Physical and Biological Responses to an Alternative Removal Strategy of a Moderate‐sized Dam in Washington,USA

Dam removal is an increasingly practised river restoration technique, and ecological responses vary with watershed, dam and reservoir properties, and removal strategies. Moderate‐sized dams, like Hemlock Dam (7.9 m tall and 56 m wide), are large enough that removal effects could be significant, but small enough that mitigation may be possible through a modified dam removal strategy. The removal of Hemlock Dam in Washington State, USA, was designed to limit channel erosion and improve fish passage and habitat by excavating stored fine sediment and reconstructing a channel in the former 6‐ha reservoir. Prior to dam removal, summer daily water temperatures downstream from the dam increased and remained warm long into the night. Afterwards, a more natural diel temperature regime was restored, although daily maximum temperatures remained high. A short‐lived turbidity pulse occurred soon after re‐watering of the channel, but was otherwise similar to background levels. Substrate shifted from sand to gravel–cobble in the former reservoir and from boulder to gravel–cobble downstream of the dam. Initially, macroinvertebrate assemblage richness and abundance was low in the project area, but within 2 years, post‐removal reaches upstream and downstream of the dam had diverse and abundant communities. The excavation of stored sediment and channel restoration as part of the dam removal strategy restored river continuity and improved benthic habitat while minimizing downstream sedimentation. This study provides a comparison of ecological effects with other dam removal strategies and can inform expectations of response time and magnitude. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

LONG‐TERM TAXON‐SPECIFIC RESPONSES OF MACROINVERTEBRATES TO DAM REMOVAL IN A MID‐SIZED SWEDISH STREAM

Dam removal to restore ecologically impaired rivers is becoming increasingly common. Although the target often is to facilitate fish migration, dam removal has also been assumed to benefit other types of organisms. Because few studies thus far deal with effects of dam removal on stream macroinvertebrates and because results have been equivocal, we investigated both short‐ and longer‐term dam‐removal effects on downstream macroinvertebrate communities. We did this in a before‐and‐after study of the removal of a dam located in a south Swedish stream. We sampled the benthic fauna 6 months before dam removal and both 6 months and 3.5 years after the dam was removed. We compared species composition, taxonomic richness, total densities and densities of macroinvertebrate groups before and after dam removal and between downstream and reference sites. We found that dam removal reduced some macroinvertebrate taxa at the downstream site, but we found no effect on community composition. Although this corroborates results from previous short‐term studies, we also found a reduction of taxonomic richness and that some dam‐removal effects persisted or even increased over time. The most likely explanation for the suppression of benthic macroinvertebrate richness following dam removal is a significantly increased sediment transport from the former reservoir and a subsequent loss of preferred substrates. Our results indicate that adverse dam‐removal effects may be long lasting but taxon specific. We therefore call for longer‐term studies on a variety of organisms to better understand how dam removal may influence downstream macroinvertebrate communities. Copyright © 2012 John Wiley & Sons, Ltd.     

Small hydropower dam alters the taxonomic composition of benthic macroinvertebrate assemblages in a neotropical river

Hydropower dams substantially modify lotic ecosystems. Most studies regarding their ecological impacts are based on large dams and provide little information about the far more abundant effects of small hydropower dams. Our aim was to characterize the ecological effects of a small hydropower dam and run‐of‐the‐river reservoir on the structure of benthic macroinvertebrate assemblages in the Pandeiros River located in the neotropical savanna of Brazil. We tested the hypothesis that benthic macroinvertebrate assemblages in sites directly affected by the dam and reservoir would show a different taxonomic structure compared with those in free‐flowing sites. We expected to find sensitive native species associated with the free‐flowing sites, whereas resistant and non‐native invasive taxa were expected in impounded sites. We also explored associations between the presence of native and non‐native invasive taxa to each habitat type. We found that the structure of benthic macroinvertebrate assemblages was significantly different between free‐flowing and impounded sites. Also, we found that the dam and reservoir facilitated colonization of non‐native invasive species (Corbicula fluminea and Melanoides tuberculata) because only in those sites they were found in high abundance, in contrast to the free‐flowing sites. Although the environmental conditions imposed by the impoundment altered the structure of benthic macroinvertebrate assemblages, the effects were limited to sites closest to the dam. Our results highlight the necessity of understanding physical habitat changes caused by the presence and management of run‐of‐the‐river dams and reservoirs.     

