Assessing Small-Stream Biotic Integrity Using Fish Assemblages across an Urban Landscape in the Puget Sound Lowlands of Western Washington

Abstract

We developed a fish index of biotic integrity (FIBI) to evaluate the relationship between urbanization and fish assemblages. The FIBI was developed with data collected from 70 sites in 30 basins and tested with a validation data set from 71 sites in 18 basins within the greater Lake Washington watershed. Fish assemblage data were evaluated at each site according to species-specific attributes and the level of human disturbance (total impervious area [TIA], impervious and vegetative coverage, mixed development, and road density) within each sampling basin. In this study TIA proved to be the most useful measure of urbanization. Approximately 50 metrics were evaluated and six were included in the final FIBI based on their response to urbanization. Each metric was scored according to standardized criteria, and the scores of all of the metrics were summed to create a final index score for each sampling location. A relationship between FIBI and TIA was expected, prompting the use of linear regression as a tool to evaluate the FIBI against five disturbance measures; correlation coefficients ranged from 0.13 to 0.68, with an overall trend of decreasing biotic integrity with increasing urbanization. Comparisons of data collected over a 10-year period from the same sampling locations demonstrated decreases in biotic integrity at most sites corresponding with an observed increase in urbanization. The FIBI developed in this study can serve as a useful management tool (in conjunction with other indicators of the condition of fish communities across Puget Sound lowland streams) as well as a metric with which to describe conditions to the public in an easily understood manner.