Habitat Simulation Research Articles

Downstream environmental effects of dam operations: Changes in habitat quality for native fish species

AbstractHydropeaking dam operation and water extractions for irrigation have been broadly stated as alterations to natural flow regimes, which have also been noticed in the Biobío Watershed, in Central Chile, since 1996. In the Biobío River, most of native fish species are endemic and very little is known about them. Their ecological and social values have never been estimated, and there is lack of information about their habitat preferences. Furthermore, changes on fish habitat availability due to natural and/or man‐made causes have not been evaluated. In this study, eight native fish species, in a representative reach of the Biobío River, were studied and their preferred habitats were surveyed and characterized. A hydrodynamic model was built and linked to the fish habitat simulation model CASiMiR. Fuzzy rules and fuzzy sets were developed for describing habitat preference of the native fish species. CASiMiR was then used to simulate how physical habitat conditions vary due to flow control (i.e. upstream dams). Results show how overall habitat quality, expressed as weighted usable area (WUA) and hydraulic habitat suitability (HHS), changes and fluctuates due to the dam operation and how the daily hydropeaking is influencing quantity, quality and location of different habitats. The study suggests that the analysed fish are highly susceptible to flow control, as dams are currently operated, and fish habitat improvement suggestions are proposed. Copyright © 2010 John Wiley & Sons, Ltd.

Linking a spatially explicit watershed model (SWAT) with an in‐stream fish habitat model (PHABSIM): A case study of setting minimum flows and levels in a low gradient, sub‐tropical river

AbstractAs changes in landuse and the demand for water accelerate, regulators and resource managers are increasingly asked to evaluate water allocation against the need for protection of in‐stream habitat. In the United States, only a small number of river basins have the long‐term hydrograph data needed to make these assessments. This paper presents an example of how to bridge the conceptual and physical divide between GIS‐based watershed modelling of basin‐discharge and in‐stream hydraulic habitat models. Specifically, we used a Soil and Water Assessment Tool (SWAT) model for the Hillsborough River to produce data for use in a Physical HABitat SIMulation (PHABSIM) model of the same river. This coupling of models allowed us to develop long‐term discharge data in ungauged river systems based on watershed characteristics and precipitation records. However this approach is not without important limitations. Results confirm that accuracy of the SWAT‐predicted hydrograph declines significantly when either the DEM resolution becomes too coarse or if DEM data are resampled to a coarser or finer resolution. This is due to both changes in the size and shape of the river basin with the varying DEMs and subsequent shifts in the proportions of land use, soils and elevation. Results show the use of 30 m DEMs produced hydrographic patterns amenable for using in‐stream habitat protocols like PHABSIM model, especially where little or no hydrographic and land use information exists. Copyright © 2010 John Wiley & Sons, Ltd.

Diel Turnover of Assemblages of Fish and Shrimp on Sandbanks in a Temperate Floodplain River

Seven sandbanks in the main channel of the Brazos River, Texas, were sampled a total of 35 times during both day (1800–1900 hours) and night (2200–2300 hours) in June and July 2008 to examine the diel turnover of fish assemblages. Day samples had an average of 10.1 species, whereas night samples had an average of 12.9 species. Average abundance (catch per unit effort, here defined as the number of individuals per 10-m seine haul) for day samples was 41.5, compared with 80.5 for night samples. Species richness and abundance were significantly greater on sandbanks at night. Additionally, nonmetric multidimensional scaling analysis and cluster analysis indicated consistent differences in assemblage structure between day and night samples. Most of the diel change in assemblage structure was due to ictalurids and palaemonids (freshwater decapod crustaceans) that were only common on sandbanks nocturnally. Catastomids, clupeids, cyprinids, and poeciliids were present on sandbanks both diurnally and nocturnally. Species that exclusively use sandbank habitats may be foraging during the day and seeking refuge from predators at night. Nocturnal species apparently move onto sandbank habitats at night for foraging and then migrate to deeper, more structurally complex habitats during the day to escape predation. Diel turnover of sandbank fish assemblages may be a general phenomenon in lotic ecosystems, but it is apparently not as pronounced in temperate rivers as in tropical rivers. Diel changes in habitat use deserve greater study because species assemblage structure and habitat use are the basis for ecological niche models used to predict species distributions based on climate change scenarios as well as physical habitat simulation models designed to determine instream flows.