DISTRIBUTION AND ABUNDANCE OF STREAM FISHES IN RELATION TO BARRIERS: IMPLICATIONS FOR MONITORING STREAM RECOVERY AFTER BARRIER REMOVAL

Dams are ubiquitous in coastal regions and have altered stream habitats and the distribution and abundance of stream fishes in those habitats by disrupting hydrology, temperature regime and habitat connectivity. Dam removal is a common restoration tool, but often the response of the fish assemblage is not monitored rigorously. Sedgeunkedunk Stream, a small tributary to the Penobscot River (Maine, USA), has been the focus of a restoration effort that includes the removal of two low‐head dams. In this study, we quantified fish assemblage metrics along a longitudinal gradient in Sedgeunkedunk Stream and also in a nearby reference stream. By establishing pre‐removal baseline conditions and associated variability and the conditions and variability immediately following removal, we can characterize future changes in the system associated with dam removal. Over 2 years prior to dam removal, species richness and abundance in Sedgeunkedunk Stream were highest downstream of the lowest dam, lowest immediately upstream of that dam and intermediate farther upstream; patterns were similar in the reference stream. Although seasonal and annual variation in metrics within each site was substantial, the overall upstream‐to‐downstream pattern along the stream gradient was remarkably consistent prior to dam removal. Immediately after dam removal, we saw significant decreases in richness and abundance downstream of the former dam site and a corresponding increase in fish abundance upstream of the former dam site. No such changes occurred in reference sites. Our results show that by quantifying baseline conditions in a small stream before restoration, the effects of stream restoration efforts on fish assemblages can be monitored successfully. These data set the stage for the long‐term assessment of Sedgeunkedunk Stream and provide a simple methodology for assessment in other restoration projects. Copyright © 2011 John Wiley & Sons, Ltd.     

Low-head dam removal increases functional diversity of stream fish assemblages

Despite the growing number of dam removals, few have been studied to understand their impacts on stream fish communities. An even smaller proportion of dam removal studies focus on the impacts of low-head dam removals, although they are the most common type of dam. Instead, the majority of removal studies focus on the impacts of larger dams. In this study, two previously impounded Illinois Rivers were monitored to assess the impacts of low-head dam removal on the functional assemblage of stream fishes. Study sites were sampled each fall from 2012–2015 (pre-dam removal) and 2018–2020 (post-dam removal) in three locations: the tailrace, impoundment, and river channel. Fishes were aggregated into habitat and reproductive guilds, relating community changes to habitat, environmental metrics, and stream quality. Prior to removal, the slackwater guild was the most prevalent habitat guild throughout both rivers, while nest builders and benthic spawners were the most abundant reproductive guilds. During the two years following removal, fish assemblage throughout both rivers shifted to a more evenly distributed representation of habitat and reproductive guilds, while restoration of lotic habitat conditions increased, as surface water temperatures decreased and QHEI, IBI, and dissolved oxygen increased. This shift in environmental metrics and increase in overall stream quality increased, particularly in the formerly impounded reaches, indicate diminished habitat homogeneity, and a shift towards natural habitat diversity. This habitat diversification likely led to the restoration of a range of potential niches, thereby increasing the array of guild types inhabiting these rivers, while simultaneously preventing single-guild dominance.     

