abstract1. The Santa Ana River in the Los Angeles region of California demonstrates common habitat degradation symptoms that are characteristic of the urban stream syndrome. These impacts have altered the Santa Ana River community structure, with few species as impacted as the native Santa Ana sucker (sucker; Pantosteus santaanae). 2. Consequently, a recovery plan developed for sucker identified the need for a population viability analysis (PVA) to assess sucker extirpation risk. However, PVAs can be data‐intensive and are subject to several sources of bias when standardized protocols are absent. 3. More than 20 years of sucker and arroyo chub (chub; Gila orcuttii) surveys using different methods were compiled to build an integrated hierarchical multi‐population PVA to estimate trends in abundance and extirpation probability of these native fishes from the Santa Ana River. 4. PVA modelling indicated similar patterns in sucker and chub abundance along the Santa Ana River, with the highest abundance of both species in the upper regions of the river during the early 2000s and downstream in recent years (2018–2022). Extirpation risk was estimated to be greatest near wastewater treatment facilities, where native fish abundance estimates have been zero since 2018. Extirpation risk was lower downstream of the wastewater treatment facilities for both species, although extinction risk was higher for sucker than chub throughout the river. 5. As the model evolves and more data are collected, the PVA could be used to assess the effects of various management actions, such as non‐native predator removals and native fish re‐introductions, on sucker and chub persistence.

Multiscale analysis predicts native species presence based on habitat and nonnative species abundance

AbstractObjectiveThe loss and degradation of aquatic habitat through fragmentation, water extraction, and climate warming contribute to declining native stream fish diversity. In response to these declines, a large‐scale repatriation program was established in Arizona and New Mexico to expand the distribution and abundance of native fishes. This program has had variable success, with imperiled fish populations establishing and persisting in some streams, while other populations failed to establish.MethodsTo identify factors contributing to the success of repatriation, we conducted a study investigating how biotic and abiotic variables affect native fish presence or absence at differing spatial scales throughout the Gila River basin. We conducted fish and habitat surveys in 29 river reaches at three spatial scales: the 1‐km, 0.1‐km, and mesohabitat scale. We used generalized linear models to identify associations among species presence, habitat characteristics, and nonnative species abundance across spatial scales. Logistic regression models were also used to test associations with heterogeneity of habitat characteristics and species presence.ResultModels indicated strong species‐ and scale‐specific responses to these variables. Nonnative abundance was less influential than other habitat variables in predicting the presence of native fish, although the majority of associations that emerged were negative. Further, we were able to identify that water velocity, depth, and substrate size were the most common variables associated with species' presence across all scales and age‐classes, with most species responding positively to elevated velocities and variable responses for depth and substrate size. Response to variation in habitat was species‐specific, with some associations positive and others negative.ConclusionCollectively, the information from this study will help identify locations with a reasonably high likelihood of supporting native fishes and broaden our understanding of species responses to habitat and nonnative species across multiple scales.

Native fish abundance and habitat selection changes in the presence of nonnative piscivores

AbstractWe compared abundance patterns and developed resource selection models for imperilled native southwestern (USA) fishes in the presence and absence of Black Bass (Micropterus spp.) to evaluate how fishes alter their selection for habitats when sympatric with a nonnative piscivore. We collected data using snorkel surveys and in‐stream habitat sampling in Fossil Creek (AZ), upstream (native fish only) and downstream (native and nonnative fish) of a fish barrier. The abundance of all Roundtail Chub (Gila robusta), small (≤127 mm total length [TL]; vulnerable to predation) Sonora Sucker (Catostomus insignis) and Speckled Dace (Rhinichthys osculus) was significantly reduced, but the abundance of both small and large (>127 mm TL; invulnerable to predation) Desert Sucker (Catostomus clarkii) was similar in sampling reaches with and without Black Bass. When sympatric with Black Bass, small Roundtail Chub increased their selection for riffles by 2.57 times and small Desert Sucker reduce their selection for pools by 6.90 times while also selecting for faster flow velocity and finer substrates in lotic mesohabitats. Large native fishes altered selection least, notwithstanding an increased selection for canopy cover in sampling reaches with Black Bass. Observed shifts in resource selection are consistent with predator avoidance strategies. Our study highlights the behavioural consequences of nonnative piscivores on native fish communities and stresses the importance of maintaining lotic mesohabitats as potential refugia for vulnerable native fishes when nonnative piscivores are present.

