Large River Fish Research Articles

Quantifying the contributions of tributaries to large‐river fish populations through mark‐recapture modeling

AbstractObjectiveTributaries may play a vital role in maintaining populations of large river fishes, although the specific contributions of tributaries toward recruitment of river‐wide populations are not often understood. Tributaries may experience fewer cumulative anthropogenic impacts relative to main‐stem rivers and may offer more natural conditions supportive of native fish populations, which may provide opportunities for fish population restoration. Thus, an improved understanding of tributary–main‐stem population dynamics may inform targeted conservation actions for spatially structured populations of large‐river fishes. Colorado River tributaries in the Grand Canyon, Arizona, USA, are a focus of imperiled Humpback Chub Gila cypha conservation, which includes translocations to enhance population redundancy and to expand the overall population. However, the fate of fish dispersed to the main stem has not been thoroughly quantified.MethodsUsing open population mark–recapture models, we quantified the relative contribution of three groups of Humpback Chub, including fish of confirmed tributary origin that were either translocated or produced in situ, and others presumed to be Colorado River main‐stem‐origin fish, to three main‐stem populations. Our specific study objectives were to (1) estimate Colorado River abundances of tributary and main‐stem‐origin fish over time, (2) compare relative group‐specific contributions to three main‐stem populations, and (3) compare group‐specific survival rates of Humpback Chub in the Colorado River and in a tributary where a recent translocation has occurred.ResultTributaries contributed 26% and 43% of the overall abundance in two tributary inflow reach populations and zero in a third, which we attributed to uncharacteristically low tributary survival immediately following translocation. In the main stem, survival of tributary‐origin fish was higher compared to main‐stem‐origin fish, suggesting an advantage of tributary residence.ConclusionOur contrasting results from three different tributary inflow populations highlight the potential role for tributaries in sustaining large‐river fish populations, which may have important implications for long‐term maintenance of river metapopulations.

Unconfined fishway design, implementation, and assessment for the emerald Shiner (Notropis atherinoides) in the Upper Niagara River, New York

An engineered seawall in need of repair along the Upper Niagara River in Buffalo, NY, was previously identified as a hydrodynamic barrier to the migration of a small but important prey fish, the emerald shiner (Notropis atherinoides), and its connection to the upstream environs of Lake Erie. This study proposed a novel unconfined fishway design to be incorporated into existing plans for rehabilitation of the seawall at Broderick Park. The proposed fishway was developed using a two-dimensional depth-averaged numerical model, it was explored by placing the target fish into a physical model, and the constructed demonstration project was assessed using field measurements and observations. The numerical model showed that modular baffles attached to the seawall reduced the nearshore flow velocities to about 0.4 to 0.6 m s−1, which was within the acceptable range of velocities for the upstream migration of the emerald shiners. The physical experiments demonstrated that most emerald shiners chose to swim toward, within, or close to the proposed fishway in the presence of higher flow velocities within the main channel region. The proposed fishway design was integrated into the seawall repair as part of a demonstration project supervised by the US Army Corps of Engineers-Buffalo District. Post-construction assessment of the installation showed that (1) the modular baffles reduced time-averaged flow velocities near the seawall and water surface, (2) while very few shiners were observed swimming along the pre-construction seawall, large numbers were observed swimming upstream within this reduced flow velocity corridor created by the baffles, and (3) the structural integrity of the installation proved to be durable and resilient to extremely harsh environmental conditions. This study explored a new approach to fishway design for the passage of relatively small fish in large rivers, and it effectively melded fishway requirements into seawall restoration in concert with the expectations of multiple stakeholders.

Diverse portfolios: Investing in tributaries for restoration of large river fishes in the Anthropocene

