Using environmental DNA metabarcoding to determine endangered redside dace occurrence and associated community composition in urban streams

Redside dace, an endangered fish native to the Greater Toronto Area (GTA), faces significant threats from urbanization, which has degraded its habitat and water quality. Understanding its current distribution is essential for guiding conservation efforts, identifying critical habitats for protection, and targeting degraded areas for restoration. We collected water samples from streams previously inhabited by redside dace in the GTA and used environmental DNA (eDNA) metabarcoding to determine its contemporary distribution and the associated fish communities. As expected, redside dace was less frequently detected in heavily urbanized streams, highlighting the negative impact of urbanization. Co-occurrence analysis identified nine positive associations with other fish species, many of which inhabit cool water streams and build nests, suggesting possible shared habitat preferences or reliance on these species as nest hosts. This study emphasizes the value of eDNA metabarcoding in assessing urbanization's effects on redside dace and freshwater ecosystems.