Changes in native fish communities in response to the presence of alien brown bullhead (Ameiurus nebulosus) in four lakes (Poland)

The aim of the study was to determine changes in the abundance of Ameiurus nebulosus and native fish in four lakes after 4–7 years. It was hypothesised that A. nebulosus would be the dominant species of fish in various types of lakes. The study was conducted in the summer of 2014 and 2018–2021, using Nordic multi-mesh gillnets. We indicated that relative numbers and biomass of A. nebulosus decreased in the oligo-mesotrophic lake, while increased in lakes of higher trophic conditions. Body weight and total length of A. nebulosus fluctuated from 3.2 to 283.3 g and 64 to 267 mm, respectively. The coefficient of condition of this species varied widely from 0.2 to 1.8 with the highest and lowest values in the meso-eutrophic lake. Ameiurus nebulosus accounted for 0.2–34.1% of the total numbers and 2.4–36.5% of the total biomass of the fish community. In the mesotrophic and meso-eutrophic lakes, A. nebulosus was a dominant component of the fish community after 7 years. The percentage contribution of A. nebulosus to the total numbers of fish decreased with the increasing trophic status of the studied lakes. Our study showed that A. nebulosus could be one of the factors responsible for a decrease in the number of native fish species and a change in the taxonomic structure of native fish in lakes of different trophic status (from oligo-mesotrophy to eutrophy).

Assessing impacts of sulfur deposition on aquatic ecosystems: A decision support system for the Southern Appalachians

AbstractWith climate change and ongoing impacts from human development and resource extraction, US federal land management agencies are acutely concerned with managing for healthy aquatic ecosystems in the Southern Appalachian Mountain (SAM) Region. Here, we describe development of a spatial decision support application to assess the biological and ecological impacts of atmospheric S and N deposition on aquatic ecosystems of the region. We first summarize foundational published work to predict continuous maps of surface water acid neutralizing capacity (ANC) and soil base cation weathering (BCw). We use the predicted ANC and BCw maps to estimate steady‐state critical loads (CLs) of atmospheric S and N deposition. We included estimated CLs of atmospheric N to get a complete picture of CLs and potential exceedances. We then present a logic‐based decision support model for assessing effects of S and N deposition based on statistically modeled stream ANC and CL exceedance. The model is easily modified for continuous monitoring of CL exceedance patterns as new S and N deposition and ANC data become available. We present mapped model results for the SAM study area and an important subset of the region, the Great Smoky Mountains National Park. ANC modeling results revealed that predicted acid sensitivity was spatially variable, with areas of relatively low stream ANC (<50 μeq · L−1) and soil BCw (<50 meq · m−2 · year−1) predominantly found in certain critical areas. Within the Great Smoky Mountains National Park, evidence for S CL exceedance based on an ANC criterion of 50 μeq · L−1 was strong at locations where ambient S deposition was at least two times the CL. We also predicted likely impacts of CL exceedances on aquatic insect species richness and native fish abundance. Responses for insect species richness and fish impact showed variability similar to CL exceedance, with increasing impact positively correlated with elevation. Finally, we discuss ways that the decision support system can be used to prioritize management across the region.

Habitat associations of imperilled fishes after conservation intervention in the Cape Fold Ecoregion, South Africa.

The aim of this study was to examine the habitat associations of a native cyprinid community of the recovering Rondegat River in the Cape Fold Ecoregion of South Africa as part of a long-term native fish abundance monitoring project. Relative abundance data were extracted from underwater video camera footage across the longitudinal gradient of the river in three sampling instances. Using multivariate methods the authors assessed community composition with respect to habitat, its overlap with a protected area and species-specific abiotic predictors of relative abundance. Distance from the uppermost site in the river was the most significant predictor of species abundance, indicating spatial segregation and varying overlap between species. The protected status of sites in the upper reaches, vegetated substrates and the size of individual sites were the most impactful on the relative abundance of the endangered fiery redfin Pseudobarbus phlegethon. The results of this study indicate that underwater video monitoring is an effective and low-cost approach that can inform conservation recommendations. Reducing agricultural runoff and sedimentation in the lower reaches may be useful further interventions to maintain key habitats of submerged vegetation.

