UNCERTAINTY IN PREDICTING THE FISH-RESPONSE TO TWO-DIMENSIONAL HABITAT MODELING USING FIELD DATA I. BOAVIDA a *, J. M. SANTOS b , C. KATOPODIS c , M. T. FERREIRA b and A. PINHEIRO a a Instituto Superior Técnico, Universidade Técnica de Lisboa, Lisboa, Portugal b Centro de Estudos Florestais, Universidade Técnica de Lisboa, Lisboa, Portugal c Katopodis Ecohydraulics Ltd., Winnipeg, Canada ABSTRACT Both water managers and researchers have the same goal when it comes to fish conservation, namely, to sustain, to improve or to restore aquatic habitat. To this aim, two-dimensional (2D) hydrodynamic models have been widely used in aquatic habitat studies because they simulate flow with high accuracy and can predict habitat dynamics. The River2D model is able to integrate the habitat suitability curves for fish life stages with the simulated depth and velocity fields and the riverbed characteristics of substrate and cover, thereby estimating the corresponding weighted usable area, and thus predicting the potential distribution of fish species in the river. However, little is known about the in situ variability associated with such predictions both for hydraulic and biological data, whereas ecological responses are known to be driven by variability. Moreover, when calculating habitat availability, differences can be found by considering in the weighted usable area formulation substrate or cover or even both. To test the level of predictive accuracy of hydraulic and biological simulations, we modelled the habitat use by two fish species, the Iberian barbel Luciobarbus bocagei and the Iberian straight-mouth nase Pseudochondrostoma polylepis, according to their requirements for depth, velocity, substrate and cover and then compared measured and simulated hydraulic and biological outcomes using the River2D model. Results indicate that 2D simulation depends on data collection, especially the density and location of bed topography points. Substantial differences were found in the biological responses. Results may differ when choosing different habitat availability variables. Similarly, habitat use may also be influenced by other biotic and abiotic interactions occurring in ecosystems, and restoration planning should be aware of such variability. Copyright © 2012 John Wiley & Sons, Ltd. key words: 2D habitat models; WUA; data collection; fish preferences; cover; substrate; Pseudochondrostoma polylepis; Luciobarbus bocagei Received 2 February 2012; Revised 25 May 2012; Accepted 24 July 2012 INTRODUCTION The use of two-dimensional (2D) habitat models has increased in the last few decades following water author- ities’ efforts and research trends to restore and conserve river ecosystems (Katopodis, 2005; Bernhardt and Palmer, 2007; Jansson et al., 2007; Nilsson et al., 2007). They made their entry in the long-standing field of ecohydraulics, aim- ing to study the interactions between the ecological and the hydraulic processes at a small spatial scale, that is, the microhabitat scale. These models are considered to be a powerful tool in simulating velocity and depth patterns (Leclerc et al., 1995; Ghanem et al., 1996; Crowder and Diplas, 2000; Katopodis, 2005) as well as the spatial phys- ical heterogeneity in a river, replacing the more limited one-dimensional models (Wu and Mao, 2007; Brown and Pasternack, 2009). When coupled with a biological model of habitat (Bovee, 1982), representing preferences of differ- ent aquatic species for spatial instream attributes at different life stages, they allow a better understanding of the processes that can limit fish occurrence, including habitat heterogeneity (Bovee, 1996; Ghanem et al., 1996), while estimating the relative abundance of habitats. These biological models integrate a Habitat Suitability Index (HSI)—that is, a habitat preference that describes the frequency of an individual of a specific species or life stage occupying a microhabitat compared with the relative frequency of that microhabitat in the environment. This is to say, given a set of available micro- habitat options in the wild, individuals will choose to occupy particular habitats in preference to others. The HSI can be represented as a function of the univariate habitat suitability curves (HSCs) that represent the degree of preference dis- played by the fish. The final outcome of these models is the weighted usable area (WUA), a measure of habitat area that represents the microhabitat availability for a target species (Stalnaker et al., 1995), which is obtained from the HSC and the physical variables found at the reach scale for a given discharge. The validity of the WUA concept has been vigor- ously debated ever since its introduction. Many researchers have demonstrated correlations between WUA and fish numbers or biomass, especially when the effects of flow or recruitment over time are considered (Jowett, 1992; *Correspondence to: I. Boavida, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: isabelboavida@ist.utl.pt RIVER RESEARCH AND APPLICATIONS River Res. Applic. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/rra.2603 Copyright © 2012 John Wiley & Sons, Ltd.