Using a process‐based catchment‐scale model for enhancing field‐based stream assessments and predicting stream fish assemblages

Catchment‐modelling techniques, although not yet widely used in biological contexts, may be a valuable tool in the management and conservation of stream fishes. This study was undertaken to evaluate the potential of a catchment‐scale hydrologic model, the Soil and Water Assessment Tool (SWAT), to explain characteristics of stream fish assemblages and to enhance current field‐based stream assessments. Stream fish assemblages, instream habitat, geomorphology, and water quality were surveyed in 16 study catchments in Ohio, USA. Subsequently, SWAT was used to model stream discharge, sediment flux and concentration, and nitrogen and phosphorus yields for each catchment. Principal component analysis (PCA) and multiple linear regression were used to explore potential relationships between environmental factors (both reach‐level measurements and SWAT‐modelled catchment‐scale processes) and fish assemblage descriptors assessed within the framework of an Ohio fish‐based Index of Biotic Integrity (IBI). Reach‐level factors contributed to three of the four significant regression models, explaining 12% to 14% of the variation in fish species richness, the number of darter (Etheostoma) species, and the number of sensitive/intolerant species. SWAT‐modelled parameters including minimum and mean annual flow and sediment yields explained significant variation in total IBI scores (57%), species richness (63%), the number of darter species (60%), and the number of sensitive/intolerant species (61%). The results support the utility of SWAT as a complement to field‐based surveys. It is proposed that process‐based catchment‐modelling techniques such as SWAT could improve stream fish management and conservation efforts when used as a screening tool to target streams and catchments for subsequent field surveys and as a method to model potential impacts to stream ecosystems and fish assemblages in highly sensitive areas and/or inaccessible catchments.