Fish carcass deposition to suppress invasive lake trout through hypoxia causes limited, non‐target effects on benthic invertebrates in Yellowstone Lake

AbstractInvasive species can have negative effects on native biodiversity and ecosystem function, and suppression is often required to minimize the effects. However, management actions to suppress invasive species may cause negative, unintended effects on non‐target taxa. Across the United States, lake trout (Salvelinus namaycush) are invasive in many freshwater ecosystems, reducing native fish abundance and diversity through predation and competition. In an integrated pest management approach, lake trout embryos in Yellowstone Lake, Wyoming, are suppressed by depositing lake trout carcasses onto spawning sites; the carcasses reduce dissolved oxygen concentrations as they decay, causing embryo mortality. We conducted a field experiment during one ice‐free season at four sites in Yellowstone Lake to investigate the non‐target effects of carcass treatment on benthic invertebrates, which could have consequences for native fish diets. While overall invertebrate density and biomass did not respond to carcass treatment, Chironomidae midges and Sphaeriidae fingernail clams decreased in abundance. Carcass treatment altered invertebrate community structure based on density, but not biomass. Carcass treatment to suppress invasive fish embryos has spatially localized, non‐target effects on some benthic invertebrate taxa. Given the small spatial extent of carcass treatment within the lake, we conclude it is unlikely that carcass treatment will alter food availability for native fishes.

Inferred Trophic Characteristics and Ecological Roles of Native and Nonnative Fishes in Odell Lake, Oregon

AbstractIntroduction and establishment of nonnative fishes can negatively affect native species and ecosystems through a variety of ecological interactions. For example, competition between species that occupy a similar trophic guild can result in extirpation or reduced abundance of native fishes. We used a combination of standard sampling methods and stable isotope analysis to evaluate relative abundance and isotopic niche overlap among native and nonnative species in Odell Lake, Oregon. Our primary focus was on interactions between native Bull Trout Salvelinus confluentus and nonnative Lake Trout Salvelinus namaycush; however, we also evaluated relative abundance and isotopic niche characteristics of other native and nonnative species. Overall, the relative abundance of nonnative fishes was greater than that of native fishes. For example, Lake Trout outnumbered Bull Trout 27:1, and the numerically most dominant species in our sample was nonnative Tui Chub Siphateles bicolor. We also observed isotopic niche overlap among a variety of native and nonnative fishes. Bull Trout and Lake Trout are both top‐level predators in Odell Lake and the probability that Bull Trout were within the isotopic niche of Lake Trout was 0.23. The probability that nonnative kokanee Oncorhynchus nerka and Tui Chub were within the isotopic niche of native Mountain Whitefish Prosopium williamsoni exceeded 0.95, and the probability that Tui Chub were within the isotopic niche of Rainbow Trout O. mykiss was 0.90. Based on data presented here and rangewide patterns for Bull Trout, we conclude that competition with Lake Trout is plausible and has the potential to influence the persistence of Bull Trout in Odell Lake. Further, we discuss ecological interactions potentially influencing the abundance of other native fishes in Odell Lake.

Patterns of species richness, abundance and individual-size distributions in native stream-fish assemblages invaded by exotic and translocated fishes.

Predicting the impacts of species introductions long has attracted the attention of ecologists yet there still is limited insight into how impacts on native assemblages vary with the degree of shared evolutionary context. Here, we used data from 535 stream-fish surveys from 15 catchments in north-eastern Spain (99,700 km2) to explore whether the relative effects on native fishes differ between fish introductions from two different ecoregions (i.e., evolutionary contexts), namely, catchments within Iberian Peninsula (i.e., 'translocated species') and catchments beyond Iberian Peninsula (i.e., 'exotic fishes'). We used hierarchical Bayesian models to relate taxon richness, abundance, and the individual-size distributions (ISDs) of native fishes to the presence, abundance, and weighted trophic level (TL) of translocated and exotic fishes, conditional on geographic and habitat covariates. Environmental covariates dominated the percentage of explained variance (≥ 65%) for all responses. Translocated fishes accounted for more of the explained variance than did exotic fishes for ISDs and abundance, but not for native fish species richness. The presence of translocated fishes was associated with lower abundance and richness of native fishes, with individuals being smaller in the presence of translocated fishes of higher TL. The presence of exotic fishes was associated with a greater abundance and richness of native fishes, with individuals generally being larger in the presence of exotic fishes. Our study suggests that translocated fishes could be as problematic as exotic fishes when angling and water transfers among catchments to deal with climate change may increase the establishment of translocated fishes. We also discuss the difficulties of using fish body size as species-blind, transferable assemblage-level trait in fish monitoring.

