Weighted Usable Area Research Articles

Spatial distribution and geomorphic condition of fish habitat in streams: an analysis using hydraulic modelling and geostatistics

AbstractReach‐scale physical habitat assessment scores are increasingly used to make decisions about management. We characterized the spatial distribution of hydraulic habitat characteristics at the reach and sub‐reach scales for four fish species using detailed two‐dimensional hydraulic models and spatial analysis techniques (semi‐variogram analyses). We next explored whether these hydraulic characteristics were correlated with commonly used reach‐scale geomorphic assessment (RGA) scores, rapid habitat assessment (RHA) scores, or indices of fish biodiversity and abundance. River2D was used to calculate weighted usable areas (WUAs) at median flows, Q50, for six Vermont streams using modelled velocity, depth estimates, channel bed data and habitat suitability curves for blacknose dace (Rhinichthys atratulus), brown trout (Salmo trutta), common shiner (Notropis cornutus) and white sucker (Catostomus commersoni) at both the adult and spawn stages. All stream reaches exhibited different spatial distributions of WUA ranging from uniform distribution of patches of high WUA to irregular distribution of more isolated patches. Streams with discontinuous, distinct patches of high score WUA had lower fish biotic integrity measured with the State of Vermont's Mixed Water Index of Biotic Integrity (MWIBI) than streams with a more uniform distribution of high WUA. In fact, the distribution of usable habitats may be a determining factor for fish communities. A relationship between predicted WUAs averaged at the reach scale and RGA or RHA scores was not found. Future research is needed to identify the appropriate spatial scales to capture the connections between usable patches of stream channel habitat. Copyright © 2008 John Wiley & Sons, Ltd.

Instream flow determination using a multiple input fuzzy‐based rule system: a case study

AbstractThe attempts made to manage water to meet human requirements should also consider the needs of freshwater species and ecosystems. There are many tools available to assess instream flow needs, one of which is the use of habitat preference models. In this study, a fuzzy approach was used for modelling habitat preferences for two life stages of Atlantic salmon (Salmo salar). Experienced fish biologists and technicians contributed to the development of fuzzy sets and fuzzy preference rules for spawning and parr habitat. Fuzzy sets were defined for water depth, velocity and substrate composition. Fuzzy preference rules for the two life stages were then defined as sets of IF–THEN rules relating the physical attributes to habitat suitability. The fuzzy suitability indices are then used to obtain weighted usable area (WUA) at different discharges and to estimate the ecologic flow required to preserve habitat. Different methods are applied to combine the membership function and rules defined by the experts. A sensitivity analysis of rules of the combined system indicated that a limited number of rules are determinant and results are highly dependent on the consequences of these rules. A modification in the consequence of these rules can significantly alter WUA estimations. It is therefore recommended to combine the knowledge of many experts in the elicitation process and to quantify the uncertainty associated with the combination of expert knowledge. Copyright © 2007 John Wiley & Sons, Ltd.

Concept and application of the usable volume for modelling the physical habitat of riverine organisms

AbstractMost physical habitat models such as PHABSIM (Physical HABitat SIMulation) calculate the Usable Area (UA) of the river surface or the riverbed as an indicator for the available habitat for a species of interest. Although these models have been widely applied, the three dimensionality of habitat hydraulics is increasingly being recognized as an essential issue for understanding the ecological needs of aquatic organisms. This paper describes the modular toolbox HaMoSOFT (Habitat Modelling SOFTware) which quantifies the available habitat as the Usable Volume (UV) for each class of depth, flow velocity, substrate and cover and for all possible class combinations. The toolbox was calibrated in an artificial river under laboratory conditions. Optimal grids for field sampling were defined by comparing model results based on reduced sampling densities. The UV of a natural river stretch, the Schwechat River in Austria, was analysed at different discharges to assess the impact of flow changes on model outputs. UA and UV percentages were modelled for five cross‐sectional datasets, one collected in the artificial river, one in the Zwalm River (Belgium) and three in the Schwechat River at different discharges. UA percentages appeared to be higher than those of UV for lower flow velocity classes in all natural rivers and for lower depth classes in all rivers. Analysis of the Usable Volumes for the three Schwechat River scenarios revealed the observed trends were independent of flow changes. The results indicate HaMoSOFT is an interesting tool to overcome some shortcomings of Weighted Usable Area (WUA) modelling. Since the UV method is a trade‐off between the practical needs for model application in river management and the complexity of the three‐dimensional hydraulic models, it may be a practical approach for realistic physical habitat quantification. Copyright © 2007 John Wiley & Sons, Ltd.