Rehabilitation of large Anthropocene rivers requires engagement of diverse stakeholders across a broad range of sociopolitical boundaries. Competing objectives often constrain options for ecological restoration of large rivers whereas fewer competing objectives may exist in a subset of tributaries. Further, tributaries contribute toward building a “portfolio” of river ecosystem assets through physical and biological processes that may present opportunities to enhance the resilience of large river fishes. Our goal is to review roles of tributaries in enhancing mainstem large river fish populations. We present case histories from two greatly altered and distinct large-river tributary systems that highlight how tributaries contribute four portfolio assets to support large-river fish populations: 1) habitat diversity, 2) connectivity, 3) ecological asynchrony, and 4) density-dependent processes. Finally, we identify future research directions to advance our understanding of tributary roles and inform conservation actions. In the Missouri River United States, we focus on conservation efforts for the state endangered lake sturgeon, which inhabits large rivers and tributaries in the Midwest and Eastern United States. In the Colorado River, Grand Canyon United States, we focus on conservation efforts for recovery of the federally threatened humpback chub. In the Missouri River, habitat diversity focused on physical habitats such as substrate for reproduction, and deep-water habitats for refuge, whereas augmenting habitat diversity for Colorado River fishes focused on managing populations in tributaries with minimally impaired thermal and flow regimes. Connectivity enhancements in the Missouri River focused on increasing habitat accessibility that may require removal of physical structures like low-head dams; whereas in the Colorado River, the lack of connectivity may benefit native fishes as the disconnection provides refuge from non-native fish predation. Hydrologic variability among tributaries was present in both systems, likely underscoring ecological asynchrony. These case studies also described density dependent processes that could influence success of restoration actions. Although actions to restore populations varied by river system, these examples show that these four portfolio assets can help guide restoration activities across a diverse range of mainstem rivers and their tributaries. Using these assets as a guide, we suggest these can be transferable to other large river-tributary systems.

Shoreline hardening affects species composition and condition of larval fishes in the Yangtze River: Implications for conservation and inshore habitat restoration

AbstractInshore habitats play a crucial role in the downstream drift of larval fish in large rivers. Natural inshore habitats of most of the world's large rivers have been heavily degraded by shoreline stabilization measures, but the knowledge about consequences for the condition and distribution of larval fishes is extremely limited. Investigations during the peak season of larval drift into species composition as well as into feeding intensity and body injuries of representative taxa were conducted at different inshore mesohabitats along semi‐natural and stabilized shores in a free‐flowing section in the middle Yangtze River. Species were unambiguously identified by a combination of DNA barcoding and morphological analyses. Overall, 37 taxa from seven families were detected from a total of 9,915 fish larvae caught. Earlier developmental stages greatly outnumbered the later stages in diversity and abundance in each investigated mesohabitat. Abundance and assemblage structure as well as the body injury rate differed significantly between mesohabitats. Generally, overall species richness and abundance tended to be lower, and divergence of the assemblage and body injury rates tended to be higher at heavily hardened shores, which could be contributed to the poor riparian vegetation and restricted suitable hydraulic conditions there. Our results may reflect the underestimated impact of inshore habitat degradation on larval fishes and highlight the demand for creating adequate areas of shallow and slow‐flow habitats with good riparian vegetation conditions along the shores. These measures will foster an appropriate connectivity between spawning areas and high‐quality nurseries for larval fishes.

РОЗВИТОК ОРГАНІЧНОГО АГРАРНОГО ВИРОБНИЦТВА ЯК ПЕРСПЕКТИВНИЙ ШЛЯХ ДО ЕКОЛОГІЧНОЇ БЕЗПЕКИ БОЛГАРСЬКОГО СЕЛА

The importance of agriculture for the Bulgarian economy is reflected in historical and economic scientific research in the field of agrarian history, economics and law, influencing the growing interest of the scientific community in agrarian problems, including agrarian-ecological ones. In the conditions of the tense European agricultural market, despite numerous agreements, the state support of the Bulgarian agricultural producer in most developed countries is steadily increasing. Bulgaria, whose economy is organically and inextricably linked with agriculture, also found itself in the conditions of an urgent need to review, reassess the system of state support for ecological agriculture, in-depth analysis of the legal basis of its functioning, establishing ways of improvement and further development of ecologically oriented agricultural production. The problem of implementing ecological aspects of agricultural production in Bulgaria in the context of European integration is highlighted. The Bulgarian model of sustainable agricultural nature management is considered, and the main environmental problems that prevented its implementation are described. Attention is focused on the main aspects of state policy regarding the support of ecological agricultural production. Agricultural production, as one of the most common types of human activity, had a harmful effect on the surrounding natural environment, since industry and transport were considered the main violators of the natural balance at one time. Currently, agriculture is in the first place in terms of environmental pollution. The condition of the agricultural lands in Bulgaria is unsatisfactory. However, it was determined that serious problems for the environment arose due to the irregular use of mineral fertilizers and agrochemicals in agricultural production, which, together with rainwater and underground water, enter rivers and lakes, causing significant damage to large river basins, fish stocks and vegetation. As a result of the scientific research, it was established that the problem of forming the concept of biological agricultural production in Bulgaria has not been solved. The model of environmentally-oriented agricultural production required organizational and economic support for the management of sustainable agricultural nature management.