LONG‐TERM IMPLICATIONS OF DAM REMOVAL FOR MACROINVERTEBRATE COMMUNITIES IN MICHIGAN AND WISCONSIN RIVERS,UNITED STATES

The removal of the numerous ageing dams in the United States has become an important stream restoration technique. The extent to which the ecological damage done to streams by dams is reversed upon removal is unknown, especially on decadal time scales. The objectives of this study were to determine if macroinvertebrate assemblages within rivers recover following the removal of a dam and to estimate the time needed for recovery. A space‐for‐time substitution approach was used on eight rivers in various stages of recovery following a dam removal, ranging from <1 to 40 years post‐removal. Within each river, macroinvertebrates were sampled in a zone unaffected by the dam removal (reference zone) and two zones impacted by the dam removal (former impoundment and downstream zone). Insects were identified to the family level and placed into functional feeding groups. Various macroinvertebrate community metrics were compared between impacted and unimpacted zones to evaluate the extent of recovery, and plotted over time since removal to develop a temporal trajectory of recovery. Generally, the macroinvertebrate community recovered 3–7 years following removal both in terms of taxonomic similarity and richness, although densities could take decades to recover. Dam removals are a beneficial restoration technique, yet the recovery of important stream components can be variable and may take longer than previous research has suggested. Having realistic expectations of the ecological ramifications of dam removal efforts is paramount in ensuring the success and thus potential of future projects. Copyright © 2011 John Wiley & Sons, Ltd.     

Using hydroacoustics to relate fluctuations in fish abundance to river restoration efforts and environmental conditions in the Penobscot River,Maine

Quantifying change after large habitat restoration efforts is critical to assessing the effort's success. After river restoration activities, fish abundance is expected to increase and also fluctuates naturally by season and with environmental conditions. A side‐looking hydroacoustic system was used to estimate fish abundance in the Penobscot River, ME, from 2010 to 2016 during predam (2010–2013) and postdam (2014–2016) removal years during nonice periods. Automated data processing enabled continuous abundance estimates from fish tracks. A threefold increase in mean fish abundance was recorded after dam removal. A fourfold increase in median fish abundance occurred in the fall relative to spring and summer of the same year, regardless of dam presence. Interannual fish abundance in every season monitored increased at least twofold after dam removal. We related variability in fish abundance to tide, discharge, temperature, diurnal cycle, daylength, moon phase, and restoration activities (focusing on dam presence). Daylength corresponding to the fall and summer was the most important indicator for higher fish abundance. Fish abundance was generally greatest in the fall during outgoing tides at night, with lesser peaks occurring during the month of June at night. Before dam removal, fish abundance peaked when water temperature was less than 9.25°C, at night, during outgoing tides. After dam removal, fish abundance peaked when daylength was less than 11.3 hr, at night, during outgoing tides, when water temperature was above 4.56°C and no full moon was present. Peak fish abundance occurred during only 0.02–2.3% of the total time sampled. The threefold increase in fish abundance recorded after dam removal was observed despite yearly stocking efforts of adult alewife increasing 24% in postdam removal years. Finally, parallel studies of fish presence in the Penobscot River were used to compare the utility of this method as an indicator of fish abundance in response to dam removal.     

An Assessment of Fish Assemblage Structure in a Large River

The Penobscot River drains the largest watershed in Maine and once provided spawning and rearing habitats to 11 species of diadromous fishes. The construction of dams blocked migrations of these fishes and likely changed the structure and function of fish assemblages throughout the river. The proposed removal of two main‐stem dams, improved upstream fish passage at a third dam, and construction of a fish bypass on a dam obstructing a major tributary is anticipated to increase passage of and improve habitat connectivity for both diadromous and resident fishes. We captured 61 837 fish of 35 species in the Penobscot River and major tributaries, through 114 km of boat electrofishing. Patterns of fish assemblage structure did not change considerably during our sampling; relatively few species contributed to seasonal and annual variability within the main‐stem river, including smallmouth bass Micropterus dolomieu, white sucker Catostomus commersonii, pumpkinseed Lepomis gibbosus, and golden shiner Notemigonus crysoleucas. However, distinct fish assemblages were present among river sections bounded by dams. Many diadromous species were restricted to tidal waters downriver of the Veazie Dam; Fundulus species were also abundant within the tidal river section. Smallmouth bass and pumpkinseed were most prevalent within the Veazie Dam impoundment and the free‐flowing river section immediately upriver, suggesting the importance of both types of habitat that supports multiple life stages of these species. Further upriver, brown bullhead Ameiurus nebulosus, yellow perch Perca flavescens, chain pickerel Esox niger, and cyprinid species were more prevalent than within any other river section. Our findings describe baseline spatial patterns of fish assemblages in the Penobscot River in relation to dams with which to compare assessments after dam removal occurs. Copyright © 2014 John Wiley & Sons, Ltd.     