Invasive peacock basses (Cichla spp.) and decreased abundance of small native fish in Brazilian reservoirs

Abstract Peacock basses (Cichla spp.) have been introduced in impoundments for decades, and their negative impacts on the native fish fauna have been documented. However, previous studies largely focused on the overall responses of fish assemblages, often neglecting specific predator–prey interactions and the underlying effects of environmental characteristics. Here, the effects of peacock bass introduction on the size structure (<120, 120–180 and >180 mm in total length) of fish assemblages in reservoirs of the Paraíba do Sul river basin were assessed. The abundance and time since the introduction of peacock basses were used as predictors, together with reservoir features, limnological variables and habitat composition as potential influencing factors. Descriptors of the overall community (richness, abundance and species composition) were only marginally associated with the occurrence of peacock basses. However, small and (to a lesser extent) medium‐sized fish were adversely affected by the abundance and introduction time of peacock basses. Two pelagic native characids – namely Astyanax cf. bimaculatus and Oligosarcus hepsetus – were the most affected. Redundancy analysis showed that reservoir age and macrophyte cover were the best predictors of fish assemblage composition. Older reservoirs and reservoirs with a higher cover of macrophytes showed a greater abundance of catfish and characids, respectively. Peacock basses appear to affect fish assemblages through predation on small fish. Therefore, analyses of assemblages by size are recommended, as the use of overall assemblage descriptors may not be sufficient to capture the adverse effects of peacock basses. Moreover, these findings underscore the importance of conserving and restoring habitats in littoral zones, which function as refuges for native fish species in reservoirs. Further educational and management measures are warranted to prevent the spread of peacock basses to other reservoirs in Brazil.

Use of Native and Nonnative Fish Hosts by the Freshwater Mussel Anodonta californiensis (California Floater) in the Columbia River Basin

Populations of many native freshwater mussels such as the California floater, Anodonta californiensis, are declining in the Columbia River Basin in the western United States. There are many possible reasons for this decline, including increased presence of nonnative fishes, especially piscivores that displace and reduce the abundance of native fishes. These nonnative fishes can negatively influence the early life history of A. californiensis, because this mussel uses native fishes to complete its life cycle. While Anodonta spp. can use nonnative fishes as hosts with limited success, the extent of this for A. californiensis is not well understood. We determined if the glochidia (larvae) of A. californiensis can successfully metamorphose using certain nonnative fishes, and we quantified differences in host effectiveness (number of juveniles produced) between native and nonnative fishes based on available fin area for attachment. Overall, native fishes hosted an average of 107.4 ± 39.9 (mean ± SE) juvenile mussels per fish while nonnative fishes hosted an average of 5.5 ± 4.9 juveniles per fish. This conclusion was unchanged when standardized for fish attachment area. Encystment of glochidia on native fishes yielded an average of 1.0 ± 0.1 juveniles/mm2 of attachable surface (total area of fins) area while nonnative fishes yielded an average of 0.16 ± 0.1 juveniles/mm2. The nonnative channel catfish, Ictalurus punctatus, did not yield any juvenile mussels and was identified as a nonhost species for A. californiensis. All other nonnative fishes tested were determined to be poor or marginal hosts. All native fishes were determined to be primary or secondary hosts for A. californiensis. The native fishes that yielded the highest number of juvenile mussels were sculpins, Cottus spp., and the redside shiner, Richardsonius balteatus, with an average of 196 and 151 juveniles per individual of each species, respectively. Our findings show that nonnative fishes are poor hosts for A. californiensis. However, nonnative fishes may contribute to the decline of native mussels in the Columbia River Basin by directly preying on and reducing the abundance of native host fishes and mussels. Future conservation plans for A. californiensis should consider the potential negative influence of nonnative fishes.

