Urban Roadway Runoff Is Lethal to Juvenile Coho, Steelhead, and Chinook Salmonids, But Not Congeneric Sockeye

We compared the sensitivity of closely related Pacific
salmon and
steelhead (Oncorhynchus spp.) to untreated urban
stormwater runoff across three storm events. Juvenile coho, sockeye,
steelhead, and Chinook were exposed for 24 h to untreated urban runoff
and then transferred to clean water for 48 h. As anticipated from
previous studies, coho were highly susceptible to runoff toxicity,
with cumulative mortality rates ranging from 92%–100% across
the three storms. By contrast, juvenile sockeye were unaffected (100%
survival), and cumulative mortality rates were intermediate for steelhead
(4%–42%) and Chinook (0%–13%). Furthermore, coho died
rapidly following the onset of stormwater exposure (generally <4
h), whereas mortality in Chinook and steelhead was delayed by 1–2
days. Similar to previous findings for coho, steelhead and Chinook
did not recover when transferred to clean water. Lastly, significant
mortality occurred in coho even when roadway runoff was diluted by
95% in clean water. Our findings extend the urban runoff mortality
syndrome in salmonids and point to a near-term need for sublethal
studies in steelhead and Chinook to more precisely understand stormwater
risks to threatened species recovery efforts in the western United
States.