The Benthic Index of Biotic Integrity (B-IBI) describes the biological condition of stream sites and their surrounding habitat based on the diversity and relative abundance of the benthic (bottom dwelling) macroinvertebrates found at the site. Ten measures of biological condition are scored and summarized as the B-IBI, which ranges from a score of 0, indicating very poor stream condition, to 100, indicating excellent condition.

• The condition of stream sites throughout Puget Sound is fairly evenly distributed over the five B-IBI biological condition categories based on the most recent sample collected since 2006. Most sites were in good (28 percent) or fair (23 percent) condition, and an additional 17 percent of sites were in excellent condition. Unfortunately, the remaining one-third of sites were in either poor (16 percent) or very poor (17 percent) condition.
• Progress toward the 2020 recovery targets was mixed based on the comparison of B-IBI scores for sites that were sampled in both 2006-2009 and 2017-2020. Overall, biological condition at some sites is declining, improving at others, and not changing at most.
• Since the 2006-2009 time period, the B-IBI scores of some streams have dropped from “excellent” to “good”, indicating that not all of the “excellent” sites have remained in excellent condition. In contrast, there has been net improvement in the biological condition of streams previously ranked “fair” by the B-IBI.
• The B-IBI is correlated with land use conversion and urbanization and scores tend to be lower in areas with greater urban development.