Modelling brown trout spatial requirements through physical habitat simulations

AbstractTerritorial behaviour is often considered the main mechanism regulating salmonid populations and territory size regarded as the proximate factor that limits abundance, being mainly determined by body size. Despite the spatial requirements of young‐of‐the‐year (YOY) brown trout, Salmo trutta, have been previously established, there is still a gap in the knowledge about the spatial needs of older individuals. Therefore, we aimed to develop an allometric territory size relationship for YOY, juvenile and adult trout. Territory size was inferred from 12‐year demographic data and physical habitat simulations performed at 10 pristine sites where populations showed high and stable densities. When compared to previous models in salmonids, results revealed not only interspecific but also intraspecific significant differences in the size of territories used by individuals of comparable size. Interestingly, the explanatory power of the derived allometric territory size relationships was similar to previous models based on individual observational data. It suggested that despite the fact that results obtained from population data cannot explicitly describe territorial behaviour of individuals, they may represent accurately their spatial requirements. Finally, results suggested that territory volume, and not territory area, scaled with body size at a rate consistent with brown trout metabolic rate, the scaling rate being highly constant at any level of simulated proportion of nonterritorial fish in the population. Consequently, if individuals must increase territories to fulfil energetic demands, the allometry of territory volume seems a better predictor of spatial requirements than territory area. Copyright © 2009 John Wiley & Sons, Ltd.

Sampling for Environmental Flow Assessments

Environmental flow studies normally involve sampling. Except for hydrologically based methods, the studies usually are conducted only on part of the reach of stream to which the study results will be applied. This assumes that the study sites, or the samples, are representative of the stream as a whole. There is no way to be sure of this without studying the whole stream, but it is possible to estimate how close the study results and the “true” results are likely to be, if the study uses a probability (random) sampling design that allows for estimating confidence intervals. Sampling designs that allow for interval estimates of statistics are normal in science, but, strangely, not in environmental flow studies. Here, I describe and discuss some appropriate sampling designs and related considerations in the context of environmental flow studies. I then describe and critique the sampling in some recent studies using the Physical Habitat Simulation System (PHABSIM), the method most commonly used in the United States, which I describe briefly for readers not familiar with it. Finally, I explain why deliberate selection of samples gives biased results, and describe the experience that persuaded statisticians of this fact early in the twentieth century.

The Potential Role of Stream Habitat Restoration in Facilitating Salmonid Invasions: A Habitat‐Hydraulic Modeling Approach

Many successful invasions have taken place in systems where harmful disturbance has changed habitat conditions. However, less attention has been paid to the role of habitat restoration, which modifies habitats and thus also has the potential to facilitate invasions. We examined whether in‐stream habitat restorations have the potential to either facilitate or resist invasion by two widely introduced non‐native stream salmonids, Salvelinus fontinalis Mitchill and Oncorhynchus mykiss Walbaum, in Finland. A physical habitat simulation system was used to calculate whether the habitat area for the target species increased or decreased following the restorations. For comparison, we also reported results for four native stream fish species. The simulations showed that the restored streams provided the highest amount of usable habitat area for the native species, particularly for Salmo salar L. and Gottus gobio L. However, it was interesting to note that the restorations significantly increased habitat quality for the two non‐native species, especially at low flows. Nevertheless, the non‐native species had the lowest amount of usable habitat area overall. The modeling results indicated that not only habitat destruction but also habitat restoration could contribute to the spread of non‐native species. Fisheries and wildlife managers should be aware of the possibility, when restoring habitats in order to preserve native ecosystems, that non‐native species could manage to gain a foothold in restored habitats and use them as population sources for further spread. Knowing the widespread negative effect of non‐native species, this risk should not be underestimated.

The basic flow method for incorporating flow variability in environmental flows

AbstractThe Basic Flow is a methodology used to calculate environmental flow needs for river regulation. It has gained increased recognition in Spain for hydrological planning. It is based on the study of irregularities in hydrological series of daily mean flows using the simple moving average model as a tool to extract the relevant information. The Basic Flow Methodology (BFM), beyond providing a unique minimum flow value, constitutes a complex management proposal for regulated rivers which includes other management aspects affecting the biological functioning of a river (such as the necessity of flow variability, bankfull flows or varying flow rates) through the establishment of monthly instream flow requirements.This paper presents a practical application of the BFM in the Silvan stream, a natural mountain stream impacted by a hydroelectric regulation project. Results are discussed in terms of physical habitat created and compared to those obtained from the application of another method based on the Instream Flow Incremental Methodology, using a set of computer programs (RHYHABSIM) for physical habitat simulation. Copyright © 2010 John Wiley & Sons, Ltd.

