Rheoreaction impacts dispersal of fish larvae in restored rivers

Connectivity of nurseries and spawning habitats for young of the year life stage is essential for successful recruitment of fish populations and therefore provides a key indicator for river restoration measures. Models for dispersal offer the potential to draw conclusions regarding restoration scenarios and to fill knowledge gaps about possible implications for fish populations. A newly developed rheoreaction‐based correlated random walk model (RCRW), in combination with a three‐dimensional numerical model and a non‐steady‐state particle tracing model, was applied for nase carp larvae (Chondrostoma nasus) before and after a restoration project on the river Danube, Austria. Spatio‐temporal patterns of dispersal of virtual larvae, attached with rheoreactive behaviour, were analysed within both scenarios. In comparison to the heavily modified river reach, the restored reach enabled a greater amount of upstream movement from the release site and showed a generally higher variability of spatio‐temporal distribution patterns. In contrast, estimated total settlement of rheoreactive larvae was substantially higher for the situation prior to the restoration measure. By comparing model results with a previously field experiment it was found that model simulations including rheoreaction as a single behaviour for navigation could not explain the whole pattern of larval dispersal. Therefore it is highly recommended for future studies to develop larval dispersal models by considering other factors (i.e., behaviour, bio‐energetics and environmental factors) of existing and future individual‐based models, which could serve as a tool to analyse the effect of restoration measures for recruitment of riverine fish populations.     

Monitoring the channel process of a stream restoration project in an urbanizing watershed: a case study of Kelley Creek,Oregon, USA

Pacific Northwest (PNW) streams in the United States were impacted by the 20th century development, when removal of instream structure and channelization degraded an aquatic habitat. The lower Kelley Creek in southeast Portland, USA was channelized during the 1930's Works Progress Administration (WPA) projects. Stream restoration reintroduced pool‐riffle sequences and heterogeneous substrates to protect salmonids while mitigating impacts from flooding. We investigated whether the restored pool‐riffle morphology changed substantially following effective discharge events. We examined channel forms for four reaches representing three time periods—pre‐development (two reference reaches), development and restoration. We conducted thalweg profiles, cross‐sections and pebble counts along the reaches to examine how channel geometry, residual pool dimensions and particle size distribution changed following effective discharge events. The effective discharge flows altered the restoration reach more substantially than the reference reaches. The restoration reach decreased in median particle size, and its cross‐sectional geometry aggraded near its margins. However, the residual pool morphology remained in equilibrium. Richardson Creek's reference reach degraded at the substrate level, while Kelley Creek's reference reach remained in equilibrium. The restoration reach's aggradation may have resulted from sedimentation along the nearby Johnson Creek. In contrast, Richardson Creek's degradation occurred as upstream land use may have augmented flows. Stream channels with low gradient pool‐riffle morphologies are ideal for salmonid spawning and rearing and should be protected and restored within urban corridors. The findings of our study suggest that the connectivity of streams and the dynamic fluvial geomorphology of stream channels should be considered for stream restoration projects in humid temperate climates. Copyright © 2008 John Wiley & Sons, Ltd.     

Influence of habitat restoration on post‐emergence displacement of brown trout (Salmo trutta L.): A case study in a Northern Swedish stream

During the 19th and 20th centuries, Swedish streams were channelized to facilitate the commercial transport of timber on water. Stream‐dwelling fish were affected by the consequent reduction in habitat quality. To mitigate the effects on salmonids, which sustain important recreational fisheries, many streams have been restored through the replacement of boulders into the channel since the early 1980s. However, the effects of restoration on salmonids remain poorly known. This study assesses the effect of habitat restoration on the post‐emergence displacement of brown trout (Salmo trutta L.) fry. Hatchery‐raised fry were released in a third‐order stream canalized for timber floating in northern Sweden, and the effect of restoration on displacement was assessed. The short‐term (24 h) displacement of trout fry was reduced from 10.1 to 2.3% of fry released following restoration. Water velocity accounted for 89.4% of the variation in fry displacement across years and sites. Post‐summer recruitment (the proportion of introduced juvenile trout remaining in the study reaches 60 days after the emergence) increased approximately three fold after restoration. These findings suggest that habitat restoration benefit trout populations through substantial reductions of fry displacement and possible contributions to juvenile recruitment. Copyright © 2009 John Wiley & Sons, Ltd.     

