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 incr emental methodology (IFIM) in the Italian regulated rivers, habitat suitabi lity curves (HSCs) have been developed with respect to several microhabitat riverine parameters. Initially, current velocity, water depth, substrate c lass size and cover were analysed with an univariate approach, then bivaria te habitat suitability models were developed from depth and velocity data. The comparison of experimental univariate HSCs with those from the literatu re outlined some differences that can essentially be explained by character istics of the investigated river, confirming the necessity of using site-sp ecific curves in relation to each experimental study area. To compare the u nivariate and bivariate approaches, the weighted usable area (WUA)-discharg e relationships were calculated using both types of HSCs. Response curves o btained from the two approaches turned out to be quite different. In PHABSI M habitat modelling. HSCs univariate functions need to be aggregated to pro duce the WUA-discharge relationship. A multiplicative criterion is generall y used for the combined suitability factor; by means of this aggregation cr iterion all variables have equal weight. According to bivariate models, dep th is much more important than velocity in defining habitat suitability req uirements. Copyright (C) 2001 John Wiley ge Sons, Ltd.