Erratum to: Relating Narrative, Inquiry, and Inscriptions: Supporting Consequential Play

In this paper we describe our research using a multi-user virtual environment, Quest Atlantis, to embed fourth grade students in an aquatic habitat simulation. Specifically targeted towards engaging students in a rich inquiry investigation, we layered a socio-scientific narrative and an interactive rule set into a multi-user virtual environment gaming engine to establish a virtual world through which students learned about science inquiry, water quality concepts, and the challenges in balancing scientific and socio-economic factors. Overall, students were clearly engaged, participated in rich scientific discourse, submitted quality work, and learned science content. Further, through participation in this narrative, students developed a rich perceptual, conceptual, and ethical understanding of science. This study suggests that multi-user virtual worlds can be effectively leveraged to support academic content learning.

Habitat evaluation using suitability index and habitat type diversity: a case study involving a shallow forest stream in central Taiwan

In recent years, the Taiwanese government has strongly promoted the concept of ecological engineering in the hope that doing so will encourage the maintenance of the ecosystem and its integrity. As a result, the riprap spur dike is one of the most commonly used measures for protecting stream banks. Traditionally, a spur dike is used at concave banks to prevent their scouring and/or to increase their stabilization. An additional benefit of deflector structures, like spur dikes, may be to increase the weighted usable area (WUA) for aquatic life survival during periods of increased flow (examples include typhoon, flood, etc.). A two-dimensional river habitat simulation program (River2D) coupled with a developed shallow water habitat type diversity module was used for the case study at a headwater stream in central Taiwan. The habitat suitability index for this study was established using substrate, depth, and velocity from field surveys for the fish family Cyprinidae by prepositioned area electrofisher. The ungauged flood conditions were calculated using digital elevation models within a watershed delineation and hydrological modeling system in accordance with local regulations. Simulated results indicate that the spur dikes currently in use on the stream in this study need be improved from a WUA point of view more effectively handle a flood event.

Instream Flow Science For Sustainable River Management1

Abstract: Concerns for water resources have inspired research developments to determine the ecological effects of water withdrawals from rivers and flow regulation below dams, and to advance tools for determining the flows required to sustain healthy riverine ecosystems. This paper reviews the advances of this environmental flows science over the past 30 years since the introduction of the Instream Flow Incremental Methodology. Its central component, Physical HABitat SIMulation, has had a global impact, internationalizing the e‐flows agenda and promoting new science. A global imperative to set e‐flows, including an emerging trend to set standards at the regional scale, has led to developments of hydrological and hydraulic approaches but expert judgment remains a critical element of the complex decision‐making process around water allocations. It is widely accepted that river ecosystems are dependent upon the natural variability of flow (the flow regime) that is typical of each hydro‐climatic region and upon the range of habitats found within each channel type within each region. But as the sophistication of physical (hydrological and hydraulic) models has advanced emerging biological evidence to support those assumptions has been limited. Empirical studies have been important to validate instream flow recommendations but they have not generated transferable relationships because of the complex nature of biological responses to hydrological change that must be evaluated over decadal time‐scales. New models are needed to incorporate our evolving knowledge of climate cycles and morphological sequences of channel development but most importantly we need long‐term research involving both physical scientists and biologists to develop new models of population dynamics that will advance the biological basis for 21st Century e‐flow science.

Development and Evaluation of a Stream Channel Classification for Estimating Fish Responses to Changing Streamflow

