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.