Habitat suitability indices development in Denmark: are international indices applicable under small lowland stream conditions?

Abstract

The transferability of habitat suitability indices (HSIs) remains a controversial issue within physical habitat-based modelling. This issue is often exacerbated when the indices are obtained from international sources without adequate transferability tests being conducted. The aim of this paper was to assess the applicability of seven sets of HSIs (three local and four international) to small lowland Danish streams applied to large parr brown trout (Salmo trutta), an important ecological indicator in these systems. Using a simple hierarchical literature review, the applied indices were ranked firstly for their biological and physical similarities to the local Danish small lowland stream conditions, secondly on the methods used to devise the HSIs and collect the field data, and thirdly if they were locally or internationally sourced. The review ranked background information hierarchically (from most important to least) provided with the HSIs literature: (1) size class of species, (2) season, (3) river type, (4) discharge level, (5) HSIs type, (6) HSIs category, (7) data processing methods, (8) sampling methods, and (9) HSIs country of origin (local or international). To test the effectiveness of the hierarchical review-based system to rank the applicability of the HSIs, the impact of using the different HSIs for physical habitat-based modelling was tested using the hydraulic-habitat model RHYHABSIM. A set of locally derived indices were used as the baseline and compared to the other HSIs. There were large differences in the hydraulic-habitat model predictions of the maximum weighted usable habitat area (WUAmax, ranging from 0.459 to 1.023 m2m−1) and the corresponding optimum discharge (Q opt, ranging from 0.170 to 0.315 m3s−1). This emphasizes that model output is very dependent on the set of HSIs chosen; so careful consideration in the selection process is needed. The review ranking system compared favourably to the physical habitat model results, indicating that it is possible to transfer HSIs from international sources if the biological and physical conditions from their source study and stream(s) were similar to the target stream, irrespective of the country of origin. The study shows promising results for water managers wanting to use already developed HSIs by transferring international HSIs even when using simple transferability tests such as a hierarchical literature review.