NOAA Coastal Storms Program
Forecasting: What does this mean for the future?
High rates of coho salmon pre-spawn mortality raise concerns for the survival and productivity of salmon populations in the future. Therefore, scientists are investigating several issues in an attempt to forecast impacts on salmon populations, including
- Relationship between land use and pre-spawn mortality,
- Effects of pre-spawn mortality over time,
- Impacts of human growth and development, and
- Possible impacts of climate change.
Are land-use and land cover patterns linked to pre-spawn mortality?
Depiction of a watershed. King County DNR.
A drainage basin, or watershed, is the area of land that drains runoff into a stream or other water body. Land-use patterns in a watershed may affect the quality of water draining into the streams and water bodies. Geographic information system (GIS) software and spatial analysis techniques were used to measure the total area of various land-use and land cover types (commercial, residential, industrial, etc.) within each drainage basin. Statistical analyses revealed how much each habitat type was correlated with pre-spawn mortality rates.
How will pre-spawn mortality affect Pacific Northwest salmon over time?
Pre-spawn mortality may have important consequences for regional coho salmon populations. For example, if only half the adults returning to a stream survive to spawn, then the number of offspring from the stream will be approximately reduced by half. Over the entire Puget Sound region, these impacts on survival rates could reduce the productivity of wild coho populations. A population model was constructed to simulate how changes in various survival rates could affect the overall coho population. Preliminary model simulations only account for adult mortality, so potential impacts on other salmon life stages caused by polluted stormwater runoff are not currently included. These models demonstrate the potential for rapid loss of local populations of coho salmon in urban streams receiving polluted stormwater runoff.
What are the potential impacts of human population growth and urban development?
Coastal counties make up only 11 percent of the land area in the U.S., but are home to over half our population. Growth estimates for the counties surrounding Puget Sound indicate that the human population will increase by up to 62 percent by 2025 (Washington State Office of Financial Management). Spatial data associating pre-spawn mortality and land-use type can be combined with growth management projections to predict where pre-spawn mortality may pose a threat to coho salmon populations.
Map showing projected growth in coastal areas of Western Washington.
A future role for climate change?
Urban stream during flood conditions
Stormwater toxicity is determined to a large extent by the timing and intensity of rainfall within a watershed. For example, in urban areas, non-point source pollutants (e.g. metals from motor vehicles) tend to acumulate on roads and other impervious surfaces. These contaminants are then washed into rivers and estuaries during storm events. Any change in climate that produces a corresponding change in regional precipitation patterns will have an important influence on stormwater quality and salmon health.
To explore some outputs from this case study, proceed to Products and Resources.