Resource selection and species interactions between native and non‐native fishes in a simulated stream system

AbstractEffective fishery management necessitates understanding of resource partitioning by fishes that inhabit complex systems composed of biotic and abiotic features. Evaluations of non‐native species introductions have continually demonstrated adverse effects associated with abundance and distribution of native fishes. Therefore, understanding resource selection and interactions between native and non‐native species is important for recovery efforts. Habitat use by two native fish species (largescale sucker Catostomus macrocheilus [Girard] and mountain whitefish Prosopium williamsoni [Girard]) and one non‐native fish species (pumpkinseed Lepomis gibbosus [Linnaeus]) of the Kootenai River, Idaho, were evaluated in a laboratory stream system. Trials were conducted in allopatry and in sympatry with and without the presence of wood to describe habitat selection in the context of on‐going habitat rehabilitation efforts. Interactions were evident between native largescale sucker and non‐native pumpkinseed concerning use of a woody structure and current velocity. Mountain whitefish used low‐velocity habitats and selected locations that were further from wood when in sympatry with pumpkinseed. Our research suggests that habitat use of native, large‐river fishes may be influenced by the presence of a non‐native species, and that considering such interactions is critical when designing and implementing habitat rehabilitation efforts in river ecosystems.

Increased juvenile native fish abundance following a major flood in an Arizona river

Spring floods trigger spawning in many native fishes of the desert Southwest (USA), but less is known about fish community response when native fishes are rare. Here, we document change to native and nonnative fish captures and instream habitat features following a decade-high flooding event (2019) in the Verde River (AZ) where native fish captures were rare in the years pre-flood. Using prepositioned areal electrofishing devices (PAEDs), we sampled the fish community at 90 sampling units pre-flood (2017) and resampled those same units post-flood (2019) to compare and identify changes to catch and habitat features. Relative abundance of native fishes increased from 0.6% pre-flood (0.01 fish/PAED) to 53.0% post-flood (1.66 fish/PAED) and was largely attributable to the presence of juvenile Roundtail Chub Gila robusta (≤ 70 mm total length (TL)) and juvenile Sonora Sucker Catostomus insignis (≤ 100 mm TL). Juvenile Desert Sucker Catostomus clarkii experienced a lesser increase. One adult native fish was captured in 2017 and adult native fishes were absent from 2019 sampling. The catch of adult/subadult Common Carp Cyprinus carpio (> 100 mm TL) declined; however, this could be related to reservoir management and not the flood. The abundance of all size-classes of Black Bass Micropterus spp., Red Shiner Cyprinella lutrensis and other nonnative fishes did not change. The majority (97%) of juvenile native fishes were captured at the uppermost sampling reach. A 54% reduction to canopy cover across all sampling reaches and an increase of fine sediments at the most downstream reach demonstrates how floods can restructure the river environment. This case-study adds evidence that protection of spring floods is vital to the persistence and recolonization of fishes native to the desert Southwest, especially where they are rare. The continued presence of nonnative species may preclude juvenile native fishes from recruiting to adults.

Joint-species analysis reveals potential displacement of native fishes by non-native fishes within the Santa Ana River, California

Accurate estimates of abundance are a cornerstone for resource managers to make effective decisions for fish conservation. However, multiple sampling methods often are required to sample fish communities and ignoring the detection process can create substantial bias in latent state parameter estimation (e.g., abundance, survival). We developed a joint-species N-mixture model that integrated snorkel, seining, and electrofishing surveys to estimate factors affecting native and non-native fish distributions in the Santa Ana River, California. We found through data integration that native Santa Ana sucker (Catostomus santaanae) and arroyo chub (Gila orcuttii) were most abundant in wide stream channels, and the abundance of both native fishes were negatively correlated with non-native largemouth bass (Micropterus salmoides). Our results highlight the power of integrating multiple data sets into a single analysis and incorporating among-species correlation into abundance modeling. Our results also highlight a pattern of native fish declines that coincide with an expanding largemouth bass population, a concern for the management of native aquatic communities within the Santa Ana River.