Evaluation of instream habitat enhancement options using fish habitat simulations: case-studies in the river Pas (Spain)

In the northern coast of Spain there are rivers with Atlantic salmon populations. In the upper reaches of one of these streams, river Pas, the effectiveness of habitat enhancement measures was evaluated, under different instream flow conditions. By means of the Instream Flow Incremental Methodology and using a two dimensional hydraulic model (River2D, Steffler P (2000) Software River2D. Two Dimensional Depth Averaged Finite Element Hydrodynamic Model. University of Alberta, Canada), the potential value of stream habitat for different salmon development stages requirements was measured by Weighted Useable Area (WUA). This habitat evaluation was carried out for the unmodified stream reach, which represent the control or natural conditions. Habitat improvement measures (alternate deflectors and low dams) were simulated in the original riverbed topography. Over this modified base, habitat was estimated running River2D again. By comparing the salmon habitat evaluations in the control conditions with those obtained under those improvement conditions we have been able to assess the effectiveness of each one, and the instream flow environment at which maximum improvement is reached. The maximum habitat improvement was obtained around 10 m3/s for the adult salmon, and for the fry and parr it was around 6 m3/s. However, the habitat simulation results show that with both improvement measures, under a natural flow regime the mean annual habitat increases around 1% of the WUA in relation to the control conditions, which is not a significant improvement. A similar small WUA increase was obtained when changing the bed topography, considering geomorphological adjustments due to the new erosion and sedimentation areas caused by the presence of these structures. Therefore, these types of habitat improvement measures are not recommended in these stream reaches.

영천댐 건설이 금호강의 어류 서식환경에 미치는 영향에 관한 평가

금번 연구에서는 금호강의 금호 수위관측소 지점에서 영천댐 건설에 따른 유황분석을 하였다. 분석 결과, 영천댐 건설 전에는 갈수랑 <TEX>$10.49\;m^3/s$</TEX>, 저수량 <TEX>$13.30\;m^3/s$</TEX>, 평수랑 <TEX>$15.65\;m^3/s$</TEX>, 풍수량 <TEX>$25.00\;m^3/s$</TEX> 이었으나, 건설 후에는 갈수량이 <TEX>$2.07\;m^3/s$</TEX>, 저수랑 <TEX>$2.89\;m^3/s$</TEX>, 평수량 <TEX>$4.0\;m^3/s$</TEX>, 풍수량 <TEX>$9.36\;m^3/s$</TEX> 으로 유황이 상당히 열악해 진 것으로 분석되었다. 유지유량 증분법의 물리적 서식처모의 모형을 적용하여 피라미의 성장단계별 가중된 가용면적-유량 관계곡선을 작성하였다. 이 관계곡선으로 유지 가능일 수를 초과확률로 가중된 가용면적의 서식처 현황곡선을 작성하고, 영천댐건설에 따른 하천의 유황변화가 어류 서식환경에 미치는 영향을 평가하였다. 평가결과, 산란기와 성어기 모두 영천댐건설에 따라 가중된 가용면적이 감소하여 어류 서식환경이 저하되었다. 그러나 산란기중 유량의 초과확률 <TEX>$90\;\%$</TEX> 이상에서는 영천댐에서 하류지역의 관개용수를 공급함으로써 유황이 개선되고 가중된 가용면적도 증가하여 어류서식환경이 개선된 것으로 평가되었다. In this study, flow duration analysis was conducted at the Gumbo stage gauging station due to construction of the Youngchun dam. The flow duration characteristics were <TEX>$10.49\;m^3/s$</TEX> of drought flow, <TEX>$13.30\;m^3/s$</TEX>. of low flow, <TEX>$15.65\;m^3/s$</TEX> of normal flow, and <TEX>$25.00\;m^3/s$</TEX> abundant flow before construction of Youngchun dam. But after construction of Youngchun dam, the flow duration characteristics were <TEX>$2.07\;m^^3/s,\;2.89\;m^3/s,\;4.0\;m^3/s,\;9.36\;m^3/s$</TEX> and they had been deteriorated. Applying the Physical Habitat Simulation Model by Instream Flow Incremental Methodology, the Weighted Usable Area(WUA)-Discharge Curve was developed for Zacco Platypus according to the growth stages. Using the WUA-Discharge Curve, the WUA Duration Curve was developed with exceedance probability of daily flow and evaluated fish habitat conditions due to the construction of Youngchun dam. As an evaluation result, the WUA was reduced and fish habitat environment was deteriorated due to the construction of Youngchun dam during the spawning and growth period of Zacco Platypus. However the exceedence probability of the <TEX>$90\;\%$</TEX>, irrigation water supply from the Youngchun dam improved flow duration characteristics and Weighted Usable Area as well as fish habitat.

