Development and evaluation of bioenergetic-based habitat suitability criteria for trout

We constructed energetic models of habitat use for 82–322 g rainbow trout (Oncorhynchus mykiss) in a large regulated river, and 8–28 g Colorado River cutthroat trout (O. clarki pleuriticus) in a small headwater stream, to determine if observed summer habitat use by these species could be attributed to net energy acquisition, and to develop habitat suitability criteria based on net energy gain. Metabolic models of energy expenditure were derived from literature sources, but measurements of energy availability were site-specific. From the energy models, we assigned a suitability value of 1.0 to the entire range of velocities where positive net energy gains were predicted, and a suitability value of zero to velocities where negative net energy gains were predicted. Predicted net energy gain velocities were compared with observed velocities used by each species. For rainbow trout, the energetic model predicted energetically profitable velocities ranging from 5 to 45 cm s⁻¹. Predicted velocities were similar to velocities used by rainbow trout. This indicated that rainbow trout, as a group, were using energetically profitable stream locations, but some rainbow trout used non-profitable velocities. For Colorado River cutthroat trout, the energetic model predicted energetically profitable velocities ranging from 5 to 45 cm s⁻¹; however, Colorado River cutthroat trout used significantly lower velocities than predicted. The dissimilarity between velocities predicted and used by Colorado River cutthroat trout may be attributed to their inability to utilize energetically profitable velocities available in the stream because of depth restrictions The results suggest that the predictive abilities of energetic models vary between streams because of differences in depth and velocity availability. © 1997 John Wiley & Sons, Ltd.     

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

We 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 & Sons, Ltd.     

Interactive effects of cover and hydraulics on brown trout habitat selection patterns

Habitat modelling results are extremely sensitive to the habitat suitability criteria (HSC) used in the simulations. HSCs are usually expressed as univariate habitat suitability curves, although such univariate approach has been long questioned, since overlooking interactions between hydraulic variables may misrepresent the complexity of fish behaviour in habitat selection. It could lead to adopt erroneous flow management decisions based on misleading results. Furthermore, the interactive effects of hydraulic variables on habitat selection may be driven by the structural features of the channel, which determine cover availability. Therefore, we compared brown trout habitat selection patterns through multivariate resource selection functions (RSFs) in structurally contrasting rivers to unveil the interactive effects of hydraulics and cover elements and their consequences in univariate HSC results. Microhabitat preferences of young‐of‐the‐year (0+) trout were similar across fast and slow waters, meanwhile juvenile (1+) and adult (>1+) preferences significantly changed. RSFs for young‐of‐the‐year trout were consistent with univariate results and did not differ among water types. However, RSFs for older trout varied among water types and revealed complex interactions among hydraulic variables and between hydraulics and structural elements, which were not described accurately by univariate curves. Therefore, results suggest that interactions between water depth and current velocity have a significant effect on habitat selection patterns in juvenile and adult brown trout, this effect being controlled by cover availability. Copyright © 2008 John Wiley & Sons, Ltd.     

Application of GIS modelling to quantify fish habitats in lakes

Quantitative measures of fish habitat in lakes and impoundments have seldom been proposed. The availability of Geographic Information Systems (GIS), however, now provides a means for researchers to assess fish habitat on a whole-lake scale. GIS modelling was used in this study to quantify the impacts of stratification on the amount and distribution of brown trout ( Salmo trutta ) habitat in a large reservoir in Victoria, Australia. Species-specific temperature and dissolved oxygen tolerance ranges for brown trout were applied to the hydrological profile and lake bathymetry. The model predicted that brown trout habitat was reduced by 82% in the summer, and the model was validated with position data from acoustically tagged adult brown trout. This study demonstrated that a quantifiable assessment of fish habitat on a whole-lake spatial scale can be made. It also could be applied to a range of fish species, with applications in ecosystem monitoring and fisheries management.     

