Estimating the future decline of wild coho salmon populations resulting from early spawner die‐offs in urbanizing watersheds of the Pacific Northwest, USA

Since the late 1990 s, monitoring efforts evaluating the effectiveness of urban stream restoration projects in the greater metropolitan area of Seattle, Washington, USA, have detected high rates of premature mortality among adult coho salmon (Oncorhynchus kisutch) in restored spawning habitats. Affected animals display a consistent suite of symptoms (e.g., disorientation, lethargy, loss of equilibrium, gaping, fin splaying) that ultimately progresses to death on a timescale of a few hours. Annual rates of prespawn mortality observed over multiple years, across several drainages, have ranged from approximately 20% to 90% of the total fall run within a given watershed. Current weight-of-evidence suggests that coho prespawn mortality is caused by toxic urban stormwater runoff. To evaluate the potential consequences of current and future urbanization on wild coho salmon, we constructed life-history models to estimate the impacts of prespawn mortality on coho populations and metapopulations. At the low (20%) and high (90%) ends of the range of observed mortality, model results indicated the mean time to extinction of localized coho populations in 115 and 8 y, respectively. The presence of productive source populations (i.e., unaffected by prespawn mortality) within a metapopulation reduced local extinction risk. However, as more populations within a metapopulation become affected by spawner die-offs prior to spawning, the source population's productivity declined. These simple models demonstrate the potential for rapid losses from coho populations in urbanizing watersheds. Because the models do not account for possible impacts of toxic runoff to other coho life stages, they likely underestimate the cumulative impacts of nonpoint source pollution on wild populations.

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