Ontogenetic Microhabitat Shifts in the Bullhead,Cottus gobio L.,in a Fast Flowing Stream

We investigated differences in microhabitat preference curves for bullheads, Cottus gobio L., of different size-classes during low flow periods, and evaluated the influence of such differences on habitat use through Weighted Usable Area (WUA) predictions in relation to river flow in a piedmont stream in Southwest France. Water depth, current velocity, and substratum composition were used to calculate proportional use values for each size-class (SC), and to quantify size-specific microhabitat preferences. Bullhead used non-cohesive and coarse mineral particles (pebbles, cobbles, boulders), but there was a spatial segregation of individuals from different size classes (SC1–SC4). Smaller bullhead (SC1, total length <60 mm) took refuge under cobbles, significantly preferred shallower areas, and were less prone to select high current velocities than larger bullhead (SC 2 to 4, >60 mm), the latter occurring below (or under) the largest particles, where current velocity is weakened and sand accumulates. SC1 bullhead had a more restricted range for each habitat descriptors, and were thus likely to require a more specific habitat type than other bullhead. The maximum WUA values and the related preferred discharges (0.15–0.75 m3 s–1) depended on the considered size-class. Our results suggest that ontogenetic niche shifts may play a role in the structure and dynamics of populations, by adjusting species' requirements to the spatial and temporal dynamics of environmental conditions, including abiotic and biotic conditions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

PHABSIM analysis of a straight trapezoidal reach and a highly sinuous reach in a low-order agricultural stream in the midwest

The PHABSIM model was applied to adjacent straight trapezoidal and highly sinuous reaches in a third order agricultural stream in East-Central Illinois. A 30-year flow record was used to generate weighted usable area (WUA) estimates for six warmwater fish species for each reach. Supplementing PHABSIM analysis, detailed surveys provided planametric maps of each reach, and regular fish sampling both supported the selection of species modeled with PHABSIM and provided data for PHABSIM evaluation. Analysis of the 30-year WUA estimates supported the development of metrics for fish community analysis, and the analysis of selected fish species populations in each reach. Six monthly fish collections in each reach provided a basis for and a detailed description of the fish communities in each reach. Metrics developed for four of the six species modeled were positively related to the relative differences in numbers of fish present between the two reaches.

Assessing discharge use by spawning Atlantic salmon: A comparison of discharge electivity indices and PHABSIM simulations

AbstractThe locations used by spawning Atlantic salmon (Salmo salar L.) in a reach of the Girnock Burn, Scotland, were monitored over three successive years. Reach discharge was estimated for each spawning observation using a conversion factor applied to continuous flow data from a gauge located in the catchment. Data on the availability and use of different discharges were used to construct a discharge electivity index for the reach. The index was compared to output from a Physical HABitat SIMulation (PHABSIM) model of the reach.Spawning fish used relatively high discharges, with the highest electivity value being for a discharge approximately three times the reach median flow. The electivity index and the PHABSIM weighted usable area (WUA) versus discharge curve were similar across the low flow range and both suggested similar optimum discharges for spawning (1.1 and 1.4 m3 s−1 respectively). However, electivity values suggested unsuitable conditions were reached at discharges greater than 1.4 m3 s−1 whereas PHABSIM predicted relatively high WUA values at discharges up to 2 m3 s−1.Electivity indices provide an insight into discharge selection that is not dependent on hydraulic simulations or assumptions about microhabitat (depth, velocity, substrate) preferences. Moreover, they can be used to assess discharge suitability in hydraulically complex streams where the one‐dimensional hydraulic models used by PHABSIM may be inappropriate. However, unlike PHABSIM, they cannot be used to predict the suitability of flows outside observed discharge ranges and so are limited in their application. Further work is required to refine the methodology and assess its transferability to other streams. Nonetheless, indices may represent a useful tool that can be used to complement other methods of assessing instream flow needs. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Investigating a major assumption of predictive instream habitat models: is water velocity preference of juvenile Atlantic salmon independent of discharge?