HYDRAULIC GEOMETRY AND LONGITUDINAL PATTERNS OF HABITAT QUANTITY AND QUALITY FOR RAINBOW TROUT (Oncorhynchus mykiss)

We developed predictions of habitat quantity and quality for three life stages of rainbow trout, Oncorhynchus mykiss, across a range of stream sizes characterized by mean annual discharge of 1 to 50 m³ s^?1. The physical habitat template was created by nesting a reach‐scale two‐dimensional hydrodynamic model (River2D) within a downstream hydraulic geometry system using published coefficients for low‐gradient and high‐gradient watersheds. This provided both longitudinal and transverse estimates of depth and velocity profiles that, when combined with habitat suitability curves for the life stages, resulted in predictions of habitat quantity (weighted usable area) and habitat quality (the proportion of the stream profile that provided useable habitat) for rainbow trout along the stream continuum. Habitat quantity increased asymptotically for all life history stages but increased more rapidly in the low‐gradient watershed. Habitat quality decreased non‐linearly for young‐of‐the‐year and peaked at intermediate stream sizes for juveniles in both low‐gradient and high‐gradient watersheds. Adult habitat quality peaked in the high‐gradient watershed but increased asymptotically in the low‐gradient watershed, presumably due to lower mean velocities at larger stream sizes. Incorporation of transverse variation in depth and velocity in our physical habitat template provides a more realistic representation of habitat quantity and quality than do earlier assessments based on simple modal estimates of depth and velocity. Copyright © 2013 John Wiley & Sons, Ltd.     

PREDICTING HABITAT RESPONSE TO FLOW USING GENERALIZED HABITAT MODELS FOR TROUT IN ROCKY MOUNTAIN STREAMS

Dams and water diversions can dramatically alter the hydraulic habitats of stream ecosystems. Predicting how water depth and velocity respond to flow alteration is possible using hydraulic models, such as Physical Habitat Simulation (PHABSIM); however, such models are expensive to implement and typically describe only a short length of stream (10² m). If science is to keep pace with development, then more rapid and cost‐effective models are needed. We developed a generalized habitat model (GHM) for brown and rainbow trout that makes similar predictions to PHABSIM models but offers a demonstrated reduction in survey effort for Colorado Rocky Mountain streams. This model combines the best features of GHMs developed elsewhere, including the options of desktop (no‐survey) or rapid‐survey models. Habitat–flow curves produced by PHABSIM were simplified to just two site‐specific components: (i) Q95h (flow at 95% of maximum habitat) and (ii) Shape. The Shape component describes the habitat–flow curves made dimensionless by dividing flow increments by Q95h and dividing habitat (weighted usable area) increments by maximum habitat. Both components were predicted from desktop variables, including mean annual flow, using linear regression. The rapid‐survey GHM produced better predictions of observed habitat than the desktop GHM (rapid‐survey model explained 82–89% variance for independent validation sites; desktop 68–85%). The predictive success of these GHMs was similar to other published models, but survey effort to achieve that success was substantially reduced. Habitat predicted by the desktop GHM (using geographic information system data) was significantly correlated with the abundance of large brown trout (p < 0.01) but not smaller trout. Copyright © 2013 John Wiley & Sons, Ltd.     

The effect of discharge and predation on habitat use by wild and hatchery brown trout (Salmo Trutta)

The effects of trout stock, discharge and predation risk on habitat use by brown trout, Salmo trutta, were studied in four artificial streams. Trout stock had no effect on habitat use as both wild and hatchery fish used similar habitats. The presence of pike (Esox lucius) caused trout to decrease their use of pools, the habitat in which pike occurred, and increase their use of other habitats. Decreasing discharge reduced available area of the stream and resulted in fewer fish in the shallow margins. Both decreased flow and increased predation risk caused more overlap in habitat use, and thus increased the potential for intraspecific competition, predation and the use of poorer habitats. The results illustrate the danger of applying habitat use relationships obtained from one stream to all other streams where habitat availability and biotic interactions may differ.     