Evaluation of nature‐like fishways for re‐establishing connectivity in fragmented salmonid populations in the River Emån

We evaluated the function of two nature‐like fishways for re‐establishing connectivity for anadromous salmonids in the regulated River Emån. Between 90 and 100% of the salmonids that entered the fishways actually passed through them, with median speeds of 180–190 m h^?1. Only 50% of the anadromous brown trout that passed the first fishway also passed the second one, indicating that the fish might have had problems locating the upstream fishway. The fishways were also observed to function as a passage for downstream post‐spawning migrants. The densities of brown trout yearlings upstream of the fishways were higher in 2002, after the fishways were built, than during pre‐fishway years. In control sites in other parts of the river as well as in a nearby river, no changes in yearling densities were observed. Thus, the fishways are working for upstream spawners, albeit at a recolonization rate that is slower than expected. Copyright © 2005 John Wiley & Sons, Ltd.     

Point source effects on density,biomass and diversity of benthic macroinvertebrates in a Mediterranean stream

We sampled benthic macroinvertebrates above and below a point source effluent in La Tordera stream (NE, Spain) over 2001–2002 to assess the effects of nutrient enrichment on the structure, and taxonomic composition of the benthic macroinvertebrate community. Below the point source, discharge, specific conductance and nutrient concentrations were higher than at the upstream reach, while dissolved oxygen (DO) decreased. Macroinvertebrate density was higher at the downstream reach than at the upstream reach on most dates but the two reaches did not differ in macroinvertebrate biomass. On average, taxa richness at the upstream reach was 20% higher than at the downstream reach. Several taxa, especially mayflies, stoneflies and caddisflies, were present only at the upstream reach. Shannon diversity was similar between the two reaches on 50% of the dates. Ordination analysis clearly separated the samples of the upstream reach from the samples of the downstream reach in the first axis and corroborated the effect of the point source on the benthic community. The two reaches followed a similar temporal pattern with respect to the distribution of taxa along the second axis of the ordination analysis. Higher similarities between the two reaches in taxa composition, densities and biomass after the spates of April and May 2002, suggest that flooding events may act as a reset mechanism for benthic communities and play an important role in stream restoration. Copyright © 2007 John Wiley & Sons, Ltd.     

Economic analysis of water temperature reduction practices in a large river floodplain: an exploratory study of the Willamette River,Oregon

This paper examines ecosystem restoration practices that focus on water temperature reductions in the upper mainstem Willamette River, Oregon, for the benefit of endangered salmonids and other native cold‐water species. The analysis integrates hydrologic, natural science and economic models to determine the cost‐effectiveness of alternative water temperature reduction strategies. A temperature model is used to simulate the effects of combinations of upstream riparian shading and flow augmentations on downstream water temperatures. Costs associated with these strategies are estimated and consist of the opportunity costs of lost agricultural production and recreation opportunities due to flow releases from an up‐stream reservoir. Temperature reductions from another strategy, hyporheic flow enhancement, are also examined. Restoration strategies associated with enhanced hyporheic cooling consist of removal/reconnection of current obstacles to the creation of dynamic river channel complexity. The observed reduction of summer water temperatures associated with enhanced channel complexity indicates that restoring hyporheic flow processes is more likely to achieve cost‐effective temperature reductions and meet the total maximum daily load (TMDL) target than conventional approaches that rely on increased riparian shading or/and combinations of flow augmentation. Although the costs associated with the hyporheic flow enhancement approach are substantial, the effects of such a long‐term ecological improvement of the floodplain are expected to assist the recovery of salmonid populations and provide ancillary benefits to society. Copyright © 2008 John Wiley & Sons, Ltd.     