The mean column velocity preference of juvenile Atlantic salmon Salmo salar (LF 30–55 mm) was investigated by observing their spatial pattern of habitat use in a laboratory flume while varying discharge (Q) over a 18-fold range (Q=2·6–46·8l s-1). Based on 341 fish observations at three discharges (Q=2·6, 15·0 and 46·8l s-1), three separate velocity preference curves were developed using standard procedures. The mean column velocities measured at 0·6 depth for the fish positions at the set low, medium and high discharges had medians of 7, 10 and 24 cm s-1, respectively, and varied significantly between the discharges. Across the range of flows, the fish utilized mean column velocities between 0 and 56 cm s-1, but the three velocity preference curves differed. Differences between juvenile Atlantic salmon use of habitat, defined according to mean column velocities at different discharges, were greatest at the lower end of the available range of velocities (<20 cm s-1). Weighted usable area (WUA), the output of the instream flow model PHABSIM that is used to describe the available habitat at a given discharge, was calculated for the flume using the preference curves built at the three set discharges. The model was highly sensitive to differences between the three preference curves and WUA varied by up to a two-fold difference. Furthermore, habitat-discharge relationships derived from the three preference curves were very different. Predicted habitat losses across the modelled range of discharges varied by up to 150% depending upon which velocity preference curve was used in the model. Thus, the assumption that a single preference curve can be applied across a range of discharges is not valid and is likely to result in large errors when employing PHABSIM and other models that use similar principles.

Investigating a major assumption of predictive instream habitat models: is water velocity preference of juvenile Atlantic salmon independent of discharge?

The mean column velocity preference of juvenile Atlantic salmon Salmo salar (LF 30–55 mm) was investigated by observing their spatial pattern of habitat use in a laboratory flume while varying discharge (Q) over a 18‐fold range (Q=2·6–46·8l s‐1). Based on 341 fish observations at three discharges (Q=2·6, 15·0 and 46·8l s‐1), three separate velocity preference curves were developed using standard procedures. The mean column velocities measured at 0·6 depth for the fish positions at the set low, medium and high discharges had medians of 7, 10 and 24 cm s‐1, respectively, and varied significantly between the discharges. Across the range of flows, the fish utilized mean column velocities between 0 and 56 cm s‐1, but the three velocity preference curves differed. Differences between juvenile Atlantic salmon use of habitat, defined according to mean column velocities at different discharges, were greatest at the lower end of the available range of velocities (&lt;20 cm s‐1). Weighted usable area (WUA), the output of the instream flow model PHABSIM that is used to describe the available habitat at a given discharge, was calculated for the flume using the preference curves built at the three set discharges. The model was highly sensitive to differences between the three preference curves and WUA varied by up to a two‐fold difference. Furthermore, habitat‐discharge relationships derived from the three preference curves were very different. Predicted habitat losses across the modelled range of discharges varied by up to 150% depending upon which velocity preference curve was used in the model. Thus, the assumption that a single preference curve can be applied across a range of discharges is not valid and is likely to result in large errors when employing PHABSIM and other models that use similar principles.

Habitat suitability curves for brown trout (Salmo trutta fario L.) in the River Adda, Northern Italy: comparing univariate and multivariate approaches

Habitat suitability of brown trout (Salmo trutta fario) was studied in the upper portion of the Adda River, Northern Italy. Measurements were made for 528 individuals distributed in two life-stage classes, adult and juvenile, based on body length. In order to provide basic biological information for the physical habitat simulation (PHABSIM) system of the instream flow incremental methodology (IFIM) in the Italian regulated rivers, habitat suitability curves (HSCs) have been developed with respect to several microhabitat riverine parameters. Initially, current velocity, water depth, substrate class size and cover were analysed with an univariate approach, then bivariate habitat suitability models were developed from depth and velocity data. The comparison of experimental univariate HSCs with those from the literature outlined some differences that can essentially be explained by characteristics of the investigated river, confirming the necessity of using site-specific curves in relation to each experimental study area. To compare the univariate and bivariate approaches, the weighted usable area (WUA)–discharge relationships were calculated using both types of HSCs. Response curves obtained from the two approaches turned out to be quite different. In PHABSIM habitat modelling, HSCs univariate functions need to be aggregated to produce the WUA–discharge relationship. A multiplicative criterion is generally used for the combined suitability factor; by means of this aggregation criterion all variables have equal weight. According to bivariate models, depth is much more important than velocity in defining habitat suitability requirements. Copyright © 2001 John Wiley & Sons, Ltd.