Using habitat guilds to develop habitat suitability criteria for a warmwater stream fish assemblage

The diversity of fish species found in warmwater stream systems provides a perplexing challenge when selecting species for assessment of instream flow needs from physical habitat analyses. In this paper we examined the feasibility of developing habitat suitability criteria (HSC) for the entire fish community of a warmwater stream using habitat guilds. Each species was placed a priori into a guild structure and habitat data were collected for depth, velocity, Froude number, distance to cover, embeddedness and dominant and subdominant substrate. Correct guild classification was tested with linear discriminant analysis for each species. Correct classification based on habitat‐use data was highest for riffle and pool‐cover guilds, whereas the fast‐generalist and pool‐run classes, the broader niche guilds, were more frequently misclassified. Variables most important for discriminating guilds were Froude number, velocity and depth in that order. Nonparametric tolerance limits were used to develop guild suitability criteria for continuous variables and the Strauss linear index was used for categorical variables. We recommend the use of a wide array of variables to establish more accurate habitat analysis. Additionally, guild HSC can be developed with similar effort to that needed to develop HSC for a small number of individual species. Results indicate that a habitat guild structure can be successfully transferred to another river basin and that habitats for a diverse fish assemblage can be adequately described by a small number of habitat guilds. This approach represents an alternative for incorporating entire fish assemblages into habitat analyses of warmwater stream systems. Copyright © 2010 John Wiley & Sons, Ltd.     

ASSESSING THE IMPACTS OF RIVER REGULATION ON NATIVE BULL TROUT (SALVELINUS CONFLUENTUS) AND WESTSLOPE CUTTHROAT TROUT (ONCORHYNCHUS CLARKII LEWISI) HABITATS IN THE UPPER FLATHEAD RIVER,MONTANA, USA

Hungry Horse Dam on the South Fork Flathead River, Montana, USA, has modified the natural flow regimen for power generation, flood risk management and flow augmentation for anadromous fish recovery in the Columbia River. Concern over the detrimental effects of dam operations on native resident fishes prompted research to quantify the impacts of alternative flow management strategies on threatened bull trout (Salvelinus confluentus) and westslope cutthroat trout (Oncorhynchus clarkii lewisi) habitats. Seasonal and life‐stage specific habitat suitability criteria were combined with a two‐dimensional hydrodynamic habitat model to assess discharge effects on usable habitats. Telemetry data used to construct seasonal habitat suitability curves revealed that subadult (fish that emigrated from natal streams to the river system) bull trout move to shallow, low‐velocity shoreline areas at night, which are most sensitive to flow fluctuations. Habitat time series analyses comparing the natural flow regimen (predam, 1929–1952) with five postdam flow management strategies (1953–2008) show that the natural flow conditions optimize the critical bull trout habitats and that the current strategy best resembles the natural flow conditions of all postdam periods. Late summer flow augmentation for anadromous fish recovery, however, produces higher discharges than predam conditions, which reduces the availability of usable habitat during this critical growing season. Our results suggest that past flow management policies that created sporadic streamflow fluctuations were likely detrimental to resident salmonids and that natural flow management strategies will likely improve the chances of protecting key ecosystem processes and help to maintain and restore threatened bull trout and westslope cutthroat trout populations in the upper Columbia River Basin. Copyright © 2011 John Wiley & Sons, Ltd.     

Linking ecological science with management outcomes on New Zealand's longest river

New Zealand's Waikato River has had a short but intense history of development, primarily through land‐use change and flow regulation in the upper river, and in the lower river through flood control works, non‐native species invasion, and land‐use intensification. The river undergoes sharp transitions across montane‐flood plain‐coastal environments over a short distance and under similar climate. Together with specialized life‐history requirements of many native fish, these features provide valuable insights into large river ecology and management. Testing approaches to determine outcomes of water quality changes have highlighted the value of functional indicators over traditional biotic measures for monitoring anthropogenic impacts. Initiatives to enhance native fish populations in the lower river have included remediation of migration barriers to improve access to tributary habitat, enhancement of tidal spawning habitat, and traps and gates to limit movement of large pest fish into flood plain lakes for spawning. This example of a southern temperate large river system highlights the importance of recruitment habitat and connectivity for native fish communities dominated by migratory species. Their slender bodies provide opportunities to create semipermeable barriers that enable access to flood plain habitats while restricting larger invasive fish. Recent initiatives have increased momentum to restore the ecological health of this river, but the underpinning science to guide priority actions is often lacking, and there is limited monitoring over the scales and time frames required to evaluate effectiveness.     