GRAVEL AUGMENTATION INCREASES SPAWNING UTILIZATION BY ANADROMOUS SALMONIDS: A CASE STUDY FROM CALIFORNIA,USA

Anadromous salmonid diversity and abundance worldwide have been adversely impacted by anthropogenic forces, and millions of dollars are spent each year on stream habitat restoration and enhancement. However, there is a paucity of data comparing site use by salmonids before and after enhancement implementation, and few studies examine the specific environmental conditions that determine whether salmonids utilize an enhanced site. This study examines the use of gravel augmentation to improve spawning site utilization by Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (Oncorhynchus mykiss) on the Lower American River, California, USA. Spawning increased across all augmentation sites for both species, although there were species‐specific and year‐specific differences in the degree to which a site was utilized and in the spatial distribution of redds in relation to substrate size, habitat features and other redds. There were also differences in redd architecture across sites that were related to differences in gravel size. This study illustrates that gravel augmentation projects can enhance spawning habitat for salmonids where spawning beds have degraded but that species‐specific and site‐specific attributes and gravel size can influence the relative effectiveness of a project. Copyright © 2013 John Wiley & Sons, Ltd.     

Effects of process-based floodplain restoration on aquatic macroinvertebrate production and community structure

Recent stream restoration approaches, such as Stage 0 restoration, aim to restore natural processes to regain lost ecosystem functions, but project implementation can also represent a reach-scale disturbance. Assumed outcomes of these restoration actions, like greater biological productivity, are rarely evaluated. In this study, we examined the short-term effects of Stage 0 floodplain restoration on the secondary production of aquatic macroinvertebrate communities in the South Fork McKenzie River, Oregon, 1–2 years following project implementation. We seasonally sampled macroinvertebrates from benthic and submerged wood surfaces in the restored reach, and two unrestored reference reaches located upstream, to estimate annual secondary production. Macroinvertebrate production estimates were 3× lower on a per-meter-squared basis in the restored reach than in the upstream unrestored reference reaches (9764 vs. 29,636 mg DM/m²/yr). However, because there was 4.5-times greater wetted area available in the restored reach, overall macroinvertebrate production per unit of valley length was 3.4× higher in the restored reach than in unrestored reference reaches (2744 vs. 802 kg DM/km). Additionally, the mosaic of aquatic habitats created via restoration (main-channel, side-channel, and wetted forest habitats) supported a diversity of macroinvertebrate assemblages both within and among reaches. Our findings suggest Stage 0 project implementation, which can include dewatering and filling incised channels, may reduce aquatic macroinvertebrate production on a per-unit-area basis for at least 1 or 2 years following restoration. However, this short-term disturbance effect may be offset by channel aggradation and widening, which can provide a more wetted area for macroinvertebrate production and may support greater macroinvertebrate community diversity. Future studies are needed to examine the longer term (2–10 years) aquatic macroinvertebrate response to Stage 0 restoration, and the impacts of shifting resource availability on stream fishes.     

Emergence timing and subsequent downstream movements of two non-native salmonids in a Lake Superior tributary

We describe patterns of emergence and downstream movement by recently emerged fry of two non-native salmonids in the Great Lakes region, North America. Our primary objectives were to describe the timing of emergence in relation to spring flooding, and to examine the effects of reach-level complexity of stream habitat on rates of movement. Emergence and movement patterns of coho salmon and brown trout fry were assessed over an eight-week period in two reaches distinguished by differences in channel woody debris. Fry emergence occurred from mid-March to early May, and peaked in early to mid-April. Movement during this period was uncorrelated with upstream densities of resident fry and fish moving downstream did not appear moribund or in poor condition. Nearly twice as many fish moved through the simple reach that lacked woody debris cover even though upstream densities of resident fry were generally greater in the complex reach. The results reported here indicate that peak emergence occurs in close association with the timing of spring floods. Variability in the timing of either emergence or spring floods could have profound effects on the size of coho salmon and brown trout populations within streams of this region. Results from this study further suggest that greater habitat complexity may reduce downstream movements of newly emerged salmonid fry in a natural system.     