Influence of Spatial Extent on Contemporary and Future Threat Evaluation for Imperiled Fluvial Fishes and Mussels

Species conservation often faces many challenges, such as addressing threats from multiple stressor sources, representing under-studied taxa, and understanding implications of spatial extent. To overcome these challenges, we assessed contemporary anthropogenic threats from stream fragmentation and landscape disturbance as well as future habitat suitability under climate change for traditionally well-studied (fishes) and under-studied (mussels) imperiled fluvial taxa in Michigan, USA. To understand how threats to species vary spatially, predicted habitat suitability was analyzed for three hierarchically nested spatial extents: statewide, within species’ biogeographic ranges, and within river patches fragmented by barriers. Comparison of current and future habitat suitability for 27 fish and 23 mussel species indicates large potential statewide gains for many warmwater and/or large river fishes and several mussel species, however these gains are greatly diminished by biogeographic range limitations and habitat fragmentation among current and future habitats. One mussel species and several cold- and coolwater fishes are projected to have significant habitat losses under climate change irrespective of spatial extent. On average, 79% of habitats for mussels and 58% for fishes were considered moderately to severely disturbed from current human landscape activities. Habitat fragmentation was greater for fishes than mussels, with large dams playing a primary role in fragmenting habitats relative to small dams and waterfalls. Results indicate that threat assessments can vary substantially according to spatial extent and taxa, and consideration of both contemporary and future threats to habitats is needed to inform conservation of imperiled fluvial organisms.

Life and death in a dynamic environment: Invasive trout, floods, and intraspecific drivers of translocated populations.

Understanding the relative strengths of intrinsic and extrinsic factors regulating populations is a long‐standing focus of ecology and critical to advancing conservation programs for imperiled species. Conservation could benefit from an increased understanding of factors influencing vital rates (somatic growth, recruitment, survival) in small, translocated populations, which is lacking owing to difficulties in long‐term monitoring of rare species. Translocations, here defined as the transfer of wild‐captured individuals from source populations to new habitats, are widely used for species conservation, but outcomes are often minimally monitored, and translocations that are monitored often fail. To improve our understanding of how translocated populations respond to environmental variation, we developed and tested hypotheses related to intrinsic (density dependent) and extrinsic (introduced rainbow trout Oncorhynchus mykiss, stream flow and temperature regime) causes of vital rate variation in endangered humpback chub (Gila cypha) populations translocated to Colorado River tributaries in the Grand Canyon (GC), USA. Using biannual recapture data from translocated populations over 10 years, we tested hypotheses related to seasonal somatic growth, and recruitment and population growth rates with linear mixed‐effects models and temporal symmetry mark–recapture models. We combined data from recaptures and resights of dispersed fish (both physical captures and continuously recorded antenna detections) from throughout GC to test survival hypotheses, while accounting for site fidelity, using joint live‐recapture/live‐resight models. While recruitment only occurred in one site, which also drove population growth (relative to survival), evidence supported hypotheses related to density dependence in growth, survival, and recruitment, and somatic growth and recruitment were further limited by introduced trout. Mixed‐effects models explained between 67% and 86% of the variation in somatic growth, which showed increased growth rates with greater flood‐pulse frequency during monsoon season. Monthly survival was 0.56–0.99 and 0.80–0.99 in the two populations, with lower survival during periods of higher intraspecific abundance and low flood frequency. Our results suggest translocations can contribute toward the recovery of large‐river fishes, but continued suppression of invasive fishes to enhance recruitment may be required to ensure population resilience. Furthermore, we demonstrate the importance of flooding to population demographics in food‐depauperate, dynamic, invaded systems.

Identifying monitoring information needs that support the management of fish in large rivers.