Occupancy Patterns of Mammals and Lentic Amphibians in the Elwha River Riparian Zone Before Dam Removal

The downstream transport of sediments and organics and upstream migration of anadromous fishes are key ecological processes in unregulated riverine ecosystems of the North Pacific coast, but their influence on wildlife habitats and populations is poorly documented. Removal of two large hydroelectric dams in Washington's Elwha Valley provides an unprecedented opportunity to study long‐term responses of wildlife populations to dam removal and restoration of these key ecological processes. We compared pre‐dam removal patterns in the relative abundance and occupancy of mesocarnivores, small mammals and lentic amphibians of the Elwha River riparian zone above, between and below the dams. Occupancy of riparian habitats by three mesocarnivore species diminished upriver but did not appear to be closely linked with the absence of salmon in the upper river. Although the importance of salmon in the lower river cannot be discounted, other gradients in food resources also likely contributed to observed distribution patterns of mesocarnivores. Abundance and occupancy patterns within congeneric pairs of new world mice (Peromyscus spp.) and shrews (Sorex spp.) indicated that closely related species were negatively associated with each other and responded to habitat gradients in the riparian zone. The availability of lentic habitats of amphibians was highly variable, and occupancy was low as a result of rapidly changing flows during the larval development period. We speculate that long‐term changes in habitat conditions and salmon availability following dam removal will elicit long‐term changes in distribution of mesocarnivores, small mammals and amphibians. Long‐term monitoring will enhance understanding of the role of fish and restored ecosystem processes on wildlife communities along salmon‐bearing rivers in the region. Copyright © 2013 John Wiley & Sons, Ltd.     

SHORT‐TERM IMPACTS OF LATERAL HYDROLOGICAL CONNECTIVITY RESTORATION ON AQUATIC MACROINVERTEBRATES

In floodplain ecosystems, the lateral hydrological connectivity between the main river channel and the secondary channels plays a major role in shaping both the habitat conditions and the macroinvertebrate diversity. Among other threats, human activities tend to reduce the lateral connectivity, which increases floodplain terrestrialization and induces a loss of aquatic biodiversity. Consequently, the restoration of lateral connectivity is of growing concern. We studied four secondary channels of the Rhône floodplain that were subjected either to no restoration or to three different restoration measures (river flow increase only, flow increase plus dredging and flow increase plus reconnection to the river). Macroinvertebrate and environmental data were analysed one year before and during a period of five years after restoration. We expected a progressive increase of lateral connectivity according to the type of restoration. Changes in macroinvertebrate assemblages were predicted to be towards more rheophilic communities and proportionally related to the changes in lateral connectivity. In the reconnected channel, lateral connectivity increased and remained high five years after restoration. In the dredged channel, the immediate increase of the lateral connectivity metric induced by sediment removal was followed by a rapid decrease. In the unrestored channel and the channel only influenced by flow increase, the metric remained constant in time. The macroinvertebrate composition and the rarefied EPT richness changes were proportionally related to the changes in lateral connectivity. Alien species richness and densities increased progressively in all channels after restoration. Our results showed that modifications of the lateral connectivity lead to predictable changes in macroinvertebrate diversity. Synergistic interactions between restoration and longer‐term changes (e.g. climatic change, invasion of alien species) encourage long‐term monitoring to assess the durability and trends of restoration measures. Copyright © 2012 John Wiley & Sons, Ltd.     

