Mitigating Impacts of Natural Hazards on Fishery Ecosystems

Poster Abstract: Impacts of Stormwater Runoff on Coho Salmon in Restored Urban Streams

Sarah G. McCarthy, Julann Spromberg, John Incardona, Blake Feist, Jana Labenia, Mark S. Myers, Linda Rhodes, Gina Ylitalo, Jenifer McIntyre, Laura Reed, Katherine Lynch, Jay Davis, Tracy K. Collier, and Nathaniel L. Scholz

doi: https://doi.org/10.47886/9781934874011.ch5

Poster Abstract.—Beginning in the late 1990s, agencies in the greater Seattle area began conducting fall surveys for spawning salmon to evaluate the effectiveness of local stream restoration efforts. These surveys detected a surprisingly high rate of mortality among migratory coho salmon Oncorhynchus kisutch females that were in good physical condition but had not yet spawned. In addition, adult coho from several different streams showed a similar progression of symptoms (disorientation, lethargy, loss of equilibrium, gaping, fin splaying) that rapidly led to death. In recent years, prespawn mortality (PSM) has been observed in many lowland urban streams, with overall rates ranging from ~25% to 90% of the fall runs.

Continuous daily surveys of wild coho spawners in a forested reference stream revealed fewer than 1% PSM. Although the precise cause of PSM in urban streams is not yet known, conventional water quality parameters and disease do not appear to be causal. Rather, the weight of evidence suggests that adult coho, which enter small urban streams following fall storm events, are acutely sensitive to nonpoint source stormwater runoff containing pollutants that originate from highly developed landscapes. These findings have important implications for salmonid restoration and conservation efforts in urban and urbanizing watersheds.