AbstractCurrent approaches to assessing the potential effects of river regulation and water use on stream fish communities are based on physical habitat simulation and are not feasible for estimating these effects over large spatial scales. We developed a channel classification for streams in the lower Flint River basin, Georgia, based on gross channel morphology and geology and evaluated its usefulness at 23 study sites representing the four channel types in the basin. Our channel classification separated stream channel types based on dominant geology (upland residuum versus Ocala limestone) and channel form (confined versus unconfined). Fish were sampled and habitat measured at the study sites in spring, summer, and winter from 2001 to 2004, a period that included some of the lowest and highest seasonal flows ever recorded. The channel types differed with respect to diurnal variability in temperature and dissolved oxygen concentration as well as habitat characteristics (substrate, large wood). Statistical habitat modeling indicated that channel unit (e.g., pool, riffle) availability could be predicted accurately for each channel type using streamflow, with errors rates that averaged 0.7% across channel unit types. Comparisons between the accuracy of species presence and fish abundance models fit using habitat characteristics to those fit using geomorphic channel characteristics indicated similar model accuracy with slightly less bias (6% on average) for the geomorphic channel characteristics models. Our results indicate that the channel types were useful for predicting changes in habitats and stream fish presence and abundance in response to changing streamflow. Channel classification is potentially a useful and cost‐effective approach to quantifying changes in the physical environment in response to varying streamflow. Additionally, channel classification could be combined with dynamic fish population models to evaluate the effects of river regulation and water use across large spatial scales.

An environmentally sensitive dynamic human model for LSS robustness studies with the V-HAB simulation

The development of efficient and safe Life Support Systems is one of the key drivers of the Global Solar System Exploration efforts. For each task performed by Life Support Systems (LSS) a great multitude of sub-system concepts exist and the challenge is to find the optimal combination of sub-systems for a given mission scenario. On a sub-system level the Equivalent Systems Mass (ESM) trade study approach is well suited to effectively compare sub-system options. On a system level in addition to ESM data time dependent sub-system performances within an overall system must be addressed. Criteria such as system stability, controllability and effectiveness must be considered in order to be able to assess the dynamic robustness of systems designed to the averages. In an effort to establish a dynamic simulation environment for this type of LSS optimizations the “Virtual Habitat” tool (V-HAB) is being developed at the Technical University of Munich (TUM). This paper introduces the most important part of the Virtual Habitat simulation, which is the human model.

Demonstration Flow Assessment and 2‐D Modeling: Perspectives Based on Instream Flow Studies and Evaluation of Restoration Projects

AbstractRailsback and Kadvany (2008) make a compelling case for the use of demonstration flow assessments (DFA) for instream flow assessments as a more robust method than traditional one‐dimensional habitat simulation techniques, such as the Physical Habitat Simulation System. But, based on experience with DFAs used for instream flow assessments and evaluation of stream restoration projects, the methods presented by Railsback and Kadvany (2008) may not give reproducible results, and DFAs have significant drawbacks relative to two‐dimensional (2‐D) hydraulic and habitat modeling. Application of the DFA methodology presented in Railsback and Kadvany (2008) to assess a stream restoration project on the Trinity River, California, did not give reproducible results, with substantial disparity between replicate surveys in the total quantity and spatial distribution of habitat. As an empirical two‐dimensional habitat modeling method, DFAs have several drawbacks. Compared to 2‐D models, DFAs require interpolation and extrapolation of results from the observed flows, and lack biological realism because they use categorical (binary) habitat suitability criteria. Further, 2‐D modeling offers significant benefits over DFA in the context of adaptive management. Advantages of DFAs cited by Railsback and Kadvany (2008), namely the feasibility of assessing habitat with complex hydraulics, the ease in using mechanistic and theoretical conceptual models, and the ability to assess long reaches, also apply to 2‐D models. Modification of the DFA methods presented in Railsback and Kadvany (2008) to give reproducible results may increase the cost of DFAs or reduce the length of stream that can be assessed with DFAs to the point where 2‐D modeling may be a more cost‐effective technique than DFAs.

