Juvenile Coho Salmon Mortality Induced by Roadway Runoff During Spring Storm Events

Abstract

Water quality-linked acute mortality has been well documented in adult coho salmon (Oncorhynchus kisutch)—often known as “Urban Runoff Mortality Syndrome” (URMS)—in Pacific Northwest creeks receiving roadway runoff during or shortly after storm events. URMS is primarily caused by exposure to an ozone transformation product, 2-((4-Methylpentan-2-yl)amino)-5-(phenylamino)cyclohexa-2,5-diene-1,4-dione (6PPD-quinone), that forms on all vehicle tires. Most research to date has focused on the high mortality rates of returning adult coho salmon spawners after fall rainstorms. Juvenile coho salmon are also sensitive to 6PPDQ, yet limited information exists concerning mortality risks to earlier life stages, especially during spring storms, when coho are hatching from spawning gravels. In laboratory studies, 6PPDQ exposure at concentrations <100 ng/L induce high rates of mortality in juveniles; these concentrations are similar to those reported in urban or roadway impacted watersheds during the spring. To assess the incidence of URMS in post-hatch juvenile coho salmon in springtime rearing habitats, we conducted a paired water quality-ecotoxicology study at a small field laboratory facility located at Miller Creek—a runoff-impacted watershed in Burien and Normandy Park, WA, USA. Juvenile coho (N=720) were exposed to either creek water or groundwater (N=120 per treatment per storm) across three spring storm events. Water quality and mortality endpoints were compared to groundwater-exposed controls. During the spring storm exposures, 6PPDQ concentrations reached 73-110 ng/L, which exceeded reported LC50 values (41-95 ng/L). Over the 24–73-hour storm exposure period, ~80% of Miller Creek-exposed juvenile salmon died with no mortality in groundwater controls. Other PPDs, transformation products and vehicle-derived chemical contaminants exhibited similar spikes in concentration and chemodynamics that were correlated to precipitation and increases in creek discharge volume. These results indicate a previously unquantified impediment to coho salmon recovery via a significant mortality risk to juvenile life stages of coho salmon that complements well-described risks to adult spawners across both spawning, rearing, and migratory habitats throughout their range.