History and review of the habitat suitability criteria curve in applied aquatic ecology

Hydraulic microhabitat assessment is a category of environmental flow tools (e.g., Physical Habitat Simulation system and other methodologically similar software) that, at its core, uses habitat suitability criteria (HSC) to link values of point hydraulic variables (usually depth, velocity, and substrate/cover) to habitat values for target life stages. Although this assessment tool has been used worldwide for decades, the history of the HSC curve is relatively unknown because the foundational information is predominantly contained in obscure and often unpublished reports. We review the history of the HSC concept in applied aquatic ecology to clarify its scientific pedigree, ensure its proper use, and build a foundation for future research. We begin the review with the formative decades of the 1950's through the 1970's, when consumptive‐based western USA water law conflicted with conservation traditions and natural resource management objectives, although water allocation issues date back at least to the 19th century. By analysing the history of the HSC concept, we aim to establish the biological, hydrologic, and geomorphological conditions that must be met for the HSC concept to be successfully employed. In spite of its documented assumptions and limitations, the HSC concept will likely continue to be a useful tool to help address water resources allocation issues in defined hydrologic and geomorphic settings. We conclude that HSC‐based methodologies should be considered as one of several environmental flow approaches involved in sustainable water resources management.     

Long‐term simulations of the dynamics of trout populations on river reaches bypassed by hydroelectric installations—analysis of the impact of different hydrological scenarios

Changes in a brown trout (Salmo trutta L.) population result from interaction among various mechanisms which are dependent on environmental conditions and biotic processes. In reaches influenced by the presence of dams, the instream flow in the bypassed section is not the only parameter which affects the population. Flood episodes, the general connectivity of the bypassed section, and the characteristics of the substrate which define the availability and quality of spawning grounds may also have a crucial impact. The design and fine‐tuning of tools which take environmental parameters into account can improve our understanding of the dynamics of such influenced populations. In this perspective, a deterministic model (MODYPOP) has been developed in an attempt to integrate all these factors and to test the effect of different long‐term scenarios of influenced flow regimes on the structure of trout populations. MODYPOP was applied to three populations and three reaches (on the Roizonne, Neste d'Aure and Lignon du Forez rivers in France). For each stream, experiments were carried out on a bypassed section downstream of a hydropower station, before and after an increase in the minimum instream flow due to relicensing. These experiments allowed integrating into MODYPOP local phenomena (impact of flood episodes, impact of flushing, impact of downstream migration of juveniles and adults) affecting the populations during the study period and then calibrating them. To estimate the change in the population due to the increase in minimum instream flow, different long‐term simulations were run, selecting discharge patterns at random. These scenarios help to evaluate the time required for the population to return to a range close to habitat saturation after an improvement in the hydraulic habitat or following a flood event. These applications have enabled determining the relative importance of changes in population density due to different types of events. Copyright © 2008 John Wiley & Sons, Ltd.     

A self‐correction technique for real‐time simulation of regulated rivers

An automatic self‐correction scheme is presented for online adjustment of stream‐flow forecasts during simulation of river systems with regulating structures, when actual flow data for interior points are available in real time. Another scheme is presented which allows for the modelling of channel losses due to physical effects such as evapotranspiration, bank storage and diversions. A previously developed river simulation/reservoir management model has been modified to include both schemes. The model is particularly useful for real‐time, short‐term flood forecasting and reservoir operation. The model application is made to the Lower Colorado River in Texas.     