용담댐 하류의 하천건강성 평가 및 어류 서식처를 고려한 최적 생태유량 산정

금강수계중 용담댐하류 앞섬 지점에서 생태유량을 산정하기 위한 생태 모니터링을 실시하여 어류종별 서식환경 특성조사를 포괄적으로 실시하였다. 이를 바탕으로 어류상, 상대풍부도 및 생물다양도 분석 등을 포함한 하천의 건강성 평가를 실시하고, 쉬리, 피라미 및 감돌고기 등에 대하여 작성한 서식처적합도 지수(HSI)와 물리적 서식처 모의시스템(PHABSIM)을 이용한 최적 생태유량을 산정하였다. 본 지점에서 채집된 종수는 20종이였으며, 우점종은 쉬리(22.4%), 아우점종은 피라미(22.0%)였다. 생태적 건강성은 50점 만점의 평가점수에서 <TEX>$34{\sim}42$</TEX>점으로 양호에서 좋은 상태로 나타났다. 3종의 대표어종에 대한 수심, 유속 및 하상재료에 대한 서식지적합도 지수를 물리적서식처 모의시스템에 적용한 결과 계절별로 유량의 차이를 보였으며, 3종에 대한 최적 생태유량은 9.0 cms로 추정하였다. In this study, a comprehensive field monitoring was conducted to understand habitat conditions of fish species in the upper Geum river. Based on the monitoring data, riverine health conditions such as composition ratio of fish species, richness and dominance indexes, bio-diversity (dominance index, diversity, evenness and richness), and index of biological integrity were assessed, and optimal ecological flowrates were estimated using the habitat suitability indexes established for three fish species Coreoleuciscus splendidus, Zacco platypus and Pseudopungtungia nigra selected as icon species using the physical habitat simulation system (PHABSIM). The total number of species sampled was 20 species, and two sensitive species of C. splendidus (22.4%) and Z. platypus (22.0%) dominated the fish community. The estimated IBI values ranged from 34 to 42 with average being 38 out of 50, rendering the site ecologically fair to good health conditions. An optimal ecological flowrate of 9.0 cms was recommended for the representative fish species at the site.

Field evaluation of a two‐dimensional hydrodynamic model near boulders for habitat calculation

AbstractTwo‐dimensional hydrodynamic models are now widely used in aquatic habitat studies. To test the sensitivity of calculated habitat outcomes to limitations of such a model and of typical field data, bathymetry, depth and velocity data were collected for three discharges in the vicinity of two large boulders in the South Platte River (Colorado) and used in the River2D model. Simulated depth and velocity were compared with observed values at 204 locations and the differences in habitat numbers produced by observed and simulated conditions were calculated. The bulk of the differences between simulated and observed depth and velocity values were found to lie within the likely error of measurement. However, the effect of flow simulation outliers on potential habitat outcomes must be considered when using 2D models for habitat simulation. Furthermore, the shape of the habitat suitability relation can influence the effects of simulation errors. Habitat relations with steep slopes in the velocity ranges found in similar study areas are expected to be sensitive to the magnitude of error found here. Comparison of habitat values derived from simulated and observed depth and velocity revealed a small tendency to under‐predict habitat values. Published in 2009 by John Wiley &amp; Sons, Ltd.

Lost in space, the sequel: spatial sampling issues with 1‐D PHABSIM

AbstractThe Physical Habitat Simulation System (PHABSIM) is a popular method for evaluating the habitat value of a stream in terms of a statistic, weighted usable area (WUA). Usually, PHABSIM is used with one‐dimensional (1‐D) hydraulic models, and curves of WUA over discharge are calculated at transects and combined to produce a composite curve. Curves of WUA over discharge are often presented as evidence in proceedings to determine instream flow requirements, so the reliability of these curves is important. Representing a reach of stream with transects introduces ordinary statistical questions: is the sample of transects unbiased, and is it large enough to produce usefully precise estimates? Unfortunately, these questions are seldom considered in PHABSIM studies, even though most PHABSIM studies characterize streams with fewer than 15 transects. Moreover, transect locations usually are selected deliberately, so estimates of WUA will usually be biased. This paper extends an earlier analysis of the uncertainty in composite WUA curves with a much larger set of transect curves, with more analysis of uncertainty in the shape of the WUA curves, and with a different method for simulating errors in estimates of WUA at the transects. The results show that even with larger than usual numbers of transect curves, the precision of composite WUA curves is likely to be poor, especially if there are errors in the transect WUA curves. I also offer suggestions regarding sampling, and for estimating the number of transects that might be needed to achieve a given level of precision in WUA estimates. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Comparison of spawning habitat predictions of PHABSIM and River2D models*

This study compared the predictions of two instream flow habitat models, the Physical Habitat Simulation System (PHABSIM) and River2D, with regards to spawning habitat for chinook salmon, Oncorhynchus tschawytscha, and steelhead trout, Oncorhynchus mykiss. Spawning habitat was simulated with both models for eight sites in the Sacramento River, five sites in the American River and one site in the Merced River, California, using habitat suitability criteria developed from data collected on redds in each of these rivers. For four out of five cases, both models correctly predicted that the combined suitability, calculated as the product of the depth, velocity and substrate suitabilities, of occupied locations was significantly greater than the combined suitability of unoccupied locations. There was little difference in the flow‐habitat relationships for each site and set of habitat suitability criteria predicted by the two models. The use of River2D, rather than PHABSIM, is still warranted given its ability to model complex flow conditions which cannot be simulated with PHABSIM.