Using Benthic Indicator Species and Community Gradients to Optimize Restoration in the Arid,Endorheic Walker River Watershed,Western USA

The challenge of restoring watersheds in arid regions often requires the development of novel scientific tools to guide management. The Walker Basin Program was created to reverse ecological decline in an arid, endorheic watershed through scientifically guided restoration. As part of this programme, 3 years of benthic macroinvertebrate samples were collected seasonally at 10 sites that represent the diversity of river environments from the high‐mountain headwaters to a desert terminal lake. Samples were analysed to quantify baseline conditions in reference and degraded reaches of river and identify opportunities and constraints for aquatic community restoration. Naturally harsh environments in the lower river characterized by high temperatures and low base flow combined with a weak understanding of reference conditions to limit the utility of commonly used indices for quantifying biotic integrity. A flexible approach was employed using a combination of indicator species analysis, cluster analysis, canonical correspondence analysis, and community tolerance indices to evaluate the variation of benthic macroinvertebrate community composition across a set of environmental gradients. Results demonstrate that benthic communities in the watershed are primarily influenced by a longitudinal gradient related to elevation. A strong secondary community gradient caused by anthropogenic nutrient loading may constrain restoration effectiveness in some parts of the watershed. Restoration activities should improve water quality conditions and initially target areas of the watershed less affected by nutrient loading. Results also demonstrate that benthic communities shift longitudinally. These shifts should be monitored to inform adaptive management of restoration actions. Copyright © 2014 John Wiley & Sons, Ltd.     

Computational Fluid Dynamics–Habitat Suitability Index (CFD–HSI) Modelling as an Exploratory Tool for Assessing Passability of Riverine Migratory Challenge Zones for Fish

We developed two‐dimensional computational fluid hydraulics–habitat suitability index (CFD–HSI) models to identify and qualitatively assess potential zones of shallow water depth and high water velocity that may present passage challenges for five major anadromous fish species in a 2.63‐km reach of the main stem Penobscot River, Maine, as a result of a dam removal downstream of the reach. Suitability parameters were based on distribution of fish lengths and body depths and transformed to cruising, maximum sustained and sprint swimming speeds. Zones of potential depth and velocity challenges were calculated based on the hydraulic models; ability of fish to pass a challenge zone was based on the percent of river channel that the contiguous zone spanned and its maximum along‐current length. Three river flows (low: 99.1 m³ sec^‐1; normal: 344.9 m³ sec^‐1; and high:792.9 m³ sec^‐1) were modelled to simulate existing hydraulic conditions and hydraulic conditions simulating removal of a dam at the downstream boundary of the reach. Potential depth challenge zones were nonexistent for all low‐flow simulations of existing conditions for deeper‐bodied fishes. Increasing flows for existing conditions and removal of the dam under all flow conditions increased the number and size of potential velocity challenge zones, with the effects of zones being more pronounced for smaller species. The two‐dimensional CFD–HSI model has utility in demonstrating gross effects of flow and hydraulic alteration, but may not be as precise a predictive tool as a three‐dimensional model. Passability of the potential challenge zones cannot be precisely quantified for two‐dimensional or three‐dimensional models due to untested assumptions and incomplete data on fish swimming performance and behaviours. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

Assessing macroinvertebrate community response to restoration of Big Spring Run: Expanded analysis of before‐after‐control‐impact sampling designs