A decision support system to diagnose factors limiting stream trout fisheries

Maintaining or restoring productive freshwater fisheries is a key challenge for resource managers. However, the inherent uncertainty and complexity of managing fisheries, often based on scant environmental data, make it difficult for managers and the public to reach consensus on appropriate actions. To help deal with this issue, we created a literature‐based decision support system to diagnose limiting factors for stream brown trout fisheries. Once limiting factors are determined, appropriate management actions can be tailored to address them. Our Bayesian belief network (BBN)‐based framework serves 2 functions: (a) It directs users to assemble a parsimonious environmental data set to inform stream fishery management, and (b) it integrates and interrogates these data to generate standardized and testable hypotheses about which environment factors are likely to limit trout productivity. The BBN has been trained on brown trout because among freshwater fish, this species has the richest literature base and is highly valued worldwide. However, the framework could be adapted for other stream fish. We applied our BBN to the Horokiri Stream, a data‐rich catchment in Wellington, New Zealand. The BBN probability outputs were comparable with the conclusions of 5 experienced fishery biologists following their detailed investigation into the factors that led to the loss of the Horokiri brown trout fishery between 1951 and 1990.     

Effects of Environmental Water Transfers on Stream Temperatures

Low streamflows and warm stream temperatures currently limit habitat and productivity of trout, including native Lahontan cutthroat trout in Nevada's Walker Basin. Environmental water transfers, which market water from willing sellers to instream uses, are evaluated to improve instream habitat. We use River Modelling System, an hourly, one‐dimensional hydrodynamic and water quality model, to estimate current and potential environmental water transfer effects on stream temperatures. Model runs simulate a range of environmental water transfers, from 0.14 to 1.41 cms, at diversions and reservoirs for wet year 2011 and dry year 2012. Results indicate that critically warm stream temperatures generally coincide with low flows, and thermal refugia exist in East Walker River, a tributary of the Walker River. Environmental water transfers reduce maximum stream temperatures by up to 3 °C in dry years and are more effective in dry years than wet years. This research suggests that environmental water transfers can enhance instream habitat by improving water quality as well as increasing instream flow. Copyright © 2015 John Wiley & Sons, Ltd.     

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

Reach‐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.     

Contrasting temperatures,waterflows, and light: seasonal habitat selection by young Atlantic salmon and brown trout in a boreonemoral river

Habitat use and habitat selection by young Atlantic salmon and brown trout were investigated by direct underwater observation. We sampled during winter and summer water temperatures (low: 3–7°C; high: 9–12°C) coinciding with low and high waterflows (12–20 and 60–80 m³ s^?1), and during day and night in winter, and on six selected stations in the river. Observations of 396 salmon and 120 trout indicated a distinct seasonal pattern in behaviours and habitat selection. Feeding was the dominant behaviour at high water temperatures during summer. In winter, there was a diurnal pattern in behaviour; both species sheltered in interstitial spaces in the substrate during daylight, but during night held positions on or close to the substrate in slower flowing stream areas. Coarse substrate providing cover was therefore an important habitat factor during daylight at low water temperatures, while slow‐flowing water was important during night. Although spatial niche overlap was considerable both in summer and winter, salmon and trout segregated with respect to meso‐ and microhabitat selection, and relatively more at low temperatures. Both species changed their use of mesohabitats towards more slow‐flowing glide/flat habitats in winter. Irrespective of season, trout preferred in general more slow‐flowing water than salmon did, but the difference was more pronounced in winter. Salmon used a wider range of water depths and in particular water velocities, than did trout. Both species were less tolerant of high water velocities at low water temperatures. The seasonal and diurnal pattern in habitat selection reported have important implications for habitat research and habitat‐hydraulic modelling. Copyright © 2001 John Wiley & Sons, Ltd.     

Stream macrophytes increase invertebrate production and fish habitat utilization in a California stream

Stable flow and thermal regimes, coupled with geologically derived nutrients, are drivers of enhanced productivity in volcanic spring‐fed rivers. However, little information exists on biotic mechanisms or species interactions contributing to elevated productivity at higher trophic levels. In a California stream, juvenile steelhead trout were observed to preferentially select macrophyte habitat at a rate three times greater, on average, than five other habitat types. To understand the potential rearing benefits associated with macrophytes, we conducted a manipulative experiment to determine how macrophytes affect invertebrate prey availability and stream water velocity. Macrophytes supported up to nine times greater abundance of invertebrates than adjacent open gravel habitats. They also doubled invertebrate drift rates and reduced water velocity by up to 42‐fold. The results show that aquatic macrophytes are an important stream habitat feature that may be bioenergetically more favourable for rearing salmonids than more traditional lotic habitats. We suggest that macrophytes have the potential to enhance growth rates of juvenile salmonids when compared with other habitat types. Habitats that confer growth and size advantages may ultimately improve fitness and contribute to conservation of imperilled salmonids.     