Management actions intended to benefit fish in large rivers can directly or indirectly affect multiple ecosystem components. Without consideration of the effects of management on non-target ecosystem components, unintended consequences may limit management efficacy. Monitoring can help clarify the effects of management actions, including on non-target ecosystem components, but only if data are collected to characterize key ecosystem processes that could affect the outcome. Scientists from across the U.S. convened to develop a conceptual model that would help identify monitoring information needed to better understand how natural and anthropogenic factors affect large river fishes. We applied the conceptual model to case studies in four large U.S. rivers. The application of the conceptual model indicates the model is flexible and relevant to large rivers in different geographic settings and with different management challenges. By visualizing how natural and anthropogenic drivers directly or indirectly affect cascading ecosystem tiers, our model identified critical information gaps and uncertainties that, if resolved, could inform how to best meet management objectives. Despite large differences in the physical and ecological contexts of the river systems, the case studies also demonstrated substantial commonalities in the data needed to better understand how stressors affect fish in these systems. For example, in most systems information on river discharge and water temperature were needed and available. Conversely, information regarding trophic relationships and the habitat requirements of larval fishes were generally lacking. This result suggests that there is a need to better understand a set of common factors across large-river systems. We provide a stepwise procedure to facilitate the application of our conceptual model to other river systems and management goals.

An Assessment of Nighttime Electrofishing in the Lower Wabash River

AbstractLarge rivers are highly important systems that require close monitoring of the ecological consequences of factors like commercial and recreational exploitation, channel alterations, and climate change. The Long‐Term Survey and Assessment of Large River Fishes in Illinois (LTEF) monitors the fish communities of the Illinois, Mississippi, Wabash, and Ohio rivers using daytime, pulsed‐DC electrofishing between June and October each year. Previous research has documented diel differences in the catch and composition of fish communities. Therefore, the addition of nighttime electrofishing may be beneficial to the overarching goals of long‐term monitoring. This study sought to determine whether there are significant diel differences in fish catch and composition in the Wabash River and whether these potential differences warrant the addition of nighttime sampling to the current protocol. We used nighttime, pulsed‐DC electrofishing at fixed sites corresponding to LTEF sites in the lower Wabash River from June 7 to November 7, 2017. We compared catch per unit effort (CPUE), length distributions, family, and species composition between our nighttime electrofishing data and LTEF daytime electrofishing data. Nonmetric multidimensional scaling showed that the differences between nighttime and daytime electrofishing samples were driven by higher catches of catostomids, centrarchids, and lepisosteids. Nighttime electrofishing had significantly higher mean CPUE than daytime electrofishing, and the four families with significant differences were Catostomidae, Cyprinidae, Lepisosteidae, and Sciaenidae. Of the families for which we compared length data, only Sciaenidae had a significantly different length distribution, with a higher proportional catch of smaller individuals (<100 mm) at night. Despite these differences, compositional shifts were minor, with overlap between daytime and nighttime sampling. Due to the compositional similarities between night and day, the costs of adding nighttime electrofishing to long‐term monitoring would not be necessary. We would recommend nighttime electrofishing for targeting specific taxa and age‐classes that are underrepresented in daytime samples.

Monitoring a minuscule madtom: Environmental DNA surveillance of the endangered pygmy madtom (Noturus stanauli Etnier & Jenkins 1980) in the Duck and Clinch rivers, Tennessee

AbstractEnvironmental DNA (eDNA) detection has been shown to be an effective biosurveillance tool for freshwater fishes, but further research is needed to apply eDNA detection tools to small and rare fishes in large rivers. We developed an eDNA surveillance assay and protocol for monitoring the presence of the pygmy madtom (Noturus stanauli), a federally protected freshwater fish endemic to the Clinch and Duck rivers in Tennessee (United States, North America). Noturus stanauli is a diminutive fish that is exceedingly rare throughout its range; it is currently known only from a 115‐river‐km section of the Duck River and a 5‐river‐km section of the Clinch River. The aim of this research was to develop an eDNA assay to detect the presence of N. stanauli in both the Duck and Clinch rivers. We used this newly developed eDNA protocol to assess detection as a function of water depth and to further delineate the distribution of N. stanauli in both the Duck and Clinch rivers. Field sampling was performed to delineate the extent of N. stanauli's range in both rivers. Our results indicated that samples collected from three areas within the water column, as well as a sediment core samples, yielded equal detection rates. Our assay detected the presence of N. stanauli at a previously unknown site outside the current distribution in the Clinch River, located approximately 2.5‐river‐km downstream from the Tennessee‐Virginia state line. We demonstrated that eDNA detection is a promising tool for delineating the distribution of N. stanauli; however, further research is needed to assess environmental and life history variables that influence eDNA detection probability of small fish in large rivers.

