Fish Assemblage Integrity and Watershed Urbanization Across National Parks of the National Capital Region, <scp>USA</scp>

ABSTRACT Protecting freshwater ecosystems is uniquely challenging because their biological integrity depends on watershed‐scale processes that rarely align with the boundaries of protected areas like national parks. The National Capital Region (NCR) of the US is a highly populated area that contains many national parks with different degrees of protection. Numerous streams flow through these parks, but little is known about how fish assemblage integrity is responding to mounting pressures from urbanization and population growth in the region. We analyzed nearly two decades of data (2004–2022) to (1) characterize spatial and temporal variation in fish assemblage diversity and integrity among NCR streams, (2) document temporal changes in fish abundance of different functional feeding groups across the NCR, and (3) compare historic and contemporary effects of watershed land use and streamwater chemistry on fish assemblage integrity in NCR streams. Generally, streams exposed to greater urbanization pressures (&gt; 30% watershed urban cover) scored lower on the Fish Index of Biotic Integrity (FIBI) than streams in parks located within highly protected and heavily forested watersheds. Variation in fish assemblages was primarily driven by differences in streamwater chemistry, suggesting that urbanization influences fish communities in the NCR mostly through its effects on water quality. In some cases, streams maintained relatively high FIBI scores (&gt; 3.5) despite rapid urban growth and increasing salinization. Across the region, fish assemblage integrity has not significantly changed over the past two decades, despite increasing population and urbanization pressures and notable variation among physiographic strata and parks in the NCR. Parks in the region include streams with distinct fish assemblages and serve as both a refugia for cold‐water species in the Highlands and as forested buffers that contribute to mitigate urbanization effects on fish diversity in the Piedmont and Coastal areas of the region.