Identifying Temperature Thresholds Associated with Fish Community Changes in British Columbia,Canada, to Support Identification of Temperature Sensitive Streams

We collected fish samples and measured physical habitat characteristics, including summer stream temperatures, at 156 sites in 50 tributary streams in two sampling areas (Upper Fraser and Thompson Rivers) in British Columbia, Canada. Additional watershed characteristics were derived from GIS coverages of watershed, hydrological and climatic variables. Maximum weekly average temperature (MWAT), computed as an index of summer thermal regime, ranged from 10 to 23 °C. High values of MWAT were associated with large, warm, low relief watersheds with a high lake influence. Measures of community similarity suggested that the fish community changed most rapidly through a lower transition zone at an MWAT of about 12 °C and an upper transition zone at an MWAT of about 19 °C. These results were confirmed using existing fisheries inventory data combined with predictions of MWAT from a landscape‐scale regression model for the Thompson River watershed. For headwater sites in the Chilcotin River watershed (which drains into the middle Fraser River), the relative dominance of bull trout versus rainbow trout (based on inventory data) decreased with increasing predicted MWAT although the distinction was not as clear as for the Thompson River sites. The fish communities in these watersheds can be characterized in terms of very cold water (bull trout and some cold water species), cold water (salmonids and sculpins) and cool water (minnows and some cold water salmonids). The two transition zones (ca 12 and 19 °C) can be used to identify thresholds where small changes in stream temperature can be expected to lead to large changes in fish communities. Such clear, quantifiable thresholds are critical components of a management strategy designed to identify and protect vulnerable fish communities in streams where poor land use practices, alone or in combination with climatic change, can lead to changes in stream temperatures. Copyright © 2015 John Wiley & Sons, Ltd.     

MACROINVERTEBRATE RESPONSE TO FLOW CHANGES IN A SUBALPINE STREAM: PREDICTIONS FROM TWO‐DIMENSIONAL HYDRODYNAMIC MODELS

Two‐dimensional hydrodynamic models are being used increasingly as alternatives to traditional one‐dimensional instream flow methodologies for assessing adequacy of flow and associated faunal habitat. Two‐dimensional modelling of habitat has focused primarily on fishes, but fish‐based assessments may not model benthic macroinvertebrate habitat effectively. We extend two‐dimensional techniques to a macroinvertebrate assemblage in a high‐elevation stream in the Sierra Nevada (Dana Fork of the Tuolumne River, Yosemite National Park, CA, USA). This stream frequently flows at less than 0.03 m³ s^?1 in late summer and is representative of a common water abstraction scenario: maximum water abstraction coinciding with seasonally low flows. We used two‐dimensional modelling to predict invertebrate responses to reduced flows that might result from increased abstraction. We collected site‐specific field data on the macroinvertebrate assemblage, bed topography and flow conditions and then coupled a two‐dimensional hydrodynamic model with macroinvertebrate indices to evaluate habitat across a range of low flows. Macroinvertebrate indices were calculated for the wetted area at each flow. A surrogate flow record based on an adjacent watershed was used to evaluate frequency and duration of low flow events. Using surrogate historical records, we estimated that flow should fall below 0.071 m³ s^?1 at least 1 day in 82 of 95 years and below 0.028 m³ s^?1 in 48 of 95 years. Invertebrate metric means indicated minor losses in response to modelled discharge reductions, but wetted area decreased substantially. Responses of invertebrates to water abstraction will likely be a function of changing habitat quantity rather than quality. Copyright © 2011 John Wiley & Sons, Ltd.