Coho Mortality from Road Stormwater

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Mounting evidence indicates that Coho Salmon are particularly sensitive to road pollution, delivered to streams through stormwater. Stormwater systems are managed by municipalities and regulated under the Clean Water Act specifically through the NPDES permit system. Diability and death have been observed in both juvenile and adult fish. Adult coho often enter streams to spawn, with the commencement of fall rains, exposing them to the first flush of road grime, built up over our dry summer. The death of these fish has been called "pre-spawn mortality."


  • Chow et al 2019 urban stormwater mortality coho salmon - studies effect of urban road runoff on caoho salmon during a juvenile stage. sub-lethal and lethal effects observed. Fish removed from stormwater did not appear to recover, and change in blood chemistry was observed.
  • Scholtz et al 2011 recurrent urban adult coho spawner death -
  • MacIntyre et al 2018 road stormwater effect on salmon species
  • Peter et al 2018 road contaminants affecting coho salmon
  • Fiest et al 2017 coho mortality across urban gradient
  • "Over the past two decades, coho salmon have served as a particularly important sentinel species for stormwater-driven impacts on surface water quality. This is exemplified by the recurring and premature deaths of adult coho when they return each autumn to spawn in urban watersheds (Scholz et al., 2011). Prior to dying, affected fish show a progression of behavioral symptoms including circular surface swimming and gaping, a loss of equilibrium, and immobility. This phenomenon has been variously termed “coho pre-spawn mortality (Scholz et al., 2011)”, the “coho urban runoff mortality syndrome (McIntyre et al., 2018)”, or the “coho acute spawner mortality syndrome (Peter et al., 2018)”. Spawner deaths may amount to as much as 90% of a fall run, and the syndrome has been consistently observed in field surveys across many years (Scholz et al., 2011) and many urban watersheds (Feist et al., 2017). There are multiple lines of evidence for toxic runoff as the cause of the syndrome. First, forensic analyses have largely ruled out conventional water quality parameters, spawner condition, disease, and other alternate hypotheses (Scholz et al., 2011; Spromberg et al., 2016). Second, the mortality syndrome, including symptoms, can be reproduced in experiments wherein adult coho are exposed to road runoff under controlled conditions (Spromberg et al., 2016; McIntyre et al., 2018). Third, adult coho deaths can be prevented by pre-filtering the same roadway runoff through experimental soil bioretention columns to remove pollutants (Spromberg et al., 2016). Finally, landscape modeling has shown that the severity of coho spawner mortality scales with the extent of imperviousness within a watershed (Feist et al., 2011) and, more specifically, the density of motor vehicle traffic near spawning habitats (Feist et al., 2017). The precise cause of death is not yet known, but blood chemistry in symptomatic fish indicates acidosis and a disruption of ion homeostasis (McIntyre et al., 2018)." From Chow et al 2019