Round goby versus marbled crayfish: alien invasive predators and competitors

Aquatic biodiversity is threatened by spread of invasive alien species. Round goby Neogobius melanostomus is an invasive fish in large European rivers as well as in coastal waters near their mouths and marbled crayfish Procambarus virginalis is a highly invasive crustacean. Both are small, bottom-dwelling species occupying similar habitat and shelters and utilizing similar food sources. We hypothesized that goby presents a threat to both native and non-native astacofauna in invaded ecosystems. We tested this through laboratory experiments designed to determine aggressiveness and competitiveness of goby against marbled crayfish as a model for other North American cambarid crayfish, assessing goby prey size selection and competition with marbled crayfish for space and shelter. Gobies showed high aggressiveness and dominance over the crayfish. Goby predation on juvenile crayfish was limited by mouth gape size. In goby/crayfish pairs of similar weight, gobies were more aggressive, although each affected the behavior of the other.

Movement ecology of imperilled fish in a novel ecosystem: River‐reservoir movements by razorback sucker and translocations to aid conservation

Abstract Reservoirs and associated river fragments are novel ecosystems not experienced by fishes in their evolutionary history, yet they are now commonplace across the globe. Understanding how fishes use these novel habitats is vital to conservation efforts in contemporary riverscapes. Movement patterns of the endangered razorback sucker (Xyrauchen texanus) synthesized from tagging efforts in the upper Colorado River basin, USA, illustrate the applications of tagging technology and data sharing by multiple agencies to better understand the spatial ecology of large river fishes. Tagging studies between 2014 and 2018 in Lake Powell and its two main tributary rivers, the Colorado (unfragmented) and San Juan (waterfall‐fragmented), were used to quantify movement of razorback sucker within this river–reservoir habitat complex. In addition, facilitated translocations of fish upstream of a waterfall barrier in the San Juan River were assessed in 2016–2017. Extensive movement of fish occurred within and across river and reservoir habitats. Of 722 fish captured in the Colorado River arm of Lake Powell, 36% of re‐encounters occurred upstream in the Colorado or Green rivers, or fish dispersed through the reservoir and were detected in the San Juan River arm. Fourteen fish moved more than 600 km. In the San Juan arm of the reservoir, 29% and 20% of fish in 2017 and 2018, respectively, had moved ~30–40 km upstream below the waterfall in the San Juan River within a year. In 2016–2017, 303 fish were translocated upstream of the waterfall into the San Juan River, but 80% were re‐encountered downstream of the waterfall within a year. Long‐distance movements by razorback sucker were common within and among rivers and reservoirs illustrating how large river fish, in general, might maintain population connectivity in highly altered ecosystems.

Black Carp in North America: a Description of Range, Habitats, Time of Year, and Methods of Reported Captures

AbstractBlack Carp Mylopharyngodon piceus are considered invasive in North America. Since the first wild capture in 2003, collection records have increased, yet information summarizing successful collection methods is lacking. Reported capture methods throughout the Black Carp's native and introduced ranges vary providing minimal aid for determining control and monitoring methods. Here, we describe the current species range and the spatial and temporal variation among captures. The size of fish can affect capture; thus, we report captured fish and gear dimensions and recommend appropriate scientific collection methods. We focus on collection data from 302 Black Carp ranging from 410 to 1,607 mm total length received from 2011 to February 2019. The reported range of Black Carp has expanded in the Cumberland, Illinois, Mississippi, Missouri, Ohio, and Tennessee rivers. Captures most frequently occurred in channel (41%), side‐channel (24%), and backwater (22%) habitat types, with increased records in May, June, and July. Most records were commercial captures, of which hoop net (51%) and gill net (26%) were most common. Results suggest that standard scientific methods for sampling fish in large rivers and standing water by hoop net and gill net may be used to monitor Black Carp, but a robust study design needs to be applied to determine gear selectivity and to determine if catch rates are density dependent or incidental.