大潮差河口闸下水流的流动特性分析

福建数座河口水闸发生剧烈淘刷,闸上游和下游的最大冲刷深度达到闸室高度的0.5~1.2倍,且屡修屡毁。有的水闸经多次除险加固后,不得不再花巨资重建。通过数学模型和物理模型揭示了河口水闸下游水位因受东海大潮差海潮影响发生剧烈波动时的变化规律,发现了水闸泄流过程中的间歇性淹没及非常工况甚至出现逆流的独特流动现象,给出了典型日孔流及堰流泄流能力动态变化过程线,分析了消能工变化对闸址水流结构的显著影响,并对水闸水毁修复前后闸下冲刷进行了预测。结果表明:水闸修复前闸下涨急最高水位4.53 m,修复后下降0.67 m,水闸修复后逆流现象减弱。修复后首次水跃位置往上游移动了35 m,平均水跃高度下降47.46%,闸后及二级池末端最大流速2.27 m/s和0.08 m/s,较修复前分别下降了40.58%和96.48%。常遇泄流条件下,水闸的淹没度和泄流能力呈现不断变化的动态过程。当西溪水闸62#先启孔全开,上游保持正常高水位3.2 m,下游为最低潮位-0.72 m时,修复后最大冲刷深度减少64.48%,消能防冲效果十分明显。研究成果对河口水闸安全运行具有一定的参考价值。     

Hydraulic and Physical Structure of Runs and Glides Following Stream Restoration

Hydraulic units are often linked to ecological habitat through geomorphic structure, and a better understanding of the turbulent characteristics of the units is needed. Our work examined the near‐bed turbulent structure of runs and glides in a restored river and investigated the physical characteristics that influenced the near‐bed hydraulics in these units. The research was completed in three restored reaches and one reference reach at the Virginia Tech Stream Research, Education, and Management Laboratory. The laboratory is unique because three different restoration treatments were applied contiguously along a stream, and the restoration practices ranged from passive to active. The passive reach included cattle exclusion, while the active reaches included cattle exclusion as well as vegetation plantings, bank sloping and the construction of inset floodplains. Three‐dimensional velocities were measured near the channel bed in run and glide biotopes within the three restored reaches, as well as an upstream reference reach. The velocities were utilized to analyse and compare near‐bed turbulent structure across the reaches. While the restoration activities did not address the channel bed directly, differences in physical structure of the two physical biotopes were observed among restoration treatments, likely because of changes in bank shape and roughness due to vegetation differences. Differences between reference and restored reaches were still evident approximately 3 years after cattle exclusion and construction activities. Few differences were observed in the hydraulic structure between runs and glides, and the near‐bed flow structure in both runs and glides was related to local roughness. Copyright © 2016 John Wiley & Sons, Ltd.     

Restoration of Hydrochory Following Dam Removal on the Elwha River,Washington

Hydrochory, seed dispersal by water, affects riparian vegetation by contributing to downstream community composition and diversity. However, dams can block hydrochory, reducing downstream species diversity and fragmenting riparian corridors. Dam removal is becoming more prevalent for economic and ecological reasons and is expected to restore hydrochory; however, this has never been documented in rivers. The largest dam removal project to date was the 2011 to 2014 removal of the Glines Canyon and Elwha dams on the Elwha River in Washington. Prior to dam removal, hydrochory was lower below Glines Canyon Dam compared with an upstream reach; our objective was to test the hypothesis that dam removal would restore downstream hydrochory to levels observed in the upstream reach. To test this, we collected seeds in nets above and below the dam during three sample periods (early July, late July and early August), growing out seeds in a greenhouse and comparing seed abundance and species richness above and below dams, before and after dam removal. We found that after dam removal, the average number of hydrochorous seeds and species increased below Glines Canyon Dam to levels similar to or higher than that of the upstream reach; hydrochory levels in the upstream reach did not change. This study is the first to document the restoration of hydrochory in rivers following removal of a large dam. Restoration of hydrochory may ultimately increase downstream vegetation diversity and play a role in the recolonization of reservoir sediments deposited in the riparian zone in the years following dam removal. Copyright © 2016 John Wiley & Sons, Ltd.     