Spatial and temporal variation of fish assemblage structure in a neotropical Mexican river

The introduction of non-native fish into the Teuchitlán River, in west-central Mexico, was suggested as a key factor behind the change in the fish assemblage, and even in the extinction of endemic species. The characterization of the fish assemblage and the relationship between fish abundance and the anthropogenic conditions were evaluatedover 2-year period. Fish assemblage structure, species replacement, and dominance were examined along a longitudinal environmental gradient. Physical and chemical parameters of the water were recorded. The relationship between fish abundance and abiotic factors was inferred from multivariate analysis. The results showed that the non-native poecilid Pseudoxiphophorus bimaculatus was dominant, and the abundance and biomass of non-native species exceeded 50%in all sites and seasons. Species diversity and native fish abundance were greater in upstream sites, which presented higher dissolved oxygen and lower nitrates. The native ichthyofauna of the Teuchitlán River has been replaced by non-native species. This decline of the native fish species seems to be due not only to the interaction with non-native species but also to the multiple human impacts on the lotic system.

Slimy sculpin depth shifts and habitat squeeze following the round goby invasion in the Laurentian Great Lakes

The collapse of Diporeia spp. and invasions of dreissenid mussels (zebra, Dreissena polymorpha; quagga, D. bugensis) and round goby (Neogobius melanostomus) have been associated with declines in abundance of native benthic fishes in the Great Lakes, including historically abundant slimy sculpin (Cottus cognatus). We hypothesized that as round goby colonized deeper habitat, slimy sculpin avoided habitat competition, predation, and aggression from round goby by shifting to deeper habitat. Accordingly, we predicted increased depth overlap of slimy sculpin with both round goby and deepwater sculpin (Myoxocephalus thompsonii) that resulted in habitat squeeze by both species. We used long-term bottom trawl data from Lakes Michigan, Huron, and Ontario to evaluate shifts in slimy sculpin depth and their depth overlap with round goby and deepwater sculpin. Lake Huron most supported our hypotheses as slimy sculpin shifted to deeper habitat coincident with the round goby invasion, and depth overlap between slimy sculpin and both species recently increased. Slimy sculpin depth trends in Lakes Michigan and Ontario suggest other ecological and environmental factors better predicted sculpin depth in these lakes.

Specialized Feeding Patterns and Marine Resource Use by Nonnative Catfishes in a Coastal River Ecosystem Revealed by Dietary and Stable Isotopic Analyses

AbstractTwo large predatory catfishes, the Flathead CatfishPylodictis olivarisand Blue CatfishIctalurus furcatus, have been introduced into multiple U.S. Atlantic slope drainages in recent decades, with concomitant reductions in native fish abundance and changes in faunal community composition. Here, we report on a study of the trophic ecology of sympatric Flathead Catfish and Blue Catfish in the lower Cape Fear River, North Carolina, to estimate trophic positions and identify possible impacts on marine and estuarine prey resources. We applied both dietary and stable isotope analysis to quantify species‐specific foraging patterns, as well as the extent of variation among tributaries, seasonal periods, and ontogeny. Both catfish species displayed limited dietary breadth and specialized feeding patterns focused on prey resources within a restricted range of trophic levels. Dietary overlap between Blue Catfish and Flathead Catfish was low, and the species were separated by more than one trophic position. Flathead Catfish relied heavily on fish prey throughout ontogeny, with macroinvertebrates (crustaceans) as a consistent secondary prey resource, and showed increased reliance on potential anadromous prey resources at the largest sizes. Blue Catfish diets were dominated by the invasive Asian clamCorbicula fluminea, with contributions from fish prey only increasing for the largest individuals. Catfish trophic ecology varied among tributaries, and to a greater extent for Flathead Catfish, in terms of both the fish fauna eaten and the relative contribution of different invertebrate prey. Isotopic signatures for Flathead Catfish aligned with stomach contents to indicate a greater contribution of marine prey resources, including anadromous fishes, in the main‐stem Cape Fear River compared with the Black and Northeast Cape Fear rivers. Our findings indicate that the predatory impacts of catfishes introduced to coastal river ecosystems will differ considerably between catfish species and may also vary within systems across fine spatial scales.