Stream restoration projects utilize a variety of approaches to improve conditions for aquatic organisms or enhance ecosystem function. Alterations to abiotic conditions to enhance certain ecosystems services may not lead to concurrent changes in the benthic macroinvertebrate community indicative of improved stream health. Big Spring Run was the location of a novel restoration project to recreate an anabranching “wet meadow” habitat typical of precolonization conditions without the primary goal of restoring a macroinvertebrate community characteristic of single‐channel lotic systems. We examined the effect of the restoration on the macroinvertebrate community using a multivariate analysis of assemblage composition, a before‐after‐control‐impact (BACI) approach, and an assessment of potential aerial migrants. We also examined subsets of the data using a BACI approach that represented restricted sampling designs often employed in stream restoration projects. Benthic macroinvertebrates were collected in the Spring twice prior to restoration (2010 and 2011) and 3 years after restoration (2012–2014). Adult stream insects were collected in 2014. Analyses of benthic macroinvertebrates and adult insects using the full dataset suggested that restoration had no effect on the macroinvertebrate community due to poor in‐stream conditions likely from sediment deposition following restoration. Dispersal barriers are likely acting as a secondary constraint on recolonization. Analyses using subsets of the data demonstrated that reference site quality and sampling extent can alter conclusions from a BACI approach. We found that a holistic approach using multiple lines of evidence required a nuanced approach to interpreting the data but was also informative for assessing project success. Robust monitoring protocols are likely the best approach for producing convincing results through a single line of evidence. The additional BACI analyses performed for this study, however, allowed the modest sampling regime employed to generate a broad narrative demonstrating that the macroinvertebrate assemblage did not respond to this type of restoration. Thus, we believe the holistic approach we employed can strengthen assessments of stream restoration projects when resources for monitoring are limited.     

Longitudinal changes in macroinvertebrate assemblages in the Meuse River: anthropogenic effects versus natural change

A collaborative study among three nations (France, Belgium, Netherlands) along the Meuse River developed a consistent approach for collecting and interpreting macroinvertebrate data. Specific mesohabitats were sampled in 16 locations along an 800‐km stretch of this lowland regulated river. The objective was to assess the ‘river health’ using macroinvertebrate communities as indicators of biological and ecological variation in space. The main changes in assemblages were investigated using multimetric and multivariate approaches. The authors examined relationships between faunal variations and both physico‐chemical gradients and man‐made disturbances. We related species traits to faunal changes and habitat characteristics. Both a gradual shift from a macroinvertebrate assemblage dominated by insects to a community dominated by crustaceans and molluscs and a drastic decrease in biotic index values were observed along the longitudinal gradient. Taxa were distributed according to oxygen, nitrate and ammonium concentrations, pH, conductivity and summer hydraulic conditions. But major faunal differences among sites could not be explained simply by physico‐chemical variables. The trait analysis underlined the role of temporary habitats in structuring the summer macroinvertebrate community of sites of the uppermost French sector, which supported the most diverse community in terms of trait combination. Downstream the macroinvertebrate community exhibited a more simple and less stable functional organization. We concluded that the Meuse River exhibited both a high biodiversity and a ‘reasonably good’ water quality in the upper reaches. Two transition zones highlighted the influence of a high degree of human impact on stream integrity. Regulation for navigation, ship traffic and heavily polluted effluent discharges influenced instream conditions via multiple processes determining a decline of both habitat stability and diversity. However, the rare occurrence of habitat‐sensitive species in the lower reaches indicated that a partial recovery of communities may be predicted if restoration and protection of disturbed (especially riparian) habitats are fulfilled. Copyright © 2002 John Wiley & Sons, Ltd.     

Sedimentology of New Fluvial Deposits on the Elwha River,Washington, USA,Formed During Large‐Scale Dam Removal