山区宽窄相间河道不同流量下水流结构研究

为了研究山区河流宽窄相间河段不同流量下水流结构特性,以雅安宝兴河上游河段为研究对象,概化实际河道,并建立三维水流计算模型。结果表明①小流量下,窄段上游无明显壅水现象,宽窄相间河道显现出与顺直河道相似的特性,随着流量的增加,窄段上游壅水剧烈,宽段到窄段沿程断面平均流速呈现“增加—相对稳定—骤减—相对稳定—增加”的分布规律;②受河宽、壅水和回流影响,不同流量工况下宽段横断面的水流特性分布较窄段更加复杂,且在宽段的壅水区与非壅水区流速分布规律差异亦较大;③不同流量状况下,宽段床面剪切力剧烈变化将造成卵石沿程运动的不连续性,宽段河床呈现周期性往复发展的复归性变形,从而维持了宽段河床结构的稳定,而在窄段,河道在较大的床面剪切力长期作用下,形成了与周围环境相适应的河床结构,以消耗窄段较大的水流能量,从而保护河床的稳定。     

Sediment storage and shallow groundwater response to beaver dam analogues in the Colorado Front Range,USA

Enthusiasm for using beaver dam analogues (BDAs) to restore incised channels and riparian corridors has been increasing. BDAs are expected to create a similar channel response to natural beaver dams by causing channel bed aggradation and overbank flow, which subsequently raise water tables and support vegetation growth. However, lack of funding for monitoring projects post‐restoration has limited research on whether BDAs actually cause expected channel change in the Front Range and elsewhere. Geomorphic and hydrologic response to BDAs was monitored in two watersheds 1 year post‐restoration. BDAs were studied at Fish Creek, a steep mountainous catchment, and Campbell Creek, a lower gradient piedmont catchment from May to October 2018. At each restoration site, the upstream‐ and downstream‐most BDAs were chosen for intensive study in comparison with unrestored reference reaches. Monitoring focused on quantifying sediment volumes in BDA ponds and recording changes to stream stage and riparian groundwater. Despite differences in physical basin characteristics, BDA pools at both sites stored similar volumes of sediment and stored more sediment than reference pools. Sediment storage is positively correlated to BDA height and pool surface area. However, BDAs did not have a significant influence on shallow groundwater. The lack of groundwater response proximal to BDAs could indicate that local watershed factors have a stronger influence on groundwater response than restoration design 1 year post‐restoration. Systematic, long‐term studies of channel and floodplain response to BDAs are needed to better understand how BDAs will influence geomorphology and hydrology.     

