Assessing the Universality of Habitat Suitability Indexes (HSI) for brown trout (Salmo trutta L.) in Relation to Ecohydraulic Variables

The purpose of this work is to present the results of a literature data re-analysis of HSI and demonstrate the existence of a universal level of similarity in the ranges of hydraulic variables for the considered species.

The decline in biodiversity within freshwater ecosystems is a critical issue influenced by many factors, including the increasing pollution levels and the proliferation of invasive species. Globally, river management strategies have increasingly incorporated river restoration efforts and actions aimed at preserving freshwater species, such as the brown trout (Salmo trutta L.). From a quantitative point of view, the efficiency of proposed restoration measures on river ecomorphology is assessed via the HSI, whose use is widely embraced by aquatic ecologists, river scientists, and engineers. For the specific case of the brown trout, the HSI takes into account several environmental factors, including flow velocity, water depth, and substrate type, which directly represent the environmental characteristics of the habitat. Despite variations in HSI curves based on geographical location and other incidental factors, a notable degree of similarity and shared dependence on flow discharge is observed among data in the literature. This prompts a crucial question about the extent of similarity in these curves for a particular species concerning ecohydraulic variables.

This work aims to address this query by considering literature data of HSIs measured in different worldwide locations and by various authors, all focused on the same fish species (i.e., the brown trout). The dependence of the HSI on particular ecohydraulic variables, such as water depth, velocity, temperature, and substrate are analysed by attributing the range of optimal values to the recommendations of the related authors. Histograms indicating the level of goodness of each range are then built and eventually smoothed by using fitting functions to reveal the universal characteristics of HSIs among worldwide rivers for the species being considered.