Measuring and Modeling the Hydraulic Environment for Assessing Instream Flows

Detailed measurements of water depth and velocity in natural channels, though rare, show that the velocity fields are complex and irregular even in streams with moderate gradients and gravel substrates. This complexity poses a challenge for instream flow studies, most of which use the physical habitat simulation (PHABSIM) model, a set of computer models that combine the results of hydraulic modeling with estimates of channel substrate or cover and habitat suitability criteria to compute weighted usable area (WUA), an index of habitat. Some recent studies have replaced the transect-based one-dimensional hydraulic modeling in PHABSIM with two-dimensional models that allow better definition of the depth and velocity fields in the modeled stream reach. The accuracy of the estimates as a function of channel geometry and data collection effort remains unclear, however, as does the utility of the estimates for evaluating instream flow needs. Here we review the assumptions, accuracy, and precision of hydraulic modeling and the measurements that provide input data for the models; we also consider some implications of the limitations of hydraulic modeling for describing fish habitat and assessing instream flows. Highly accurate hydraulic modeling seems infeasible for streams with complex channel geometry, and in any event practical hydraulic modeling cannot resolve flow patterns at the short length scales at which fish often respond to the hydraulic environment. Information on depth, velocity, and substrate is important for assessing instream flows, but information from hydraulic models should be treated with great caution and is not a substitute for biological understanding.

Salmonid habitat modelling studies and their contribution to the development of an ecologically acceptable release policy for Kielder Reservoir, North-east England

1. Kielder Reservoir regulates the Rivers North Tyne and Tyne. It provides a regular supply of water for downstream users, supports abstractions for a major water transfer scheme and provides hydroelectric power (HEP). Kielder's release regime typically alternates between a 1.3 m3 s−1 compensation flow and 10–15 m3 s−1 HEP releases of between 3 and 7 days in duration. Occasionally releases of up to 30 m3 s−1 are made for the purpose of encouraging fish runs, for recreational events or to help in water quality management. The impacts of this release regime on Atlantic salmon (Salmo salar) and brown trout (S. trutta) habitat at four sites on the North Tyne are assessed and alternative regimes, designed to minimize impacts, are presented. 2. There is no evidence that the compensation flow results in extreme loss of instream habitat. A discharge of 1.3 m3 s−1 ensures that water is maintained over most of the channel area at sites representative of upper, middle and lower sections of the North Tyne. This discharge lies above breaks in slope of respective site discharge versus wetted area curves; thus, disproportionate increases in discharge would be needed to increase wetted area. Simulations using the Physical Habitat Simulation System (PHABSIM) suggest that the compensation flow provides between 50% and 90% of the maximum possible weighted usable area (WUA) for juvenile (0+) salmonids. 3. During HEP releases, juvenile salmonid habitat (WUA) apparently falls to between 20 and 40% of site maxima. Newly emerged juvenile fish (March and April) are most affected by HEP releases because they are relatively small (25 mm in length) and water temperatures are relatively low at this time of year. During March and April, critical near-bed displacement velocities for newly emerged fish may be exceeded across large parts (80%) of sites up to 8 km downstream from Kielder Reservoir; fish would either be displaced downstream or forced to relocate to flow refuge areas. 4. The availability of Atlantic salmon spawning habitat (WUA) at a key site is limited by the compensation flow; 1.3 m3 s−1 provides approximately one third of the habitat available at the optimum discharge (4 m3 s−1). At this site, a discharge of approximately 2 m3 s−1 is needed to ensure most of the bed is inundated by water. Regulation has reduced the duration of flows exceeding 2 m3 s−1 from 90 to 60% of the spawning season. 5. Simulations suggest that when discharge drops from 30 m3 s−1 to the compensation flow, up to 60% of the optimum spawning habitat available at the former discharge may be left stranded (dry). This could potentially lead to egg or alevin mortality. 6. PHABSIM simulations suggest that increasing the compensation flow to 4 m3 s−1 during the spawning period (November and December) is likely to increase the availability of suitable spawning habitat. Also, increasing the compensation flow to 2 m3 s−1 during the incubation period (January through March) would minimize redd stranding. Reductions in the number of HEP releases in March and April would limit the extent to which newly emerged fish are exposed to velocities that potentially displace them. Such changes to the Kielder release regime may have implications for water resource management. While it is important that the biological instream flow requirements of the North Tyne are incorporated into the Kielder operating policy, these should be integrated along with the need for channel maintenance flows, downstream water supply abstractions and HEP generation, as well as for transfers of water to other catchments. Copyright © 2000 John Wiley & Sons, Ltd.