Downstream benthic responses to small dam removal in a coldwater stream

Increased awareness of the negative effects of dams combined with an aging dam infrastructure has led to an increase in dam removals. However, ecological responses of downstream biota to such actions are poorly understood. We examined the influence of a pair of small dam removals on downstream periphyton and macroinvertebrates in Boulder Creek, WI (USA). The dams were 180 m apart and both were removed on 9 July 2003. We monitored algae and macroinvertebrates as well as habitat characteristics for approximately 2 months before and after the removals upstream and downstream from the two dams, and continued our observations over a similar period (mid‐May to mid‐July) the following summer. After the removals, an initial release of sediment significantly increased the proportion of fine sediments in the downstream reach and buried benthic substrate. This burial led to a 60% reduction in chlorophyll in the downstream reach the week following removal, while concentrations increased in the upstream reference reach. Similarly, macroinvertebrate densities 2 weeks post‐removal were lower relative to pre‐removal densities and were associated with declines of ephemeropterans, trichopterans and dipterans. Examination of Trichoptera genera demonstrated substantial changes in this assemblage associated with diminished densities of the formerly dominant genus Brachycentrus. Algal and invertebrate populations increased in the weeks after the dam removal, but did not reach densities similar to the upstream reference reach. In the following year, both periphyton and invertebrate densities were lower in the downstream reach, suggesting a long‐term effect of the removal. Thus, effects of the dam removal were alternatively positive or negative over time, and indicate that the time scale of consideration has a strong effect on the interpretation of the consequences of this management activity. Copyright © 2008 John Wiley & Sons, Ltd.     

感潮河段洪潮耦合双驱动力水动力模型

分析了感潮河段水流运动特点和感潮河段上下边界水力作用要素差异,在完整感潮河段概念和完整感潮河段上下边界的特殊水流边界条件下,把水流水力要素分离为洪水要素和潮水要素,提出洪水水流要素和潮水水流要素耦合模拟的双驱动力水动力模型,利用长江河口感潮河段实际水流过程进行了模拟检验,获得了比圣维南方程组更合理、模拟效果更好的结果,并指出了进一步需要重点研究的问题。     

Effects of water transfer on aquatic insects in a stream in Hong Kong

Hong Kong streams serve as indirect catchments for storage reservoirs to which they are connected by a network of undergound tunnels. Water transfer to reservoirs reduces stream flow downstream of the extraction point, and surface flows may disappear in affected reaches during the dry season. The ecological effects of periodic dewatering on aquatic insects were investigated in Tai Po Kau Forest Stream, Hong Kong, where water extraction at a weir gives rise to intermittent flow immediately downstream of a well studied perennial stream section. Benthic samples were taken from the intermittent reach on 48 occasions over a two year period encompassing two dry seasons. Trends in total population densities were unclear, with considerable fluctuations over the study period. In contrast, species richness declined markedly after surface flow disappeared, reaching the lowest values at the end of both dry seasons. A rapid increase accompanied flow resumption on the onset of the summer monsoon, and maximum wet season values were up to five times greater than species richness during the dry season. Dry season declines in Ephemeroptera and Trichoptera were accompanied by a dramatic increase in Coleoptera (Elmidae and especially Helodidae). The situation was reversed during the wet season when Trichoptera (especially Cheumatopsyche and Chimarra) and Ephemeroptera (Leptophlebiidae, Baetidae and Heptageniidae) were numerous. Community structure in the intermittent reach recovered quickly from the dewatering disturbance, due largely to recolonization by drifting animals. At the scale investigated, the community was resilient to disturbance because a source of mobile colonists was available upstream. However, if dewatering affects reaches far downstream, the presence of suitable colonists in upstream reaches cannot be guaranteed, and recovery from disturbance will be delayed. Lowland streams in Hong Kong are often polluted and host to exotic species; in such circumstances the effects of water transfers are exacerbated and recolonization is hindered.     

MODELLING THE EFFECTS OF RIVER FLOW ON POPULATION DYNAMICS OF PIPING PLOVERS (CHARADRIUS MELODUS) AND LEAST TERNS (STERNULA ANTILLARUM) NESTING ON THE MISSOURI RIVER

