Estimating Impacts on Coho Salmon Populations from Land Use-Related Spawner Mortality in Urban Streams

Since the late 1990s, monitoring efforts evaluating the effectiveness of urban stream restoration projects in the greater metropolitan area of Seattle, Washington, 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 pre-spawn mortality (PSM) observed over multiple years across several drainages have ranged from ~20% to 90% of the total fall run within a given watershed.  The current understanding of coho PSM is that it occurs when pollutants accumulate on impervious surfaces (i.e. roads, roofs, parking lots) during summer and early fall dry periods are then washed into coho-bearing streams by fall storm events. The phenomenon seems to require both specific land uses and precipitation patterns to occur. To evaluate the relationships between PSM, land use and precipitation patterns, we ran a series of spatial analyses to detect correlations between land cover (roadways, impervious surfaces, forests, etc.) patterns, seasonal rainfall patterns and PSM rates in 6 different streams. We found that the relative proportion of local roads, impervious surfaces, and commercial property in the catchment was most strongly correlated with coho PSM rates.  Further, our analyses suggest that as urban expansion continues, areas that once supported coho salmon may experience PSM rates that could jeopardize the coho salmon population viability. The same conditions that existed in streams experiencing significant PSM rates cluster around the major metropolitan areas of eastern Puget Sound. In addition, the large size of these areas suggests that the conditions associated with PSM events are not limited to our 6 study sites. So, to evaluate the potential consequences of current and future urbanization on wild coho salmon, we constructed life-history models to estimate the impacts of pre-spawn 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 eight years, respectively.  The presence of productive source populations (i.e., unaffected by pre-spawn mortality) within a metapopulation reduced local extinction risk. However, as more populations within a metapopulation become affected by spawner die-offs, the source population’s productivity declined. These simple models demonstrate the potential for rapid losses from coho populations in urbanizing watersheds.