Relationship between chinook salmon (Oncorhynchus tshawytscha) redd densities and PHABSIM-predicted habitat in the Merced and Lower American rivers, California

An index of chinook salmon (Oncorhynchus tshawytscha) spawning habitat predicted using the physical habitat simulation system (PHABSIM) component of the instream flow incremental methodology was compared with redd densities and locations for sites in the Merced River, California, during 1996 and with redd numbers in sites in the Merced and Lower American rivers, California, from 1989 through 1996. Predicted weighted useable area (WUA) was significantly correlated with chinook salmon spawning density and location at five of seven sites in the Merced River. At the microhabitat level, in the Merced River during 1996, there was a significant relationship between chinook salmon redd location and predicted WUA. Cells with more WUA in the Merced River tended to have more redds. At the mesohabitat level, there was a significant relationship between redd density and predicted WUA in both rivers. Transect areas in the Merced River with higher predicted WUA had more redds. Sites with higher numbers of redds had more predicted WUA. Significant correlations between predicted WUA and spawning locations increase confidence in the use of PHABSIM modeling results for fisheries management in the Merced and Lower American rivers as well as in other rivers.

Field sampling design and spatial scale in habitat–hydraulic modelling: comparison of three models

Habitat–hydraulic models simulating habitat productive capacity for fish have met with limited success. Limitations of hydraulic modelling, particularly related to spatial scale relevant for hydraulic field data collection and model simulations, have attracted little attention. The hypothesis that hydraulic field sampling procedures and modelling scale per se affect results was tested using three habitat–hydraulic models that employed the same fish habitat data and similar hydraulic models.The PHABSIM, EVHA and HABITAT habitat–hydraulic models were compared on a 5.56 km long, 12–35 m wide, river segment in Newfoundland, Canada. Approaches to hydraulic data collection allowed higher spatial resolution in the EVHA and HABITAT models on selected subsegments representing habitat types (with from 12 to 14 transects per subsegment), while the PHABSIM models covered the entire segment but at lower resolution (a total of 14 transects representing four habitat types placed along the entire segment).Habitat–hydraulic modelling results were similar between the PHABSIM and EVHA with respect to flow vs. suitable habitat curves, but there were important differences between HABITAT and PHABSIM/EVHA. These differences were attributed mainly to different biological models because the hydraulic models performed similarly. Weighted usable area (WUA) curves gave less information than separate suitability curves for habitat variables. It is important that habitat–hydraulic data be collected, and model simulations conducted, at scales that are relevant to habitat selection by species and age classes of interest.

Presentation and first applications of a dynamic population model for brown trout, Salmo trutta L.: aid to river management

A mathematical model representing the long‐term change in a trout population under different river management scenarios is presented. It describes the structure of a population broken down into age classes based on the Leslie matrix; if the population structure for any given month is known, the model should be able to estimate that of the following month. The passage from one month to the next takes into account various relevant factors: survival rate of individuals in the different age classes; fertility rate of females; linear and weighted growth rates; displacement linked to habitat fluctuations using weighted usable area (WUA) values. The model was applied to two French rivers. Regular monitoring of trout populations on the River Kernec enabled comparison of the response of the model with no displacement, with actual variations in fish stocks on the first river. In addition, the knowledge of WUA chronologies on the River Echez made it possible to carry out initial simulations of the response of a fish population to different river management scenarios at the second site.