Removal of two dams 32 m and 64 m high on the Elwha River, Washington, USA, provided the first opportunity to examine river response to a dam removal and controlled sediment influx on such a large scale. Although many recent river‐restoration efforts have included dam removal, large dam removals have been rare enough that their physical and ecological effects remain poorly understood. New sedimentary deposits that formed during this multi‐stage dam removal result from a unique, artificially created imbalance between fluvial sediment supply and transport capacity. River flows during dam removal were essentially natural and included no large floods in the first two years, while draining of the two reservoirs greatly increased the sediment supply available for fluvial transport. The resulting sedimentary deposits exhibited substantial spatial heterogeneity in thickness, stratal‐formation patterns, grain size and organic content. Initial mud deposition in the first year of dam removal filled pore spaces in the pre‐dam‐removal cobble bed, potentially causing ecological disturbance but not aggrading the bed substantially at first. During the second winter of dam removal, thicker and in some cases coarser deposits replaced the early mud deposits. By 18 months into dam removal, channel‐margin and floodplain deposits were commonly >0.5 m thick and, contrary to pre‐dam‐removal predictions that silt and clay would bypass the river system, included average mud content around 20%. Large wood and lenses of smaller organic particles were common in the new deposits, presumably contributing additional carbon and nutrients to the ecosystem downstream of the dam sites. Understanding initial sedimentary response to the Elwha River dam removals will inform subsequent analyses of longer‐term sedimentary, geomorphic and ecosystem changes in this fluvial and coastal system, and will provide important lessons for other river‐restoration efforts where large dam removal is planned or proposed. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

Importance of Reservoir Tributaries to Spawning of Migratory Fish in the Upper Paraná River

Regulation of rivers by dams transforms previously lotic reaches above the dam into lentic ones and limits or prevents longitudinal connectivity, which impairs access to suitable habitats for the reproduction of many migratory fish species. Frequently, unregulated tributaries can provide important habitat heterogeneity to a regulated river and may mitigate the influence of impoundments on the mainstem river. We evaluated the importance of tributaries to spawning of migratory fish species over three spawning seasons, by comparing several abiotic conditions and larval fish distributions in four rivers that are tributaries to an impounded reach of the Upper Paraná River, Brazil. Our study confirmed reproduction of at least 8 long‐distance migrators, likely nine, out of a total of 19 occurring in the Upper Paraná River. Total larval densities and percentage species composition differed among tributaries, but the differences were not consistent among spawning seasons and unexpectedly were not strongly related to annual differences in temperature and hydrology. We hypothesize that under present conditions, densities of larvae of migratory species may be better related to efficiency of fish passage facilities than to temperature and hydrology. Our study indicates that adult fish are finding suitable habitat for spawning in tributaries, fish eggs are developing into larvae, and larvae are finding suitable rearing space in lagoons adjacent to the tributaries. Our findings also suggest the need for establishment of protected areas in unregulated and lightly regulated tributaries to preserve essential spawning and nursery habitats. Copyright © 2014 John Wiley & Sons, Ltd.     

Population effect of a large‐scale stream restoration effort on Chinook salmon in the Pahsimeroi River,Idaho

Stream habitat restoration is an important tool for fisheries management in impaired lotic systems. Although small‐scale benefits of stream habitat restoration are commonly investigated, it is difficult to demonstrate population effects. The Pahsimeroi River Chinook salmon Oncorhynchus tshawytscha population was previously restricted to the lower portion of the river by multiple irrigation structures. To address fish passage issues, a combination of restoration projects was initiated including barrier removals, instream flow enhancements and installation of fish screens on diversions. The largest barrier was removed in 2009, more than doubling the amount of accessible linear habitat. We hypothesized restoration efforts would expand the distribution of spawning salmon in the Pahsimeroi River watershed, leading to a broader distribution of juveniles. We also hypothesized a broader juvenile distribution would have population effects by reducing the prevalence of density‐dependent growth and survival. Redds were documented in newly accessible habitat immediately following barrier removal and accounted for a median of 42% of all redds in the Pahsimeroi River watershed during 2009–2015. Snorkel surveys also documented juvenile rearing in newly accessible habitat. Juvenile productivity increased from a median of 64 smolts/female spawner for brood years 2002–2008 to 99 smolts/female spawner for brood years 2009–2014. Overall, results suggested increased habitat accessibility in the Pahsimeroi River broadened the distribution of spawning adult and rearing juvenile salmon and reduced the effects of density‐dependent survival. Large‐scale stream restoration efforts can have a population effect. Despite the large‐scale effort and response, habitat restoration alone is likely not sufficient to restore this population.     

