Hydraulic habitat models based on the preferences of species for the hydraulic characteristics of their microhabitats are frequently used to evaluate the impact on the habitat of a change in river flow regime. Their application in a tropical insular environment is still limited as little is known about the hydraulic preferences of species. Hydraulic preference models have been developed for 15 taxa (diadromous shrimps and fishes) sampled in 52 rivers in the Caribbean (the French West Indies) and the Indian Ocean (the Reunion island). Five datasets were used and group 8353 samples collected by electrofishing during 320 surveys (reach × date) performed between 1999 and 2011. Generalized additive models were used to link variations of taxa density within surveys to the hydraulic characteristics of the microhabitat (velocity, depth and substrate). Hydraulic preferences within each region (Caribbean and Indian Ocean) are significant for most of the taxa and vary little between rivers and surveys. The hydraulic variables explain up to 18.1% (univariate models) and 30.0% (multivariate models) of the deviance of densities within survey. Of the taxa selected, Atya scabra, Macrobrachium heterochirus, Xiphocaris elongata and the Sicydiinae are the most demanding. Copyright © 2013 John Wiley & Sons, Ltd.

[1] Statistical hydraulic models predict the frequency distributions of point hydraulic variables, relative to their reach-averaged values, in a stream reach based on its average characteristics (e.g., discharge, depth, width, average particle size). The models initially developed in Europe have not been tested for steeper streams (>4%) with coarse grain size. We recorded water velocities and depths in 44 reaches of steep streams in tropical islands and the Alps during 69 surveys. We fitted the observed distributions of velocities and depths using a mixture of two distributions, one with low variance and the other with a high variance. Then, we predicted the mixing parameter on the basis of the reach-averaged characteristics. We compared the observed and predicted frequencies for five classes of velocities, including a class of negative velocities, and four classes of water depths. The predictions of class frequencies have a bias of 5%. Our statistical model of velocity distribution predicts the frequencies of velocity classes with an explained variance between 33 and 72% for four classes of velocity and null for a class of intermediate velocity. The statistical model of depth distributions was less efficient with an explained variance between 25 and 38% for three classes of depth and null for large depths. The average Froude number, the total height of large drops relative to the reach length and the average slope are the main explanatory variables of velocity and depth distributions.

We tested how European statistical hydraulic models developed in France and Germany predicted the frequency distributions of water depth and point-velocity measured in 14 reaches in Ecuador during 25 surveys. We first fitted the observed frequency distributions to parametric functions defined in Europe and predicted the parameters from the average characteristics of reaches (e.g. discharge rate, mean depth and width) using European regressions. When explaining the frequency of three classes of velocity and three classes of depth among reach surveys, the fitted and predicted distributions had a low absolute bias (< 3%). The residual variance of fits relative to the mean class variance was < 18%. The residual variance of predicted frequencies was 30-61% for velocity classes and 20-36% for depth classes. Overall, the European models appeared appropriate for Ecuadorian stream reaches but could be improved. Our study demonstrates the transferability of statistical hydraulic models between widely-separated geographic regions.

Résumé -Deux types d'approches techniques complémentaires sont utilisées pour guider l'établissement des débits écologiques, à l'échelle des tronçons de cours d'eau (ex. : débits réservés) comme à l'échelle de bassins versants (ex. : débits objectifs d'étiage). Les approches « hydrologiques » visent à quantifier les altérations de multiples caractéristiques du régime hydrologique et reposent sur l'identification (délicate) de relations empiriques entre altérations hydrologiques et biologiques. Les approches « habitat hydraulique », ciblées sur les débits bas à moyens, couplent des modèles hydrauliques et des modèles biologiques pour traduire certaines modifications hydrologiques en modification de qualité de l'habitat hydraulique pour les organismes. Elles ont parfois apporté des prédictions convaincantes des effets biologiques des modifications de débits d'étiage. Ces deux approches techniques ne fournissent pas directement de valeurs de débits écologiques. Nous formalisons ici une démarche technique de définition des débits écologiques, basée sur la comparaison de scé-narios de gestion et une meilleure combinaison des deux approches. La démarche comprend quatre étapes : (1) la description du contexte hydrologique naturalisé et actuel, des usages actuels et des scénarios de gestion envisagés (2) la description du contexte écologique au sens large, (3) l'identification des métriques pertinentes (hydrologiques et/ou habitats et/ou autres) pour décrire les effets des scénarios (modifications des usages, effets sur le milieu) et (4)

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