Landscape-Scale Estimates of Puget Sound Coho Salmon Spawner Mortality in Relation to Urbanization, Precipitation and Toxic Stormwater Runoff

Since the late 1990s, urban stream surveys have shown that coho salmon spawners experience high rates (e.g., 40-90% across a total run) of pre-spawn mortality (PSM).  Evidence to date suggests that toxic urban stormwater runoff is the likely causative agent. PSM at these high rates may threaten wild coho populations, particularly in urbanizing basins.  We previously used survey data from a limited number of Seattle-area urban streams and found a close correlation between PSM and the degree of urbanization.  These relationships were strengthened when rainfall patterns were qualitatively incorporated into the analysis.

To improve our analyses, we added spawner survey data from 45 monitoring sites, incorporated seasonal precipitation patterns, and used landscape indicators of urbanization that coincided temporally with monitoring data. Our approach identified the underlying gradients of urbanization related to PSM, which we used to predict PSM risk throughout the Puget Sound basin.  The refined analyses captured a larger geographic area within Puget Sound and improved our understanding of the interplay between urbanization and climatic drivers of PSM. Our results are consistent with other studies that suggest motor vehicles are the likely source of a chemical mixture that washes off urban landscapes into coho spawning streams.