Review of Methods and Variables Used in the Assessment of Stream Condition and Capacity

In order to minimize the impacts on fish of human activities such as logging, agriculture, and urbanization, managers must understand and be able to accurately monitor how these activities affect fish habitat. Specifically, it is necessary to understand what habitat factors are affected, what habitat factors are limiting to fish abundance, how these factors interact with each other and which are the best indicators of stream condition for monitoring purposes. Several general approaches have been developed to assess stream condition. One approach is to monitor stream condition in terms of water quality. A widely used water quality monitoring method is the Index of Biotic Integrity (IBI), which examines the composition and relative abundance of a set of indicator species (usually invertebrates or fish) that are sensitive to impaired water quality. Another widely used measure of stream condition is the production of fish, usually species that are popular for recreational or commercial fisheries, or that have cultural value. Fish populations can be directly measured, but these measurements require significant time and effort to collect. More rapid assessment methods involve predicting fish production based on easily measured physical or chemical habitat factors. One frequently used method, the Instream Flow Incremental Methodology (IFIM), determines the area of habitat available at different flow levels. Other methods involve simple capacity modelling using multiple regression – correlating measurements of habitat factors (e.g. pool frequency, cover, substrate size, etc.) with fish abundance to create models that predict fish abundance. Capacity models are often species-specific, and are most accurate for streams in the same geographic area and morphology to those of the streams used to create them. These models tend to include habitat variables from three broad categories: physical, chemical, and biological. These variables explain different proportions of the total variation in fish abundance, but also have different costs in terms of the time and effort required for their measurement.