NOAA Coastal Storms Program
Lines of Evidence: Preliminary Results
The exact cause of coho salmon pre-spawn mortality in urban streams remains a mystery at this time. Initial evidence suggests that urban stormwater quality may be related to spawner die-offs. Additionally, contaminant levels of stormwater may vary according to weather patterns, therefore affecting rates of mortality. Evidence also suggests that stormwater runoff may impact other life stages of coho salmon.
The case for water quality
Pre-spawn mortality salmon open to reveal eggs.
- Pre-spawn mortality is closely associated with small streams that are receiving waters from urban stormwater discharges.
- Symptomatic fish show signs of severe distress, and coho salmon often die within hours of entering spawning habitat.
- Dead fish show no unusual signs of disease or potentially lethal pathogens.
- Conventional water quality parameters (temperature, dissolved oxygen) were within an acceptable range for salmon.
Pre-spawn mortality and weather patterns
Preliminary data suggest that the timing and intensity of rain events can affect pre-spawn mortality rates in urban streams. Coho salmon are dying during and after storm events in many urban streams in the Puget Sound region. In Longfellow Creek, pre-spawn mortality varies throughout the year, and mortality rates may be inversely related to the cumulative amount of rainfall during the spawning season. Further investigation will be necessary to determine whether this weather pattern holds true over multiple years.
Impacts of stormwater on other life stages of coho salmon
Salmon embryos in tray
To determine if stormwater pollutants affect other life stages of salmon, developing coho and chinook salmon embryos are being exposed to filtered and unfiltered urban stream water. Sheds were constructed adjacent to Longfellow Creek to house developing embryos. Embryos will be subsampled regularly during the study period to describe any differences in size and development, and to quantify mortality.
To learn about next steps and implications of this study, proceed to Forecasting: What Does This Mean for the Future?