Microhabitat use by three endemic Iberian cyprinids in Mediterranean rivers (Tagus River Basin, Spain)

Abstract Microhabitat use by three endemic Iberian cyprinids, Barbus bocagei (Steindachner), Pseudochondrostoma polylepis (Steindachner), and Squalius pyrenaicus (Günther) was studied in terms of depth, mean water column velocity, focal height, focal velocity, distance to shore and substrate. Data were obtained by snorkelling during spring and summer at nine sites of the Tagus River Basin, Spain. Habitat suitability criteria (HSC) were calculated, including fish position and focal velocity in the water column. Species comparison showed differences in depth and focal height (indicating a vertical segregation), and greater water velocities for Pseudochondrostoma. Size‐class comparisons mainly showed differences in depth and focal height (correlated with fish size). The fish groups (3 species × 3 length classes) were assigned to microhabitat functional types. The results are essential for environmental flow assessments and allow 2‐ and 3‐dimensional habitat simulations in Mediterranean rivers; they are also useful to define critical habitats for the conservation of native fish populations.

Effects of variation in streamflow and channel structure on smallmouth bass habitat in an alluvial stream

AbstractWe evaluated the effects of streamflow‐related changes in channel shape and morphology on the quality, quantity, availability and spatial distribution of young‐of‐year and adult smallmouth bass Micropterus dolomieu habitat in an alluvial stream, the Baron Fork of the Illinois River, Oklahoma. We developed Habitat Suitability Criteria (HSC) for young‐of‐year and adult smallmouth bass to assess changes in available smallmouth bass habitat between years, and compare predicted smallmouth bass Weighted Usable Area (WUA) with observed WUA measured the following year. Following flood events between 1999 and 2000, including a record flood, changes in transect cross‐sectional area ranged from 62.5% to 93.5% and channel mesohabitat overlap ranged from 29.5% to 67.0% in study three study reaches. Using Physical HABitat SIMulation (PHABSIM) system analysis, we found that both young‐of‐year and adult smallmouth bass habitat were differentially affected by intra‐ and inter‐annual streamflow fluctuations. Maximum WUA for young‐of‐year and adults occurred at streamflows of 1.8 and 2.3 m3 s−1, respectively, and WUA declined sharply for both groups at lower streamflows. For most microhabitat variables, habitat availability was similar between years. Habitat suitability criteria developed in 1999 corresponded well with observed fish locations in 2000 for adult smallmouth bass but not for young‐of‐year fish. Our findings suggest that annual variation in habitat availability affects the predictive ability of habitat models for young‐of‐year smallmouth bass more than for adult smallmouth bass. Furthermore, our results showed that despite the dynamic nature of the gravel‐dominated, alluvial Baron Fork, HSC for smallmouth bass were consistent and transferable between years. Published in 2008 by John Wiley &amp; Sons, Ltd.

Regulated discharge produces substantial demographic changes on four typical fish species of a small salmonid stream

A hydroelectric power plant (HPP) started operation in December 2002 on the River Lhomme, (mean annual flow: 1.78 m3 s−1; mean annual water temperature: 9.9°C). The new HPP bypasses the river over a length of 1.2 km. The minimum flow allowed in the bypassed section is currently fixed at 0.220 m3 s−1. Before the construction of the HPP, two contrasted 150-m-long reaches of the Lhomme were selected to estimate their total fish population abundance and to analyse their fish population dynamics. Electrofishing was carried out in each of these two reaches on 23 April 2002 in a natural flow situation to remove the fish. Other inventories were carried out in late April or early May in 2003, 2004, 2005 and 2006 in minimum flow conditions. The results revealed a prompt and severe decrease in the total fish biomass (up to 81% for grayling from 2002 to 2006) combined with severe changes in the fish community structure that were not observed in a reference site. The effects of the flow reduction varied considerably depending on the size of the individuals, the species concerned and their habitat availability, which was modelled using a classical physical habitat simulation (EVHA method).