Reservoir management on the Missouri River has changed the flow regime that once created dynamic emergent sandbar habitat (ESH) for the interior least tern (Sternula antillarum) and piping plover (Charadrius melodus). High flows that create large amounts of ESH are now rare, but the remaining interannual variability in river stage has strong effects on the amount of ESH available for nesting shorebirds. The scarcity of habitat has led the United States Army Corps of Engineers to develop an adaptive management plan for the restoration of ESH to support nesting terns and plovers. We describe the stochastic simulation models of ESH, plover populations and tern populations used in the adaptive management process, and examine the effects of river flow on projected outcomes of habitat restoration. The population models are most sensitive to uncertainty in adult survival rates. Model validation against historical amounts of ESH and population sizes suggests the model is a reasonable predictor of future dynamics. Flow variability contributes as much uncertainty as parameter estimation error to plover model projections but negligible uncertainty to the tern model. Autocorrelation in flow between years has stronger effects on population outcomes than the intensity of habitat restoration effort does. We compared population outcomes after a habitat‐creating flow with population outcomes following habitat restoration and found that large pulses of habitat creation produced similar or better outcomes in the short term than low but consistent habitat restoration. However, bird populations fared better in the long term with low levels of restoration when habitat‐forming flows were rare. Copyright © 2013 John Wiley & Sons, Ltd.     

Effects of dam removal on brook trout in a Wisconsin stream

Dams create barriers to fish migration and dispersal in drainage basins, and the removal of dams is often viewed as a means of increasing habitat availability and restoring migratory routes of several fish species. However, these barriers can also isolate and protect native taxa from aggressive downstream invaders. We examined fish community composition two years prior to and two years after the removal of a pair of low‐head dams from Boulder Creek, Wisconsin, U.S.A. in 2003 to determine if removal of these potential barriers affected the resident population of native brook trout (Salvelinus fontinalis). Despite the presence of other taxa in the downstream reaches, and in other similar streams adjacent to the Boulder Creek (including the brown trout, Salmo trutta), no new species had colonized the Boulder Creek in the two years following dam removal. The adults catch per unit effort (CPUE) was lower and the young‐of‐the‐year catch per unit effort (YOY CPUE) was higher in 2005 than in 2001 in all reaches, but the magnitude of these changes was substantially larger in the two dam‐affected sample reaches relative to an upstream reference reach, indicating a localized effect of the removal. Total length of the adults and the YOY and the adult body condition did not vary between years or among reaches. Thus, despite changes in numbers of adults and the YOYs in some sections of the stream, the lack of new fish species invading Boulder Creek and the limited extent of population change in brook trout indicate that dam removal had a minor effect on these native salmonids in the first two years of the post‐removal. Copyright © 2007 John Wiley & Sons, Ltd.     

Body mass and growth of overwintering brown trout in relation to stream habitat complexity

In winter, juvenile salmonids hide within the substrate during the day and emerge to feed on drifting invertebrates at night. In channelized streams, where the streambed heterogeneity has been artificially reduced, suitable microhabitats (low‐flow refugia) may be in short supply. Therefore, restoration of stream habitat by enhancement structures might improve the overwintering conditions of juvenile salmonids. We used a set of artificial streams to test whether individually‐marked juvenile brown trout of two age‐classes (age‐0 and age‐1 trout) loose mass during the winter differently in channelized and semi‐natural streams. Fish of both age‐classes lost mass early in the winter (November to January), but age‐0 fish in the channelized streams lost more of their initial mass than did the restored‐stream fish (ca. 10% vs. 2.5% on average, respectively). They then exhibited zero‐growth in both treatments in late winter (January to April), and by early spring (May), the channelized‐stream fish had completely caught up for their greater initial mass loss. In control tanks where the fish were fed continuously, age‐0 trout exhibited zero‐growth from November to January, then gaining mass constantly through the rest of the experiment. Significant time*treatment interaction was also detected for age‐1 trout, but all differences were caused by the faster growth of fish in the control tanks, whereas the two channel treatments did not differ significantly. The shortage of suitable sheltering sites in the channelized streams apparently intensified competition and caused greater initial mass loss in age‐0 trout. Furthermore, growth compensation exhibited by juvenile trout may have negative impacts on the long‐term fitness of individuals. Therefore, by increasing the amount of sheltering sites, in‐stream restoration may have potential to enhance the overwintering success of juvenile salmonids. Copyright © 2010 John Wiley & Sons, Ltd.