Over 50 years of fish community monitoring in Illinois’ large rivers: The evolution of methods used by the Illinois Natural History Survey’s Long-term Survey and Assessment of Large-River Fishes in Illinois

This document describes the methodological evolution of the Long-Term Illinois Rivers Fish Population Monitoring Program since its inception. The principal changes include a shift from fixed-site sampling to stratified random sampling at the reach scale to benefit statistical robustness and a concurrent shift from AC electrofishing to pulsed-DC electrofishing to increase the breadth of the diversity and size classes of fishes encountered. These changes in methodology were implemented during the 2016 field season and are based on quantitative assessments of program data and other results from recent peer-reviewed publications. These refinements will likely improve the program’s ability to detect and describe patterns and trends in the fish assemblages of the Illinois River and other large rivers of Illinois and the Midwest.

Ecological Niche Space of Fish Communities in Impounded Sections of Large Rivers: Its Application to Assessment of the Impact of Weirs on River Ecosystems

To estimate the impact of weirs on large river fish communities, we applied the ecological niche space (ENS) measured quantitatively using carbon and nitrogen stable isotope ratios to the monitoring of effects of weirs constructed on four major rivers in Korea. ENS was calculated using the Bayesian stable isotope in R statistics. The ENS of fish communities showed persistent differences between upstream and downstream areas of all studied weirs. The ENSs of omnivores were larger in upstream impounded areas but the ENSs of predator species, both endemic (Erythroculter erythropterus) and exotic (Micropterus salmoides) predators had decreased ENSs in upstream areas. E. erythropterus showed horizontal variations in a wide range of carbon stable isotopes, whereas M. salmoides showed vertical variations in nitrogen stable isotopes. The results suggest that weir construction may have species-specific impact on the ENS of fish community by impoundment and increase niche overlap in upstream areas of the weir. The measured ENS of fish community was significantly correlated with the relative abundance of tolerant species negatively, whereas correlated with that of endemic species positively, suggesting that the ENS can be used as a comprehensive indicator of habitat conditions.

Intensive Sampling Reveals Underreported Use of Great-River Tributaries by Large-River Fishes in Missouri

Large tributaries may help sustain large-river fish populations by mitigating fish-habitat losses within the highly modified great rivers of the Mississippi River basin. These tributaries are likely most beneficial for fish species specializing on non-degraded large-river habitat for some portion of their life histories. Few great-river tributaries, however, have been surveyed using methods that comprehensively target all fish species, resulting in uncertainty or bias in the reported composition of many tributary fish assemblages. We report important distributional records, including 23 new accounts, for 12 large-river specialist fishes in Missouri—Alosa alabamae (Alabama Shad), Cycleptus elongatus (Blue Sucker), Pimephales vigilax (Bullhead Minnow), Notropis wickliffi (Channel Shiner), Polyodon spathula (Paddlefish), Hybognathus placitus (Plains Minnow), N. blennius (River Shiner), Macrhybopsis hyostoma (Shoal Chub), Scaphirhynchus platorynchus (Shovelnose Sturgeon), M. storeriana (Silver Chub), Ichthyomyzon unicuspis (Silver Lamprey), and Alosa chrysochloris (Skipjack Herring)—following 38 comprehensive fish surveys in tributaries of the Missouri and Mississippi rivers. New accounts collectively demonstrate tributaries support more large-river specialists than historically documented and thus may be currently undervalued sources of habitat for large-river fishes.

Tracking the Trajectory of Change in Large River Fish Communities Over 50 Y

Multivariate statistics are commonly used by ecologists to study spatial and temporal community dynamics to better inform management decisions. Since these methods are a universal tool to analyze data, it is important to assess their effectiveness using long-term datasets in well-studied systems. The objectives of this study were to identify trends in the fish communities and to characterize the community trajectory (i.e., directional or nondirectional) of the Illinois River, a large tributary to the Mississippi River, using nonmetric multidimensional scaling (NMDS). Although NMDS has been used to evaluate community trajectory in multivariate space over time in smaller rivers, there has been less attention given to larger rivers, which may exhibit different dynamics. To evaluate changes in large river fish communities, we used a long-term electrofishing monitoring dataset (1957–2013) that includes fish abundance data from six reaches of the Illinois River, Illinois, U.S.A. Temporal changes in Illinois River fish communities were evident in NMDS ordinations. Larger changes in NMDS ordinations were associated with improving water quality conditions, whereas the invasion of bigheaded carp (silver carp Hypophthalmichtys molitrix and bighead carp H. nobilis) produced smaller and less variable changes. Across all six reaches of the Illinois River, the trajectory of fish communities was directional in multivariate space, with no return to an initial or predisturbance condition. Community trajectory in larger and more heavily-modified rivers may be substantially different and more complex than that of smaller and less heavily modified rivers.