Seasonal variation in the habitat available for 0+Rutilus rutilus (L.) in a regulated river

1. Habitat availability for 0+ Rutilus rutilus (L.) was determined at four sites in the regulated River Great Ouse, Eastern England, using a modified version of the Instream Flow Incremental Methodology. Discharge–habitat relationships were then calculated using a modified version of the Physical Habitat Simulation (PHABSIM) computer package. A weighted usable area (WUA) time series was generated using the discharge–habitat models. 2. Habitat availability varied between sites and with discharge, although at three of the four sites some suitable habitat remained throughout the year. In general, more habitat was available for juvenile fishes than for young larval fishes and during the winter. 3. As discharge increased habitat availability decreased, primarily as a result of increased velocity and depth. This was exacerbated by the trapezoidal shape of the main channel. In a marina, which was thought to be acting as a flood refuge, WUA decreased as a result of increasing water depth. Only in a modified backwater did WUA increase with discharge. Owing to the morphology of the backwater, there was still a WUA minimum at intermediate discharges. 4. The output of the model indicates the variation in habitat available for small fish in both temporal and spatial dimensions, and reinforces the requirement for spatial heterogeneity in riverine habitats for the preservation of fish stocks. © 1997 John Wiley & Sons, Ltd.

A Spatial and Temporal Evaluation of PHABSIM in Relation to Measured Density of Juvenile Atlantic Salmon in a Small Stream

We evaluated the relationship between weighted usable area (WUA) predicted by the physical habitat simulation (PHABSIM) model and the population density of juvenile Atlantic salmon Salmo salar in Catamaran Brook, New Brunswick, Canada. Various temporal and spatial scales of study were used to establish whether a positive linear relationship existed. The PHABSIM model was applied to 19 sites representing four habitat types, and various streamflow scenarios were used to calculate the amount of available WUA. Maximum WUA values for different habitat types and different reaches usually occurred at flows representing 85% of mean annual flow. Fish densities at the 19 sites were estimated by electrofishing in the summer and late autumn for 3 years. Few positive, significant relations were established between Atlantic salmon density and WUA; r2 values ranged from 0.18 to 0.95, with the best relations occurring at the scale of habitat type (5 of 16 comparisons were significant, P < 0.05). The WUA values calculated from the 15-d average flow before fish sampling displayed the best associations with fish density.

Effects of reduced flow on brown trout (Salmo trutta L.) populations downstream dams in french pyrenees

AbstractThe effects of 16 hydroelectric power plants, in operation for 75 years, were studied in 15 non‐polluted Salmonid streams in the Pyrenean mountains. The populations of brown trout (Salmo trutta L.) and the physical habitat characteristics were compared between two sections, one upstream of the dams (control section) and one below the dams with constant reduced flow (residual section). The average velocity, depth and area of cover decreased significantly below the dams. Similarly, the total abundance of brown trout calculated per linear metre of stream decreased at nine sites for biomass and eight sites for densities. The reductions of biomass and densities per unit area were less at seven sites. The abundances of the main stages of brown trout were modified differently; adults were affected more than fry by constant reduced flow. The modifications of biomass and densities of the age classes were significantly related to the instream flow below the dam, expressed as a proportion of the mean annual flow at the control section. Similarly, the differences of total, adult, juvenile and fry abundances between the control and residual section were significantly related to the differences in weighted usable area (WUA), average depth, average velocity and area of cover. A multiple linear regression model using differences of WUA and area‐of cover explained 84% of the difference of biomass per linear metre of stream and 68% of the difference of density. Estimating the relative capacity of a stream to support fish after a reduction in flow by measuring the variations of WUA is a promising approach for predicting the development of brown trout populations. The results are discussed in terms of modifications and the relative capacity of a stream to support a brown trout population.

A new tool to interpret magnitude and duration of fish habitat variations

AbstractThe main result of habitat simulation procedures is a static relationship between an index of potential habitat, e.g. weighted usable area (WUA), versus discharge in a study reach representative of a stream. A new methodology was developed to analyse the timing and magnitude of physical habitat variations. Three options are presented: (i) the habitat time series; (ii) the habitat duration curves; and (iii) the continuous under threshold habitat duration curves. The last option is a new procedure to interpret habitat chronicles. It determines continuous durations during which the total WUA in a study reach was lower than a given threshold. The assumption according to which some durations/threshold values could represent limiting events for fish population dynamics is illustrated with surveys of two wild brown trout populations. The relationship between spawning habitat conditions and the relative density of 0 + the year after was studied. A continuous duration of more than 20 days with spawning habitat conditions lower than 80% of the optimum conditions seemed to limit the number of 0 + trout. This procedure is presented as a tool to interpret natural discharge time series for management.