Effects of dams in river networks on fish assemblages in non‐impoundment sections of rivers in Michigan and Wisconsin,USA

Regional assessment of cumulative impacts of dams on riverine fish assemblages provides resource managers essential information for dam operation, potential dam removal, river health assessment and overall ecosystem management. Such an assessment is challenging because characteristics of fish assemblages are not only affected by dams, but also influenced by natural variation and human‐induced modification (in addition to dams) in thermal and flow regimes, physicochemical habitats and biological assemblages. This study evaluated the impacts of dams on river fish assemblages in the non‐impoundment sections of rivers in the states of Michigan and Wisconsin using multiple fish assemblage indicators and multiple approaches to distinguish the influences of dams from those of other natural and human‐induced factors. We found that environmental factors that influence fish assemblages in addition to dams should be incorporated when evaluating regional effects of dams on fish assemblages. Without considering such co‐influential factors, the evaluation is inadequate and potentially misleading. The role of dams alone in determining fish assemblages at a regional spatial scale is relatively small (explained less than 20% of variance) compared with the other environmental factors, such as river size, flow and thermal regimes and land uses jointly. However, our results do demonstrate that downstream and upstream dams can substantially modify fish assemblages in the non‐impoundment sections of rivers. After excluding river size and land‐use influences, our results clearly demonstrate that dams have significant impacts on fish biotic‐integrity and habitat‐and‐social‐preference indicators. The influences of the upstream dams, downstream dams, distance to dams, and dam density differ among the fish indicators, which have different implications for maintaining river biotic integrity, protecting biodiversity and managing fisheries. Copyright © 2010 John Wiley & Sons, Ltd.     

ECOLOGICAL FUNCTIONS OF FISH BYPASS CHANNELS IN STREAMS: MIGRATION CORRIDOR AND HABITAT FOR RHEOPHILIC SPECIES

The introduction of weirs into stream ecosystems resulted in modifications of serial continuity and in the decline of riverine fish species. Successful river restoration requires information on the ecological functionality of fish bypass channels that are considered an ecological improvement according to the European Water Framework Directive. In this study, we compared the functionality of three nature‐oriented fish passes as compensatory habitats and migration corridors for fishes. Fish passes differed significantly from upstream and downstream reaches of the weirs, revealing higher current speed, lower water depth, smaller channel width and greater habitat variability. Following these structural differences, they provided key habitats for juvenile, small and rheophilic fishes that are typically underrepresented in highly modified water bodies. All fish passes were used as migration corridors, with increased fish movements during high discharge and at spawning periods. Because river stretches with high variability of current speed and water depth are scarce in highly modified water bodies, fish passes can play an important role as compensatory habitats and should thus be considered more intensively in habitat assessments and river restoration. Ideally, fish bypasses should mirror the natural discharge dynamics and consider all occurring fish species and sizes. Copyright © 2011 John Wiley & Sons, Ltd.     

The Effects of DAM Removal on River Colonization by Sea Lamprey Petromyzon Marinus

Habitat fragmentation is an important cause of biodiversity loss in freshwater systems, as worldwide rivers have been fragmented by dams and other hydraulic structures. To restore freshwater fish populations, some barriers have been removed, but the long‐term ecological effects of this removal have been rarely quantified. In the present study, we quantified the effects of barrier removal on river colonization by anadromous sea lamprey (Petromyzon marinus) by analyzing the spatial distribution and nest density in a small coastal river (France) from 1994 to 2011. Our results demonstrated the benefit of dam removal within few years after restoration. Indeed, the spatial distribution of nests shifted significantly upstream and was more uniform throughout the river after removal. Our results also suggest that the spatial patterns of habitat colonization were affected by the density of nests, river flow and connectivity. Finally, although the number of nests was significantly higher after removal, it was not possible to clearly identify the contribution of intrinsic versus external factors involved in this pattern. Further investigations are therefore needed to quantify the potential subsequent effects on juvenile recruitment and the overall population dynamics. Copyright © 2014 John Wiley & Sons, Ltd.