Discontinuities and functional resilience of large river fish assemblages

AbstractFunctional composition of communities across scales is increasingly used to infer resilience of biotic communities to environmental change. To assess the relevance of these concepts to management of large rivers, analyses were applied to fish community data of the Upper Mississippi River. First, to evaluate whether there was evidence for structural patterns in fish size distributions, a discontinuity analysis was performed. Using long‐term fish data, consistent discontinuities were identified across 14 reaches, suggesting similar structuring processes occur throughout the nearly 1300 river kilometers represented by those reaches. Increased variability in species abundance in relation to proximity to edges of body size aggregations supports the discontinuity hypothesis that body size aggregations are structured by key processes. Functional richness and redundancy were quantified within and across identified scales for each of 14 river reaches. Diversity of trophic and spawning guilds was generally greater in the upstream reaches in comparison with downstream reaches, with the exception of the diversity of large‐bodied spawning guilds. Evidence of functional shifts in the composition of fishes occurred, but differed by size aggregations, likely reflecting scale‐specific resource availability. Redundancy of spawning and trophic groups across body size aggregations suggested downstream reaches of this system may be less resilient to disturbances and were weakly associated with reduced habitat diversity. These findings suggest that discontinuities in large river fish assemblages do occur and may provide indication of shifting resource availability. Further investigation of the underlying processes and scales that support resource availability will be critical to managing for resilience in large river ecosystems.

Large river fish functional diversity responses to improved water quality over a 28 year period

The enactment of environmental legislation and subsequent improvements in wastewater treatment during the late 20th century resulted in improved water quality for many large rivers in developed countries. Yet, little is known about how these improvements affected the functional diversity of aquatic assemblages, or of its relationship with species diversity during recovery. We examined species richness (SR), functional dispersion (FDis), and functional evenness (FEve) changes in four reaches of the Illinois River, USA, during a recovery period (1983–2010). The functional indices were based on fish habitat and foraging traits and weighted with either abundance or biomass. Though most correlations of the indices with SR were weak, additional species tended to increase FDis and decrease FEve. Biomass-weighted functional dispersion (FDisw) and SR consistently increased during the recovery, while the responses of FEve and abundance weighted FDis were much more variable. FDisw was better explained by improvements in water quality in all but the least impacted reach, where SR was best explained. Un-ionized ammonia, phosphorus, phenols, and water clarity were among the most important predictors of biodiversity changes. Increases in habitat and foraging specialized species appeared to contribute equally to functional changes in the three more degraded reaches, whereas increases in habitat specialization had a greater impact on FDisw changes in the least impacted reach. Later stage recovery of habitat specialist fishes in the absence of major habitat alterations indicates habitat improvement projects should be undertaken following water quality remediation. Functional diversity and species richness are complementary and both should be assessed to detect the responses of fish assemblages to environmental alterations. Biomass may better reflect functional changes in response to disturbance than abundance and should be assessed when estimating functional diversity.

The influence of discharge, current speed, and development on the downstream dispersal of larval nase (Chondrostoma nasus) in the River Danube

We investigated the mode (active versus passive) of larval downstream dispersal and its influencing factors in the nase carp (Chondrostoma nasus). Marked larvae (early and later stages), together with equivalent numbers of passive particles, were released in the main channel of the River Danube (Austria) at different flow (low, high) and current (over-critical, under-critical) conditions. Larvae and particles were recaptured with stationary nets at varying distances from release. We assumed that differences in the spatial dispersal patterns between larvae and particles were due to fish activity. We hypothesized that river discharge, developmental stage, current speed, and distance from release would influence these differences. We found that activity was independent of developmental stage or current speed at release, although activity was higher during low flow conditions. It may be that larvae deliberately enter the current during low flow, because the hydraulic conditions facilitate active dispersal. Furthermore, activity was greatest near the release site. This might be due to an intrinsically greater activity when fish are placed into novel surroundings or a result of rheoreaction. The discharge-dependent dispersal patterns observed represent an important ecological link between flow and recruitment and demonstrate the importance of inshore conditions for the early life stages of fish in large rivers, especially with regard to river modification and restoration schemes.