Do exotic poeciliids affect the distribution or trophic niche of native fishes? Absence of evidence from Hong Kong streams

Abstract Exotic poeciliids have been reported to reduce abundance and richness of native fishes and, more generally, constrain the trophic niches of native counterparts. However, understanding of the consequences of poeciliids on native fishes in tropical streams is limited. We conducted a survey of lowland streams in Hong Kong during 2018–19 to document the prevalence of poeciliids and look for signs that they had affected the distribution of native fishes. Overall, 31 native and 15 exotic fishes were recorded during the survey. Exotics occurred at 37 out of 45 survey sites, with poeciliids present at 35 sites. Richness of poeciliids or other exotic fishes had no correlation with the number of native species at each site. Moreover, native fish assemblages appeared unaffected by poeciliids, which might reflect a legacy of stream pollution. By comparing nine streams where the presence and composition of poeciliids varied, we investigated whether the mosquitofish (Gambusia affinis) alone, and in combination with the guppy (Poecilia reticulata), swordtail (Xiphophorus hellerii), and variable platy (Xiphophorus variatus), affected the trophic niche of native half‐banded barb (Puntius semifasciolatus: Cyprinidae) which has a similar body size to the poeciliids, as quantified by gut content and stable‐isotope analysis. We also tested whether the trophic niche of the mosquitofish was affected by the three other poeciliids. Gut‐content analysis indicated that the half‐handed barb fed on aquatic invertebrates, while the mosquitofish ate mainly terrestrial insects. The three other poeciliids were predominately primary consumers. The mosquitofish, alone or in combination with other poeciliids, had no detectable effects on the dietary or isotopic niche of the barb, nor was the niche of the mosquitofish influenced by the presence of other poeciliids. Limited dietary overlap and differences in microhabitat use were likely to account for the absence of a detectable niche shift by the barb where poeciliids were present.

Could spatial heterogeneity in flow disturbance drive temporal stability of native–invasive species co‐occurrence in riverscapes?

Abstract Spatial heterogeneity of abiotic influences like disturbance in riverscapes could play an important role in dispersal‐aided community stability. We tested if higher spatial variability in conditions around river confluences caused by different flood disturbance regimes in branches could possibly influence the stability of a fish assemblage dominated by strong negative interactions between non‐native trout and native galaxiid fishes. We used field surveys to evaluate the effects of disturbance‐driven differences between river branches on fish densities and temporal variation in the relative abundance of galaxiids. Three repeated electrofishing surveys were conducted in two reaches of all branches of eight confluences in Canterbury, New Zealand. We found the differences in flood disturbance regimes between the two stream branches joining at a confluence were associated with higher mean fish abundance and more temporal stability in the relative abundance of native fish. Conversely, when flood disturbance conditions in the two streams joining were similar, resulting in a homogeneous confluence, fish abundance was lower and temporal variability was higher. For homogeneous confluences, strong interactions between native galaxiids and invasive trout probably produced lower fish densities where rivers with low flow disturbance met, whereas when two highly flow‐disturbed rivers met, environmental harshness probably lowered fish densities. By comparison, enhanced galaxiid temporal stability at confluences containing both disturbed and stable river branches probably reflected the combined effects of refugia from trout predation in disturbed branches and refugia from flooding in stable branches. Overall, these results are likely to reflect the combined effects of population‐specific responses to physical heterogeneity and spatially variable interspecific interactions. They may mean that river network configuration, and confluences especially, could influence community stability through a habitat portfolio effect. Furthermore, it will be worth investigating further whether preservation or restoration of differences in flow disturbance conditions in riverscapes encourages higher ecosystem resilience in the face of global change.

Remarkable response of native fishes to invasive trout suppression varies with trout density, temperature, and annual hydrology

Recovery of imperiled fishes can be achieved through suppression of invasives, but outcomes may vary with environmental conditions. We studied the response of imperiled desert fishes to an invasive brown (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) suppression program in a Colorado River tributary, with natural flow and longitudinal variation in thermal characteristics. We investigated trends in fish populations related to suppression and tested hypotheses about the impacts of salmonid densities, hydrologic variation, and spatial–thermal gradients on the distribution and abundance of native fish species using zero-inflated generalized linear mixed effects models. Between 2012 and 2018, salmonids declined 89%, and native fishes increased dramatically (∼480%) once trout suppression surpassed ∼60%. Temperature and trout density were consistently retained in the top models predicting the abundance and distribution of native fishes. The greatest increases occurred in warmer reaches and in years with spring flooding. Surprisingly, given the evolution of native fishes in disturbance-prone systems, intense, monsoon-driven flooding limited native fish recruitment. Applied concertedly, invasive species suppression and efforts to mimic natural flow and thermal regimes may allow rapid and widespread native fish recovery.